Essential Fructosuria

Mendelian MONDO:0009252 Pathograph 18 Show in embeddings browser Inborn Error of Carbohydrate Metabolism Disorder of Fructose Metabolism

Essential fructosuria is a rare, clinically benign autosomal recessive disorder of fructose metabolism caused by biallelic loss-of-function variants in KHK, which encodes ketohexokinase (fructokinase), the enzyme that phosphorylates dietary fructose to fructose 1-phosphate as the first step of hepatic fructose catabolism. Without hepatic phosphorylation, ingested fructose (free, or released from sucrose or sorbitol) escapes first-pass liver metabolism, so blood fructose rises higher and falls more slowly than normal after a fructose load, and part of it is excreted unchanged in the urine. The rest is phosphorylated slowly by hexokinase in muscle and adipose tissue. Because fructose 1-phosphate is never formed, there is none of the phosphate trapping and ATP depletion that make fructose toxic in hereditary fructose intolerance. The condition causes no symptoms and is usually found incidentally, when a reducing sugar that is not glucose is detected on urinalysis; its practical importance is avoiding a misdiagnosis of diabetes mellitus or of a harmful disorder of fructose metabolism. No treatment or dietary restriction is needed.

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1
Inheritance
6
Pathophys.
4
Phenotypes
2
Gaps
18
Pathograph
1
Genes
3
Variants
1
Medical Actions
4
Differentials
4
Models
1
Deep Research
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Classifications

ICIMD (Inherited Metabolic Disorders)
galactose and fructose
👪

Inheritance

1
Autosomal recessive HP:0000007
Affected individuals carry two deficient KHK alleles. In the one family characterized at the molecular level, three affected siblings were compound heterozygotes for p.Gly40Arg and p.Ala43Thr. Heterozygous carriers excrete no more fructose after an oral load than unaffected people.
Autosomal recessive inheritance
Show evidence (4 references)
PMID:33621267 SUPPORT BACKGROUND Human Clinical
"Essential fructosuria (EF) is a benign, asymptomatic, autosomal recessive condition caused by loss-of-function variants in the ketohexokinase gene and characterized by intermittent appearance of fructose in the urine."
States autosomal recessive inheritance and the loss-of-function KHK mechanism. The sentence opens the abstract of a UK Biobank genotype study, so it restates the established picture rather than reporting a new finding.
PMID:7833921 SUPPORT Human Clinical
"In a well-characterized family, in which three of eight siblings have fructosuria, all affected individuals are compound heterozygotes for two mutations Gly40Arg and Ala43Thr."
Affected siblings carry two different KHK variants, one on each allele, the genotype expected under autosomal recessive inheritance.
PMID:28368361 SUPPORT REVIEW SYNTHESIS Human Clinical
"Heterozygotes appear to excrete no more fructose after an oral load than normal subjects"
Carriers of one deficient allele have no biochemical phenotype, which is consistent with recessive inheritance.
+ 1 more reference
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Discussions and Knowledge Gaps

2
Is essential fructosuria a hepatic-selective KHK-C deficiency with KHK-A preserved, and does it matter whether KHK-A is lost too?
KNOWLEDGE GAP khk_a_preservation
Attached to
pathophysiology#Hepatic Ketohexokinase-C Deficiency genetic#KHK
The only molecularly characterized family is compound heterozygous for a null allele and a thermolabile allele that, in recombinant enzyme, loses KHK-C activity at body temperature while KHK-A survives. Whether patients really retain peripheral KHK-A, and whether a genotype that removes both isoforms would still be benign, has not been examined in humans. Mouse knockouts lacking both isoforms are healthy, but mouse and human fructose handling are not interchangeable.
Show evidence (1 reference)
PMID:12941785 SUPPORT In Vitro
"Further mutation analysis in this rare disorder could illuminate the question of whether ketohexokinase-A activity is, unlike that of ketohexokinase-C, physiologically indispensable."
The authors pose the question explicitly.
Does lifelong KHK deficiency carry any long-term cardiometabolic consequence in humans?
KNOWLEDGE GAP long_term_outcome
Attached to
disease#Essential Fructosuria
Essential fructosuria is regarded as benign mostly from case reports and one family. A UK Biobank study found no participant with the known disease genotype, so it could not test long-term outcomes in affected people. This matters beyond the disease itself because KHK inhibition is proposed as a treatment for fatty liver disease on the premise that KHK loss is harmless.
Show evidence (2 references)
PMID:33621267 SUPPORT Human Clinical
"No participants with the compound heterozygous genotype present in subjects exhibiting the EF phenotype in the literature (Gly40Arg/Ala43Thr) were identified."
The largest population study could not assess affected genotypes.
PMID:29870677 SUPPORT Model Organism
"inhibition of KHK offers the potential to protect from diet-induced hepatic steatosis and insulin resistance."
States the therapeutic premise that draws on essential fructosuria being benign.
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Pathophysiology

6
Biallelic KHK Loss-of-Function Variants
Both KHK alleles carry deficient variants. In the index family one allele is effectively null (p.Gly40Arg) and the other encodes a thermolabile enzyme (p.Ala43Thr).
KHK hgnc:6315 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves KHK (hgnc:6315). hgnc:6315 is a gene from the HUGO Gene Nomenclature Committee.
Genetic context variant_origin: GERMLINE zygosity: COMPOUND_HETEROZYGOUS functional_impact_category: LOSS_OF_FUNCTION
Downstream
Hepatic Ketohexokinase-C Deficiency The null and thermolabile alleles together leave too little active KHK-C, the hepatic isoform, at body temperature.
Show evidence (1 reference)
PMID:7833921 SUPPORT Human Clinical
"In a well-characterized family, in which three of eight siblings have fructosuria, all affected individuals are compound heterozygotes for two mutations Gly40Arg and Ala43Thr."
Establishes the biallelic KHK genotype in affected individuals.
Hepatic Ketohexokinase-C Deficiency
Ketohexokinase activity, which phosphorylates fructose at C1 to fructose 1-phosphate, is deficient in liver. KHK-C is the isoform expressed in hepatocytes and in the proximal renal tubule. Recombinant-enzyme data suggest that the low-affinity KHK-A isoform of other tissues may be spared in at least one genotype, so the deficiency may be hepatic-selective rather than complete.
hepatocyte CL:0000182 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves hepatocyte (CL:0000182). CL:0000182 is a cell type from the Cell Ontology.
KHK hgnc:6315 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves KHK (hgnc:6315). hgnc:6315 is a gene from the HUGO Gene Nomenclature Committee.
ketohexokinase activity GO:0004454 ↓ DECREASED Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves decreased ketohexokinase activity (GO:0004454). GO:0004454 is a molecular function from the Gene Ontology.
liver UBERON:0002107 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in liver (UBERON:0002107). UBERON:0002107 is an anatomical location from the Uberon multi-species anatomy ontology.
Animal models
Khk isoform-selective and complete knockout mice (Diggle 2010) Mouse recapitulates The complete knockout removes hepatic KHK, and the mice have only minimal biochemical abnormalities on a basal diet.
Downstream
Failure of Hepatic Fructose Phosphorylation Without ketohexokinase, the liver cannot carry out the first step of fructose catabolism, so no fructose 1-phosphate is formed after a fructose load.
Show evidence (3 references)
PMID:12941785 SUPPORT BACKGROUND Human Clinical
"Hepatic ketohexokinase deficiency causes the benign disorder essential fructosuria."
Identifies hepatic ketohexokinase deficiency as the disease lesion.
PMID:12941785 SUPPORT In Vitro
"Ketohexokinase-A has much poorer substrate affinity than ketohexokinase-C for fructose but is considerably more thermostable."
KHK-C is the high-affinity fructose kinase, which is why its loss rather than that of KHK-A governs fructose clearance.
PMID:19365088 SUPPORT Model Organism
"The highly expressed KHK-C isoform localized to hepatocytes in the liver and to the straight segment of the proximal renal tubule."
Localizes KHK-C to hepatocytes and proximal tubule in rodent tissue, including knockout controls. The human localization is inferred from the conserved splicing pattern, not shown here.
Failure of Hepatic Fructose Phosphorylation
Hepatocytes cannot convert fructose to fructose 1-phosphate, so the normal rapid first-pass hepatic uptake and catabolism of dietary fructose does not occur. Because fructose 1-phosphate does not accumulate, hepatic ATP and inorganic phosphate are not depleted after a fructose load, unlike the transient depletion in healthy people and the persistent depletion in hereditary fructose intolerance.
hepatocyte CL:0000182 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves hepatocyte (CL:0000182). CL:0000182 is a cell type from the Cell Ontology.
fructose catabolic process GO:0006001 ↓ DECREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased fructose catabolic process (GO:0006001). GO:0006001 is a biological process from the Gene Ontology.
Downstream
Accumulation of Unmetabolized Fructose in the Circulation Fructose that escapes hepatic phosphorylation stays in the blood, so blood fructose rises higher and clears more slowly after a load.
Impairment of fructose metabolism Loss of hepatic fructose phosphorylation is the impaired fructose metabolism that defines the disorder.
Show evidence (3 references)
PMID:7833921 SUPPORT BACKGROUND Human Clinical
"This enzyme catalyses the first step of metabolism of dietary fructose, conversion of fructose to fructose-1-phosphate."
Defines the metabolic step that is lost.
PMID:7816517 SUPPORT Human Clinical
"In the three healthy controls after fructose (200 mg/kg, 20% solution, 2.5 min), the fructokinase-mediated increase of F-1-P was rapid, reaching 4.9 mmol/L within 3 min"
The normal response that is absent in essential fructosuria: rapid hepatic fructose 1-phosphate formation after intravenous fructose.
PMID:28368361 SUPPORT REVIEW SYNTHESIS Human Clinical
"indicating that unmetabolized fructose is innocuous, while fast partial metabolism in the liver seems to be the genesis of fructose toxicity."
Explains why loss of the first, rather than the second, enzymatic step is harmless: unphosphorylated fructose is not toxic.
Accumulation of Unmetabolized Fructose in the Circulation
After ingestion of fructose, sucrose or sorbitol, unphosphorylated fructose accumulates in the blood (hyperfructosemia) and is cleared slowly. The fructose load is distributed to the kidney and to extrahepatic tissues, which metabolize it poorly.
Animal models
Khk knockout mouse (Miller 2018) Mouse recapitulates Reproduces the elevated plasma and urinary fructose and the reduced conversion of fructose to glucose and lactate after an oral load.
KHK knockout mouse on fructose diets (Patel 2015) Mouse recapitulates KHK deletion converts dietary fructose into marked hyperfructosemia without fructose-induced hyperglycemia.
Downstream
Renal Excretion of Unmetabolized Fructose Fructose that remains in the blood is filtered and partly excreted unchanged in the urine.
Extrahepatic Hexokinase-Mediated Fructose Phosphorylation Circulating fructose that is not excreted is taken up and phosphorylated to fructose 6-phosphate by hexokinase in muscle and adipose tissue.
Increased erythrocyte fructose 3-phosphate Erythrocytes exposed to more fructose form more fructose 3-phosphate; the red-cell formation rate itself is normal, so the increase follows from substrate supply.
Show evidence (2 references)
ORPHA:2056 SUPPORT Other
"It is characterized by elevated fructosemia and presence of fructosuria following ingestion of fructose and related sugars (sucrose, sorbitol)."
Orphanet names elevated blood fructose after fructose-containing sugars.
PMID:29870677 SUPPORT BACKGROUND Human Clinical
"Hereditary deficiency of ketohexokinase (KHK), the first enzymatic step in fructose metabolism, leads to essential fructosuria in humans, characterized by elevated levels of blood and urinary fructose following fructose ingestion but is otherwise clinically benign."
Summarizes the human phenotype in the introduction of a mouse study, so it is graded as human clinical background rather than as the study's own finding.
Renal Excretion of Unmetabolized Fructose
A substantial fraction of a fructose load (10 to 20 percent, against 1 to 2 percent normally) is lost unchanged in the urine. Fructosuria is therefore intermittent and depends on recent intake of fructose, sucrose or sorbitol.
kidney UBERON:0002113 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in kidney (UBERON:0002113). UBERON:0002113 is an anatomical location from the Uberon multi-species anatomy ontology.
Computational models
Whole-body fructose metabolism model of essential fructosuria (Allen and Musante 2018) PHYSIOLOGICAL Reproduces urinary fructose loss in essential fructosuria and predicts that renal fructose reabsorption is mostly abolished.
Downstream
Fructosuria Fructose excreted unchanged appears in the urine.
Non-glucose reducing substance in urine Fructose is a reducing sugar, so urinary fructose gives a positive reducing-substance test while the glucose-oxidase strip stays negative.
Show evidence (1 reference)
PMID:28368361 SUPPORT REVIEW SYNTHESIS Human Clinical
"Between 10% and 20% of the administered dose is excreted in the urine as compared with 1%-2% in normal subjects"
Quantifies urinary fructose loss after a load relative to controls.
Extrahepatic Hexokinase-Mediated Fructose Phosphorylation
The fructose that is neither metabolized in liver nor excreted is phosphorylated at C6 by hexokinase in skeletal muscle and adipose tissue, entering glycolysis as fructose 6-phosphate. This route is slow, which is consistent with the slow clearance of blood fructose. It is an alternative pathway rather than a disease mechanism, and no clinical consequence of it is known.
skeletal muscle fiber CL:0008002 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves skeletal muscle fiber (CL:0008002). CL:0008002 is a cell type from the Cell Ontology. adipocyte CL:0000136 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves adipocyte (CL:0000136). CL:0000136 is a cell type from the Cell Ontology.
hexokinase activity GO:0004396 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves hexokinase activity (GO:0004396). GO:0004396 is a molecular function from the Gene Ontology.
Animal models
Khk knockout mouse (Miller 2018) Mouse recapitulates Shows fructose 6-phosphate in skeletal muscle of knockout but not wild-type mice, consistent with hexokinase disposal of fructose.
Show evidence (2 references)
PMID:28368361 SUPPORT REVIEW SYNTHESIS Human Clinical
"Ingested fructose is partly excreted unchanged in the urine and the rest is metabolised by an alternative pathway, namely, conversion to fructose-6-phosphate by hexokinase in adipose tissue and muscle"
Names hexokinase in muscle and adipose tissue as the alternative route.
PMID:29870677 SUPPORT Model Organism
"In addition, the observation of significant levels of fructose-6-phosphate in skeletal muscle tissue of Khk KO, but not WT, mice suggests a potential mechanism, whereby fructose is metabolized via muscle hexokinase in the absence of KHK."
Khk knockout mice accumulate fructose 6-phosphate in skeletal muscle, consistent with hexokinase-mediated disposal.
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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for Essential Fructosuria Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
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Phenotypes

4
Blood 1
Increased erythrocyte fructose 3-phosphate Abnormal erythrocyte metabolite concentration HP:0034737 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Increased erythrocyte fructose 3-phosphate, annotated with Abnormal erythrocyte metabolite concentration (HP:0034737). HP:0034737 is a phenotype from the Human Phenotype Ontology.
Show evidence (3 references)
PMID:1308858 SUPPORT Human Clinical
"Erythrocytes of 3 adult siblings with essential fructosuria contained 45-200 mumol/l fructose 3-phosphate (Fru-3-P), i.e. 3-15 times the concentration in normal controls."
Measured erythrocyte fructose 3-phosphate in three affected siblings.
PMID:1308858 SUPPORT Human Clinical
"HbA1 and HbA1c were normal."
Glycated hemoglobin is unaffected despite the raised fructose 3-phosphate.
PMID:28368361 SUPPORT REVIEW SYNTHESIS Human Clinical
"Nevertheless, neither cataracts nor diabetes have been reported in subjects with essential fructosuria and is likely of a benign nature."
The elevation has no reported clinical consequence, despite the earlier hypothesis that fructose 3-phosphate is diabetogenic.
Genitourinary 2
Fructosuria VERY_FREQUENT Elevated urine fructose level HP:6000804 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Fructosuria, annotated with Elevated urine fructose level (HP:6000804). HP:6000804 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:33621267 SUPPORT BACKGROUND Human Clinical
"Essential fructosuria (EF) is a benign, asymptomatic, autosomal recessive condition caused by loss-of-function variants in the ketohexokinase gene and characterized by intermittent appearance of fructose in the urine."
Fructosuria, intermittent, is the defining feature.
ORPHA:2056 SUPPORT Other
"HP:0031979 | Abnormal urine carbohydrate level | Very frequent (99-80%)"
Orphanet annotates the parent term, abnormal urine carbohydrate level, as very frequent. Urinary fructose is the urine carbohydrate abnormality in this disease, and this row is the source of the frequency band.
Non-glucose reducing substance in urine HP:6000389 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Non-glucose reducing substance in urine (HP:6000389). HP:6000389 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:28368361 SUPPORT REVIEW SYNTHESIS Human Clinical
"Affected persons are usually discovered on routine urinalysis by the presence of reducing sugars with a negative reaction with glucose oxidase."
Describes the urinalysis finding through which cases are ascertained.
Metabolism 1
Impairment of fructose metabolism OBLIGATE HP:0011033 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Impaired fructose tolerance, annotated with Impairment of fructose metabolism (HP:0011033). HP:0011033 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
ORPHA:2056 SUPPORT Other
"HP:0011033 | Impairment of fructose metabolism | Obligate (100%)"
Orphanet annotates impaired fructose metabolism as obligate.
PMID:28368361 SUPPORT REVIEW SYNTHESIS Human Clinical
"After an oral or IV load of fructose (1 g/kg body weight), blood fructose increases rapidly, far the level seen in controls and declines slowly."
Describes the abnormal fructose-load response.
🧬

Genetic Associations

1
KHK
Gene: KHK hgnc:6315 gene page HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is KHK (hgnc:6315). hgnc:6315 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE
Show evidence (4 references)
PMID:7833921 SUPPORT Human Clinical
"Direct evidence that mutation of the KHK structural gene is the cause of essential fructosuria was also obtained."
First molecular demonstration that KHK variants cause essential fructosuria.
PMID:7833921 SUPPORT Human Clinical
"Alternative mRNA species and alternative KHK isozymes are produced by alternative polyadenylation and splicing of the KHK gene."
Supports the existence of alternatively spliced KHK isoforms.
PMID:9799106 SUPPORT In Vitro
"tissues expressing high levels of KHK (liver, kidney and duodenum) utilise exclusively the 3c exon, while other tissues use only 3a."
RNA analysis of human and rat tissues showing that the high-activity fructose-metabolizing tissues use exon 3c (KHK-C) and other tissues use exon 3a (KHK-A).
+ 1 more reference
Variants (3)
p.Gly40Arg
Gene: KHK hgnc:6315 HUGO Gene Nomenclature Committee (hgnc) Relation: this variant is in this gene This variant is in KHK (hgnc:6315). hgnc:6315 is a gene from the HUGO Gene Nomenclature Committee. missense variant
Missense variant in a conserved region of KHK, found in trans with p.Ala43Thr in the affected siblings of the index family and absent from 52 unrelated controls. Recombinant KHK-A and KHK-C carrying Gly40Arg are inactive and largely insoluble, so the allele behaves as a null.
Show evidence (2 references)
PMID:7833921 SUPPORT Human Clinical
"Neither mutation was seen in a sample of 52 unrelated control individuals."
Gly40Arg and Ala43Thr were absent from the control sample.
PMID:12941785 SUPPORT In Vitro
"The Gly40Arg mutation seems null, rendering both ketohexokinase-A and ketohexokinase-C inactive and largely insoluble."
Recombinant-enzyme study showing Gly40Arg abolishes activity of both isoforms.
p.Ala43Thr
Gene: KHK hgnc:6315 HUGO Gene Nomenclature Committee (hgnc) Relation: this variant is in this gene This variant is in KHK (hgnc:6315). hgnc:6315 is a gene from the HUGO Gene Nomenclature Committee. missense variant
Missense variant in the same conserved region, on the allele that also carries the common p.Val49Ile change in the index family. The mutant enzyme retains activity but is thermolabile; at physiological temperature KHK-C activity is substantially lost while KHK-A activity is not.
Show evidence (2 references)
PMID:7833921 SUPPORT Human Clinical
"An additional conservative amino acid change (Val49IIe) was present on the KHK allele bearing Ala43Thr."
Places Val49Ile in cis with Ala43Thr in the index family. "Val49IIe" is the abstract's own spelling.
PMID:12941785 SUPPORT In Vitro
"The Ala43Thr mutant retains activity, but this mutation decreases the thermal stability of both ketohexokinase-A and ketohexokinase-C. At physiologic temperature, this results in significant loss of ketohexokinase-C activity but not of ketohexokinase-A."
Recombinant-enzyme data defining Ala43Thr as a thermolabile, isoform-selective deficiency.
p.Val49Ile
Gene: KHK hgnc:6315 HUGO Gene Nomenclature Committee (hgnc) Relation: this variant is in this gene This variant is in KHK (hgnc:6315). hgnc:6315 is a gene from the HUGO Gene Nomenclature Committee. missense variant
Common KHK missense variant (rs2304681), present on more than a third of chromosomes in UK Biobank participants. It does not produce fructosuria in the homozygous state and is not a disease allele on its own.
Show evidence (2 references)
PMID:33621267 SUPPORT Human Clinical
"The rs2304681:G>A (p.Val49Ile) variant was present on more than one-third (36.8%) of chromosomes; other variant alleles were rare (<1%)."
Population frequency of Val49Ile in UK Biobank.
PMID:33621267 SUPPORT BACKGROUND Human Clinical
"In contrast, Val49Ile, another common variant in normal Europeans, does not produce fructosuria when present in homozygous state [1]."
Homozygosity for Val49Ile alone does not cause the disease.
💊

Medical Actions

1
No treatment; diagnosis and reassurance
Action: Supportive CareNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Supportive Care (NCIT:C15747). NCIT:C15747 is a clinical intervention from the NCI Thesaurus. NCIT:C15747
Platform: Other
No treatment and no dietary restriction is needed. The value of the diagnosis is to reassure, and to avoid treatment for presumed diabetes or an unnecessary fructose-restricted diet for presumed hereditary fructose intolerance.
Show evidence (3 references)
ORPHA:2056 SUPPORT Other
"Essential fructosuria is clinically asymptomatic and harmless. Dietary restriction is not indicated."
Orphanet states that the disorder is harmless and no dietary restriction is indicated.
PMID:28368361 SUPPORT REVIEW SYNTHESIS Human Clinical
"Essential fructosuria | Fructokinase | 229800 | Asymptomatic | KHK/AR | No treatment"
The review's summary table lists no treatment for essential fructosuria.
PMID:18035330 SUPPORT REVIEW SYNTHESIS Human Clinical
"essential fructosuria caused by a deficiency of fructokinase, which has no clinical consequence and requires no dietary treatment"
From the review section of a hereditary fructose intolerance case report, contrasting the two disorders: essential fructosuria needs no dietary treatment.
🔬

Diagnosis

1
Urinalysis for reducing substances
A positive reducing-substance test with a negative glucose-oxidase strip prompts identification of the urinary sugar as fructose. Recognizing that the sugar is not glucose is what prevents a misdiagnosis of diabetes mellitus.
Urinalysis NCIT:C17241 NCI Thesaurus (NCIT)
Show evidence (2 references)
PMID:28368361 SUPPORT REVIEW SYNTHESIS Human Clinical
"Affected persons are usually discovered on routine urinalysis by the presence of reducing sugars with a negative reaction with glucose oxidase."
Urinalysis is the usual route to diagnosis.
PMID:28368361 SUPPORT REVIEW SYNTHESIS Human Clinical
"The misdiagnosis of diabetes is avoided only when the non-glucose nature of the sugar is recognized."
Identifying the sugar is what separates the disorder from diabetes mellitus.
📊

Prevalence

3
Worldwide
Unknown 0.77 per 100,000 1–9 per 1,000,000
Published estimate of 1 in 130,000 (about 0.77 per 100,000), quoted by Johnston et al. 2021 from an earlier source; the measure type is not stated. The condition is asymptomatic and found only incidentally, so the figure is expected to underestimate true occurrence; Orphanet and the 2017 review by Tran record the prevalence as unknown.
Show evidence (3 references)
PMID:33621267 SUPPORT BACKGROUND Human Clinical
"Its prevalence has been estimated to be 1:130,000 [2]."
Source of the 1 in 130,000 estimate, cited by the authors from earlier work rather than measured in the UK Biobank cohort.
PMID:33621267 SUPPORT BACKGROUND Human Clinical
"However, given that EF is asymptomatic, presumed benign, and does not require treatment, it may be more prevalent than reported."
Explains why the published estimate is likely an undercount.
PMID:28368361 SUPPORT REVIEW SYNTHESIS Human Clinical
"Since the disorder is asymptomatic and harmless, many cases may remain undetected and the detected ones unpublished. Prevalence is, therefore, unknown."
A review that declines to give a prevalence figure for the same reason, supporting the caveat on the numeric estimate.
Worldwide (Orphanet)
Point Prevalence Not yet documented
Orphanet records the worldwide point-prevalence class as unknown.
Show evidence (1 reference)
ORPHA:2056 SUPPORT Other
"Unknown | Worldwide | Point prevalence | ORPHANET"
Orphanet epidemiology row for essential fructosuria.
Published case reports to the 1960s, by ancestry
Cases In Literature
Laron's survey of the literature in the 1960s found 50 published cases, 18 of them in people of Jewish descent. This is an ascertainment count from historical case reports, not a population frequency.
Show evidence (1 reference)
PMID:33621267 SUPPORT BACKGROUND Human Clinical
"Laron identified 50 published cases of EF and noted that 18 of those were of Jewish decent [3]"
Reports the historical case count and its ancestry distribution as summarized from Laron's work. The misspelling "decent" is in the source.
🔀

Differential Diagnoses

4

Conditions with similar clinical presentations that must be differentiated from Essential Fructosuria:

Hereditary fructose intolerance MONDO:0009249
Overlapping Features Aldolase B (ALDOB) deficiency blocks the step after ketohexokinase, so fructose 1-phosphate accumulates and traps phosphate, causing hypoglycemia, vomiting and liver and kidney injury after fructose intake. Fructosuria can occur in both disorders, but only hereditary fructose intolerance is symptomatic and needs lifelong dietary restriction. See the Hereditary_Fructose_Intolerance entry.
Show evidence (2 references)
PMID:30675358 SUPPORT Human Clinical
"Repeated measurements of urinary fructose revealed pathologic fructosuria, but genetic testing for the three most common mutations in ALDOB resulted negative."
Fructosuria also occurs in hereditary fructose intolerance, here accompanied by hypoglycemia, so fructosuria alone does not distinguish the two.
PMID:28368361 SUPPORT REVIEW SYNTHESIS Human Clinical
"indicating that unmetabolized fructose is innocuous, while fast partial metabolism in the liver seems to be the genesis of fructose toxicity."
States the mechanistic distinction: a block at the first step leaves fructose unmetabolized and harmless, while a block after fructose 1-phosphate formation is toxic.
Fructose-1,6-bisphosphatase deficiency MONDO:0009251
Overlapping Features The third inherited disorder of the fructose pathway. It is a defect of gluconeogenesis and, unlike essential fructosuria, is symptomatic. See the Fructose-1,6-Bisphosphatase_Deficiency entry.
Show evidence (1 reference)
PMID:28368361 SUPPORT REVIEW SYNTHESIS Human Clinical
"Three inborn errors are known in the pathway of fructose metabolism; (1) essential or benign fructosuria due to fructokinase deficiency; (2) hereditary fructose intolerance; and (3) fructose-1,6-bisphosphatase deficiency."
Places the three inherited fructose-pathway disorders side by side.
Diabetes mellitus MONDO:0005015
Overlapping Features A positive urine sugar test is easily attributed to diabetic glycosuria. In essential fructosuria the glucose-oxidase reaction is negative and hyperglycemia is absent.
Show evidence (2 references)
PMID:28368361 SUPPORT REVIEW SYNTHESIS Human Clinical
"The misdiagnosis of diabetes is avoided only when the non-glucose nature of the sugar is recognized."
Names diabetes as the misdiagnosis to avoid.
ORPHA:2056 SUPPORT Other
"HP:0003074 | Hyperglycemia | Excluded (0%)"
Orphanet records hyperglycemia as excluded in essential fructosuria.
Renal tubular hexose reabsorption defect
Overlapping Features Fructosuria can also arise in the kidney, from a proximal tubular defect in hexose reabsorption. There fructosuria comes with glucosuria and galactosuria and without fructosemia, the reverse of essential fructosuria, in which blood fructose is high.
Show evidence (2 references)
PMID:40428357 SUPPORT Human Clinical
"We examined two sisters, aged 6 and 8 years, who presented with hypercalciuria, glucosuria, fructosuria, galactosuria, and atypical proteinuria."
Fructosuria together with glucosuria and galactosuria in a renal tubular disorder.
PMID:40428357 SUPPORT Human Clinical
"Primary diabetes, galactosemia, and fructosemia were excluded, suggesting a defect in cellular hexose transport in the proximal tubule."
A renal cause of fructosuria is distinguished from a metabolic one by excluding fructosemia.
🐁

Animal Models

3
Khk knockout mouse (Miller 2018)
A constitutive Khk knockout characterized with an oral [6-13C] fructose tracer. The authors propose it as a model for essential fructosuria; on a standard chow diet the mice show no metabolic abnormality.
Species
Mouse
Genotype
Khk-/- (both KHK isoforms ablated)
Publication
PMID:29870677
Pathograph links
Accumulation of Unmetabolized Fructose in the Circulation recapitulates fidelity: moderate observes: organism Reproduces the elevated plasma and urinary fructose and the reduced conversion of fructose to glucose and lactate after an oral load.
Limitations: The knockout removes both KHK-C and KHK-A, whereas recombinant-enzyme data suggest that at least one human genotype spares KHK-A. Mouse fructose handling, including intestinal fructose metabolism, may differ from human.
Readouts
Plasma and urinary 13C-fructose after oral fructose
Direction: INCREASED
Interpretation: The knockout leaves ingested fructose unmetabolized in plasma and urine.
Show evidence (1 reference)
PMID:29870677 SUPPORT Model Organism
"Significantly elevated urine and plasma 13C-fructose levels were observed in Khk KO vs. wild-type (WT) control mice, as was reduced conversion of 13C-fructose into plasma 13C-glucose and 13C-lactate."
Measured plasma and urine fructose in knockout and wild-type mice.
Show evidence (1 reference)
PMID:29870677 SUPPORT Model Organism
"Together, these results suggest that Khk KO mice may serve as a good model for essential fructosuria in humans"
The authors propose the knockout as a model of the human disorder.
Extrahepatic Hexokinase-Mediated Fructose Phosphorylation recapitulates fidelity: unknown observes: tissue Shows fructose 6-phosphate in skeletal muscle of knockout but not wild-type mice, consistent with hexokinase disposal of fructose.
Limitations: Fructose 6-phosphate in muscle is a correlate; hexokinase flux was not measured directly, and the corresponding human measurement has not been reported.
Readouts
Skeletal muscle fructose 6-phosphate
Direction: INCREASED
Interpretation: Fructose 6-phosphate appears in muscle only when KHK is absent.
Show evidence (1 reference)
PMID:29870677 SUPPORT Model Organism
"In addition, the observation of significant levels of fructose-6-phosphate in skeletal muscle tissue of Khk KO, but not WT, mice suggests a potential mechanism, whereby fructose is metabolized via muscle hexokinase in the absence of KHK."
Measured muscle fructose 6-phosphate in knockout versus wild-type mice.
KHK knockout mouse on fructose diets (Patel 2015)
KHK-/- mice fed 20 or 40 percent fructose diets, with portal and systemic blood fructose measured by HPLC. The authors state that the findings explain the hyperfructosemia of human essential fructosuria.
Species
Mouse
Genotype
KHK-/-
Publication
PMID:26316589
Pathograph links
Accumulation of Unmetabolized Fructose in the Circulation recapitulates fidelity: moderate observes: organism KHK deletion converts dietary fructose into marked hyperfructosemia without fructose-induced hyperglycemia.
Limitations: Measured under sustained high-fructose feeding in mice rather than a single oral load, and the knockout removes both KHK isoforms.
Readouts
Systemic blood fructose on a fructose diet
Direction: INCREASED
Interpretation: Without KHK, dietary fructose accumulates in the circulation.
Show evidence (1 reference)
PMID:26316589 SUPPORT Model Organism
"Deletion of KHK prevented fructose-induced hyperglycemia, but caused dramatic hyperfructosemia (>1 mM) with reversed portal to systemic gradients."
Measured blood fructose and glucose in KHK-/- versus wild-type mice.
Show evidence (1 reference)
PMID:26316589 SUPPORT Model Organism
"These findings explain the hyperfructosemia of human hereditary fructosuria as well as the hyperglycemia of fructose-induced metabolic syndrome."
The authors relate the knockout phenotype to the human disorder.
Khk isoform-selective and complete knockout mice (Diggle 2010)
Two targeted alleles, one ablating both KHK isoforms and one ablating only the minor KHK-A isoform. Both homozygous lines are healthy and fertile, so neither isoform is needed for normal growth and development, consistent with the benign human phenotype.
Species
Mouse
Genotype
Khk(delta/delta) (both isoforms) and Khk(3a/3a) (KHK-A only)
Publication
PMID:20841500
Pathograph links
Hepatic Ketohexokinase-C Deficiency recapitulates fidelity: moderate observes: organism The complete knockout removes hepatic KHK, and the mice have only minimal biochemical abnormalities on a basal diet.
Limitations: The complete knockout also removes KHK-A, which the human Ala43Thr-containing genotype may spare. Fructose challenge readouts are not reported in the abstract.
Show evidence (1 reference)
PMID:20841500 SUPPORT Model Organism
"Both Khk(3a/3a) and Khk(Δ/Δ) homozygous mice were healthy and fertile and displayed minimal biochemical abnormalities under basal dietary conditions."
Absence of KHK is compatible with health in the mouse, as in the human disorder.
🧮

Computational Models

1
Whole-body fructose metabolism model of essential fructosuria (Allen and Musante 2018) PHYSIOLOGICAL
A mathematical model of oral and intravenous fructose disposition, calibrated to healthy subjects and then adapted to reproduce published essential fructosuria data. To fit those data the model needed adaptations beyond hepatic KHK loss: altered portal absorption, slower peripheral metabolism, near-absent renal fructose reabsorption, and upregulated alternative hepatic pathways. These are model predictions, not measurements.
PMID:29664676
Pathograph links
Renal Excretion of Unmetabolized Fructose partially recapitulates fidelity: unknown observes: organism Reproduces urinary fructose loss in essential fructosuria and predicts that renal fructose reabsorption is mostly abolished.
Limitations: Calibrated to small, historical essential fructosuria datasets; the predicted adaptations have not been tested experimentally.
Divergences from the human mechanism
calibration provenance qualifying
The parameters for the fructosuria state were fitted to the few published fructose-load studies in affected people, so the predicted loss of renal reabsorption is constrained only by those data.
Show evidence (1 reference)
PMID:29664676 SUPPORT Computational
"Specifically, we predict that fructose absorption into the portal vein is altered, peripheral metabolism is slowed, renal reabsorption of fructose is mostly ablated, and alternate pathways for hepatic metabolism of fructose are upregulated."
The model's predictions for how the fructosuria state differs from normal.
{ }

Source YAML

click to show 
name: Essential Fructosuria
creation_date: "2026-10-14T20:49:40Z"
category: Mendelian
synonyms:
- fructokinase deficiency
- ketohexokinase deficiency
- hepatic fructokinase deficiency
- benign fructosuria
description: >-
  Essential fructosuria is a rare, clinically benign autosomal recessive
  disorder of fructose metabolism caused by biallelic loss-of-function variants
  in KHK, which encodes ketohexokinase (fructokinase), the enzyme that
  phosphorylates dietary fructose to fructose 1-phosphate as the first step of
  hepatic fructose catabolism. Without hepatic phosphorylation, ingested
  fructose (free, or released from sucrose or sorbitol) escapes first-pass
  liver metabolism, so blood fructose rises higher and falls more slowly than
  normal after a fructose load, and part of it is excreted unchanged in the
  urine. The rest is phosphorylated slowly by hexokinase in muscle and adipose
  tissue. Because fructose 1-phosphate is never formed, there is none of the
  phosphate trapping and ATP depletion that make fructose toxic in hereditary
  fructose intolerance. The condition causes no symptoms and is usually found
  incidentally, when a reducing sugar that is not glucose is detected on
  urinalysis; its practical importance is avoiding a misdiagnosis of diabetes
  mellitus or of a harmful disorder of fructose metabolism. No treatment or
  dietary restriction is needed.
disease_term:
  preferred_term: essential fructosuria
  term:
    id: MONDO:0009252
    label: essential fructosuria
parents:
- Inborn Error of Carbohydrate Metabolism
- Disorder of Fructose Metabolism
classifications:
  icimd_category:
  - classification_value: galactose_and_fructose
    notes: >-
      ICIMD group "Galactose and fructose" under category "Disorders of
      carbohydrate metabolism", matching the MONDO parent MONDO:0800152
      disorder of galactose and fructose metabolism.
inheritance:
- name: Autosomal recessive
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  description: >-
    Affected individuals carry two deficient KHK alleles. In the one family
    characterized at the molecular level, three affected siblings were compound
    heterozygotes for p.Gly40Arg and p.Ala43Thr. Heterozygous carriers excrete
    no more fructose after an oral load than unaffected people.
  evidence:
  - reference: PMID:33621267
    reference_title: "Prevalence and cardiometabolic correlates of ketohexokinase gene variants among UK Biobank participants."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: BACKGROUND
    snippet: "Essential fructosuria (EF) is a benign, asymptomatic, autosomal recessive condition caused by loss-of-function variants in the ketohexokinase gene and characterized by intermittent appearance of fructose in the urine."
    explanation: >-
      States autosomal recessive inheritance and the loss-of-function KHK
      mechanism. The sentence opens the abstract of a UK Biobank genotype study,
      so it restates the established picture rather than reporting a new
      finding.
  - reference: PMID:7833921
    reference_title: "Molecular basis of essential fructosuria: molecular cloning and mutational analysis of human ketohexokinase (fructokinase)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In a well-characterized family, in which three of eight siblings have fructosuria, all affected individuals are compound heterozygotes for two mutations Gly40Arg and Ala43Thr."
    explanation: >-
      Affected siblings carry two different KHK variants, one on each allele,
      the genotype expected under autosomal recessive inheritance.
  - reference: PMID:28368361
    reference_title: "Inborn Errors of Fructose Metabolism. What Can We Learn from Them?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "Heterozygotes appear to excrete no more fructose after an oral load than normal subjects"
    explanation: >-
      Carriers of one deficient allele have no biochemical phenotype, which is
      consistent with recessive inheritance.
  - reference: ORPHA:2056
    reference_title: "Essential fructosuria"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Essential fructosuria is a rare autosomal recessive disorder of fructose metabolism caused by a deficiency of fructokinaseenzyme activity."
    explanation: >-
      The Orphanet definition records autosomal recessive inheritance. The
      run-together "fructokinaseenzyme" is Orphanet's own text.
prevalence:
- population: Worldwide
  measure_type: UNKNOWN
  prevalence_class: BAND_1_9_PER_1000000
  rate_per_100000: 0.77
  notes: >-
    Published estimate of 1 in 130,000 (about 0.77 per 100,000), quoted by
    Johnston et al. 2021 from an earlier source; the measure type is not stated.
    The condition is asymptomatic and found only incidentally, so the figure is
    expected to underestimate true occurrence; Orphanet and the 2017 review by
    Tran record the prevalence as unknown.
  evidence:
  - reference: PMID:33621267
    reference_title: "Prevalence and cardiometabolic correlates of ketohexokinase gene variants among UK Biobank participants."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: BACKGROUND
    snippet: "Its prevalence has been estimated to be 1:130,000 [2]."
    explanation: >-
      Source of the 1 in 130,000 estimate, cited by the authors from earlier
      work rather than measured in the UK Biobank cohort.
  - reference: PMID:33621267
    reference_title: "Prevalence and cardiometabolic correlates of ketohexokinase gene variants among UK Biobank participants."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: BACKGROUND
    snippet: "However, given that EF is asymptomatic, presumed benign, and does not require treatment, it may be more prevalent than reported."
    explanation: Explains why the published estimate is likely an undercount.
  - reference: PMID:28368361
    reference_title: "Inborn Errors of Fructose Metabolism. What Can We Learn from Them?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "Since the disorder is asymptomatic and harmless, many cases may remain undetected and the detected ones unpublished. Prevalence is, therefore, unknown."
    explanation: >-
      A review that declines to give a prevalence figure for the same reason,
      supporting the caveat on the numeric estimate.
- population: Worldwide (Orphanet)
  measure_type: POINT_PREVALENCE
  prevalence_class: NOT_YET_DOCUMENTED
  notes: Orphanet records the worldwide point-prevalence class as unknown.
  evidence:
  - reference: ORPHA:2056
    reference_title: "Essential fructosuria"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Unknown | Worldwide | Point prevalence | ORPHANET"
    explanation: Orphanet epidemiology row for essential fructosuria.
- population: Published case reports to the 1960s, by ancestry
  measure_type: CASES_IN_LITERATURE
  notes: >-
    Laron's survey of the literature in the 1960s found 50 published cases, 18
    of them in people of Jewish descent. This is an ascertainment count from
    historical case reports, not a population frequency.
  evidence:
  - reference: PMID:33621267
    reference_title: "Prevalence and cardiometabolic correlates of ketohexokinase gene variants among UK Biobank participants."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: BACKGROUND
    snippet: "Laron identified 50 published cases of EF and noted that 18 of those were of Jewish decent [3]"
    explanation: >-
      Reports the historical case count and its ancestry distribution as
      summarized from Laron's work. The misspelling "decent" is in the source.
genetic:
- name: KHK
  gene_term:
    preferred_term: KHK
    term:
      id: hgnc:6315
      label: KHK
  relationship_type: CAUSATIVE
  notes: >-
    KHK is alternatively spliced into KHK-C (exon 3c), the high-affinity
    isoform of liver, kidney and small intestine, and KHK-A (exon 3a), a
    low-affinity isoform expressed at low levels in other tissues. Only one
    family has been characterized at the molecular level (compound
    heterozygosity for p.Gly40Arg and p.Ala43Thr), so genotype-phenotype
    correlation beyond that family is undefined. No ClinGen gene-disease
    validity record for KHK is cached in references_cache.
  evidence:
  - reference: PMID:7833921
    reference_title: "Molecular basis of essential fructosuria: molecular cloning and mutational analysis of human ketohexokinase (fructokinase)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Direct evidence that mutation of the KHK structural gene is the cause of essential fructosuria was also obtained."
    explanation: First molecular demonstration that KHK variants cause essential fructosuria.
  - reference: PMID:7833921
    reference_title: "Molecular basis of essential fructosuria: molecular cloning and mutational analysis of human ketohexokinase (fructokinase)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Alternative mRNA species and alternative KHK isozymes are produced by alternative polyadenylation and splicing of the KHK gene."
    explanation: Supports the existence of alternatively spliced KHK isoforms.
  - reference: PMID:9799106
    reference_title: "Structure and alternative splicing of the ketohexokinase gene."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "tissues expressing high levels of KHK (liver, kidney and duodenum) utilise exclusively the 3c exon, while other tissues use only 3a."
    explanation: >-
      RNA analysis of human and rat tissues showing that the high-activity
      fructose-metabolizing tissues use exon 3c (KHK-C) and other tissues use
      exon 3a (KHK-A).
  - reference: ORPHA:2056
    reference_title: "Essential fructosuria"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "KHK | ketohexokinase | hgnc:6315 | Disease-causing germline mutation(s) in"
    explanation: Orphanet records KHK as the disease-causing gene.
  variants:
  - name: p.Gly40Arg
    description: >-
      Missense variant in a conserved region of KHK, found in trans with
      p.Ala43Thr in the affected siblings of the index family and absent from
      52 unrelated controls. Recombinant KHK-A and KHK-C carrying Gly40Arg are
      inactive and largely insoluble, so the allele behaves as a null.
    gene:
      preferred_term: KHK
      term:
        id: hgnc:6315
        label: KHK
    type: missense_variant
    evidence:
    - reference: PMID:7833921
      reference_title: "Molecular basis of essential fructosuria: molecular cloning and mutational analysis of human ketohexokinase (fructokinase)."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Neither mutation was seen in a sample of 52 unrelated control individuals."
      explanation: Gly40Arg and Ala43Thr were absent from the control sample.
    - reference: PMID:12941785
      reference_title: "Properties of normal and mutant recombinant human ketohexokinases and implications for the pathogenesis of essential fructosuria."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "The Gly40Arg mutation seems null, rendering both ketohexokinase-A and ketohexokinase-C inactive and largely insoluble."
      explanation: Recombinant-enzyme study showing Gly40Arg abolishes activity of both isoforms.
  - name: p.Ala43Thr
    description: >-
      Missense variant in the same conserved region, on the allele that also
      carries the common p.Val49Ile change in the index family. The mutant
      enzyme retains activity but is thermolabile; at physiological temperature
      KHK-C activity is substantially lost while KHK-A activity is not.
    gene:
      preferred_term: KHK
      term:
        id: hgnc:6315
        label: KHK
    type: missense_variant
    evidence:
    - reference: PMID:7833921
      reference_title: "Molecular basis of essential fructosuria: molecular cloning and mutational analysis of human ketohexokinase (fructokinase)."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "An additional conservative amino acid change (Val49IIe) was present on the KHK allele bearing Ala43Thr."
      explanation: >-
        Places Val49Ile in cis with Ala43Thr in the index family. "Val49IIe" is
        the abstract's own spelling.
    - reference: PMID:12941785
      reference_title: "Properties of normal and mutant recombinant human ketohexokinases and implications for the pathogenesis of essential fructosuria."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "The Ala43Thr mutant retains activity, but this mutation decreases the thermal stability of both ketohexokinase-A and ketohexokinase-C. At physiologic temperature, this results in significant loss of ketohexokinase-C activity but not of ketohexokinase-A."
      explanation: Recombinant-enzyme data defining Ala43Thr as a thermolabile, isoform-selective deficiency.
  - name: p.Val49Ile
    description: >-
      Common KHK missense variant (rs2304681), present on more than a third of
      chromosomes in UK Biobank participants. It does not produce fructosuria in
      the homozygous state and is not a disease allele on its own.
    gene:
      preferred_term: KHK
      term:
        id: hgnc:6315
        label: KHK
    type: missense_variant
    evidence:
    - reference: PMID:33621267
      reference_title: "Prevalence and cardiometabolic correlates of ketohexokinase gene variants among UK Biobank participants."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The rs2304681:G>A (p.Val49Ile) variant was present on more than one-third (36.8%) of chromosomes; other variant alleles were rare (<1%)."
      explanation: Population frequency of Val49Ile in UK Biobank.
    - reference: PMID:33621267
      reference_title: "Prevalence and cardiometabolic correlates of ketohexokinase gene variants among UK Biobank participants."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      quote_role: BACKGROUND
      snippet: "In contrast, Val49Ile, another common variant in normal Europeans, does not produce fructosuria when present in homozygous state [1]."
      explanation: Homozygosity for Val49Ile alone does not cause the disease.
pathophysiology:
- name: Biallelic KHK Loss-of-Function Variants
  description: >-
    Both KHK alleles carry deficient variants. In the index family one allele is
    effectively null (p.Gly40Arg) and the other encodes a thermolabile enzyme
    (p.Ala43Thr).
  biological_scale: MOLECULAR
  genes:
  - preferred_term: KHK
    term:
      id: hgnc:6315
      label: KHK
  genetic_context:
    variant_origin: GERMLINE
    zygosity: COMPOUND_HETEROZYGOUS
    functional_impact_category: LOSS_OF_FUNCTION
  evidence:
  - reference: PMID:7833921
    reference_title: "Molecular basis of essential fructosuria: molecular cloning and mutational analysis of human ketohexokinase (fructokinase)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In a well-characterized family, in which three of eight siblings have fructosuria, all affected individuals are compound heterozygotes for two mutations Gly40Arg and Ala43Thr."
    explanation: Establishes the biallelic KHK genotype in affected individuals.
  downstream:
  - target: Hepatic Ketohexokinase-C Deficiency
    causal_link_type: DIRECT
    description: >-
      The null and thermolabile alleles together leave too little active
      KHK-C, the hepatic isoform, at body temperature.
    evidence:
    - reference: PMID:12941785
      reference_title: "Properties of normal and mutant recombinant human ketohexokinases and implications for the pathogenesis of essential fructosuria."
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: IN_VITRO
      snippet: "Affected individuals who carry both mutations therefore probably have a selective deficiency of hepatic ketohexokinase, with peripheral ketohexokinase-A being preserved."
      explanation: >-
        Infers from recombinant-enzyme properties that the compound
        heterozygous genotype produces hepatic KHK deficiency. The authors
        state it as probable rather than measured in patient tissue.
- name: Hepatic Ketohexokinase-C Deficiency
  description: >-
    Ketohexokinase activity, which phosphorylates fructose at C1 to fructose
    1-phosphate, is deficient in liver. KHK-C is the isoform expressed in
    hepatocytes and in the proximal renal tubule. Recombinant-enzyme data
    suggest that the low-affinity KHK-A isoform of other tissues may be spared
    in at least one genotype, so the deficiency may be hepatic-selective rather
    than complete.
  biological_scale: MOLECULAR
  genes:
  - preferred_term: KHK
    term:
      id: hgnc:6315
      label: KHK
  molecular_functions:
  - preferred_term: ketohexokinase activity
    term:
      id: GO:0004454
      label: ketohexokinase activity
    modifier: DECREASED
  cell_types:
  - preferred_term: hepatocyte
    term:
      id: CL:0000182
      label: hepatocyte
  locations:
  - preferred_term: liver
    term:
      id: UBERON:0002107
      label: liver
  evidence:
  - reference: PMID:12941785
    reference_title: "Properties of normal and mutant recombinant human ketohexokinases and implications for the pathogenesis of essential fructosuria."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: BACKGROUND
    snippet: "Hepatic ketohexokinase deficiency causes the benign disorder essential fructosuria."
    explanation: Identifies hepatic ketohexokinase deficiency as the disease lesion.
  - reference: PMID:12941785
    reference_title: "Properties of normal and mutant recombinant human ketohexokinases and implications for the pathogenesis of essential fructosuria."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Ketohexokinase-A has much poorer substrate affinity than ketohexokinase-C for fructose but is considerably more thermostable."
    explanation: >-
      KHK-C is the high-affinity fructose kinase, which is why its loss rather
      than that of KHK-A governs fructose clearance.
  - reference: PMID:19365088
    reference_title: "Ketohexokinase: expression and localization of the principal fructose-metabolizing enzyme."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "The highly expressed KHK-C isoform localized to hepatocytes in the liver and to the straight segment of the proximal renal tubule."
    explanation: >-
      Localizes KHK-C to hepatocytes and proximal tubule in rodent tissue,
      including knockout controls. The human localization is inferred from the
      conserved splicing pattern, not shown here.
  downstream:
  - target: Failure of Hepatic Fructose Phosphorylation
    causal_link_type: DIRECT
    description: >-
      Without ketohexokinase, the liver cannot carry out the first step of
      fructose catabolism, so no fructose 1-phosphate is formed after a
      fructose load.
    evidence:
    - reference: PMID:7816517
      reference_title: "Changes of liver metabolite concentrations in adults with disorders of fructose metabolism after intravenous fructose by 31P magnetic resonance spectroscopy."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "In the patient with essential fructosuria, the concentrations of F-1-P, ATP, and Pi remained unchanged, confirming that fructokinase was indeed inactive."
      explanation: >-
        In vivo liver 31P MRS after intravenous fructose shows no fructose
        1-phosphate formation, the direct consequence of absent hepatic
        fructokinase activity.
- name: Failure of Hepatic Fructose Phosphorylation
  description: >-
    Hepatocytes cannot convert fructose to fructose 1-phosphate, so the normal
    rapid first-pass hepatic uptake and catabolism of dietary fructose does not
    occur. Because fructose 1-phosphate does not accumulate, hepatic ATP and
    inorganic phosphate are not depleted after a fructose load, unlike the
    transient depletion in healthy people and the persistent depletion in
    hereditary fructose intolerance.
  biological_scale: CELLULAR
  cell_types:
  - preferred_term: hepatocyte
    term:
      id: CL:0000182
      label: hepatocyte
  biological_processes:
  - preferred_term: fructose catabolic process
    term:
      id: GO:0006001
      label: fructose catabolic process
    modifier: DECREASED
  chemical_entities:
  - preferred_term: fructose 1-phosphate
    term:
      id: CHEBI:78737
      label: fructose 1-phosphate
    modifier: DECREASED
  evidence:
  - reference: PMID:7833921
    reference_title: "Molecular basis of essential fructosuria: molecular cloning and mutational analysis of human ketohexokinase (fructokinase)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: BACKGROUND
    snippet: "This enzyme catalyses the first step of metabolism of dietary fructose, conversion of fructose to fructose-1-phosphate."
    explanation: Defines the metabolic step that is lost.
  - reference: PMID:7816517
    reference_title: "Changes of liver metabolite concentrations in adults with disorders of fructose metabolism after intravenous fructose by 31P magnetic resonance spectroscopy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In the three healthy controls after fructose (200 mg/kg, 20% solution, 2.5 min), the fructokinase-mediated increase of F-1-P was rapid, reaching 4.9 mmol/L within 3 min"
    explanation: >-
      The normal response that is absent in essential fructosuria: rapid
      hepatic fructose 1-phosphate formation after intravenous fructose.
  - reference: PMID:28368361
    reference_title: "Inborn Errors of Fructose Metabolism. What Can We Learn from Them?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "indicating that unmetabolized fructose is innocuous, while fast partial metabolism in the liver seems to be the genesis of fructose toxicity."
    explanation: >-
      Explains why loss of the first, rather than the second, enzymatic step is
      harmless: unphosphorylated fructose is not toxic.
  downstream:
  - target: Accumulation of Unmetabolized Fructose in the Circulation
    causal_link_type: DIRECT
    description: >-
      Fructose that escapes hepatic phosphorylation stays in the blood, so
      blood fructose rises higher and clears more slowly after a load.
    evidence:
    - reference: PMID:28368361
      reference_title: "Inborn Errors of Fructose Metabolism. What Can We Learn from Them?"
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      quote_role: REVIEW_SYNTHESIS
      snippet: "After an oral or IV load of fructose (1 g/kg body weight), blood fructose increases rapidly, far the level seen in controls and declines slowly."
      explanation: >-
        Describes the exaggerated and prolonged rise in blood fructose after a
        load. The missing word after "far" is in the source.
  - target: Impairment of fructose metabolism
    causal_link_type: DIRECT
    description: >-
      Loss of hepatic fructose phosphorylation is the impaired fructose
      metabolism that defines the disorder.
    evidence:
    - reference: ORPHA:2056
      reference_title: "Essential fructosuria"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "It is characterized by elevated fructosemia and presence of fructosuria following ingestion of fructose and related sugars (sucrose, sorbitol)."
      explanation: >-
        Orphanet characterizes the disorder by fructose that is not metabolized
        normally after ingestion of fructose, sucrose or sorbitol.
- name: Accumulation of Unmetabolized Fructose in the Circulation
  description: >-
    After ingestion of fructose, sucrose or sorbitol, unphosphorylated fructose
    accumulates in the blood (hyperfructosemia) and is cleared slowly. The
    fructose load is distributed to the kidney and to extrahepatic tissues,
    which metabolize it poorly.
  biological_scale: ORGANISM
  chemical_entities:
  - preferred_term: fructose
    term:
      id: CHEBI:28757
      label: fructose
    modifier: INCREASED
  evidence:
  - reference: ORPHA:2056
    reference_title: "Essential fructosuria"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "It is characterized by elevated fructosemia and presence of fructosuria following ingestion of fructose and related sugars (sucrose, sorbitol)."
    explanation: Orphanet names elevated blood fructose after fructose-containing sugars.
  - reference: PMID:29870677
    reference_title: "Ketohexokinase knockout mice, a model for essential fructosuria, exhibit altered fructose metabolism and are protected from diet-induced metabolic defects."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: BACKGROUND
    snippet: "Hereditary deficiency of ketohexokinase (KHK), the first enzymatic step in fructose metabolism, leads to essential fructosuria in humans, characterized by elevated levels of blood and urinary fructose following fructose ingestion but is otherwise clinically benign."
    explanation: >-
      Summarizes the human phenotype in the introduction of a mouse study, so
      it is graded as human clinical background rather than as the study's own
      finding.
  downstream:
  - target: Renal Excretion of Unmetabolized Fructose
    causal_link_type: DIRECT
    description: >-
      Fructose that remains in the blood is filtered and partly excreted
      unchanged in the urine.
    evidence:
    - reference: PMID:28368361
      reference_title: "Inborn Errors of Fructose Metabolism. What Can We Learn from Them?"
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      quote_role: REVIEW_SYNTHESIS
      snippet: "The loss of fructose into the urine in this condition illustrates well the fact that fructose, having escaped hepatic metabolism, is poorly metabolized in extrahepatic tissues."
      explanation: Links urinary fructose loss to fructose escaping hepatic metabolism.
  - target: Extrahepatic Hexokinase-Mediated Fructose Phosphorylation
    causal_link_type: DIRECT
    description: >-
      Circulating fructose that is not excreted is taken up and phosphorylated
      to fructose 6-phosphate by hexokinase in muscle and adipose tissue.
    evidence:
    - reference: PMID:28368361
      reference_title: "Inborn Errors of Fructose Metabolism. What Can We Learn from Them?"
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      quote_role: REVIEW_SYNTHESIS
      snippet: "Ingested fructose is partly excreted unchanged in the urine and the rest is metabolised by an alternative pathway, namely, conversion to fructose-6-phosphate by hexokinase in adipose tissue and muscle"
      explanation: Describes the alternative extrahepatic route taken by the fructose that is not excreted.
  - target: Increased erythrocyte fructose 3-phosphate
    causal_link_type: DIRECT
    description: >-
      Erythrocytes exposed to more fructose form more fructose 3-phosphate; the
      red-cell formation rate itself is normal, so the increase follows from
      substrate supply.
    evidence:
    - reference: PMID:1308858
      reference_title: "Essential fructosuria: increased levels of fructose 3-phosphate in erythrocytes."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "An oral load with 50 g of fructose produced an additional 40 mumol/l increase of erythrocyte Fru-3-P after 5 h."
      explanation: A fructose load raises erythrocyte fructose 3-phosphate further in affected adults.
- name: Renal Excretion of Unmetabolized Fructose
  description: >-
    A substantial fraction of a fructose load (10 to 20 percent, against 1 to 2
    percent normally) is lost unchanged in the urine. Fructosuria is therefore
    intermittent and depends on recent intake of fructose, sucrose or
    sorbitol.
  biological_scale: ORGANISM
  locations:
  - preferred_term: kidney
    term:
      id: UBERON:0002113
      label: kidney
  evidence:
  - reference: PMID:28368361
    reference_title: "Inborn Errors of Fructose Metabolism. What Can We Learn from Them?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "Between 10% and 20% of the administered dose is excreted in the urine as compared with 1%-2% in normal subjects"
    explanation: Quantifies urinary fructose loss after a load relative to controls.
  downstream:
  - target: Fructosuria
    causal_link_type: DIRECT
    description: Fructose excreted unchanged appears in the urine.
    evidence:
    - reference: PMID:29664676
      reference_title: "A mathematical analysis of adaptations to the metabolic fate of fructose in essential fructosuria subjects."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      quote_role: BACKGROUND
      snippet: "The only reported symptom of EF is the appearance of fructose in the urine following either oral or intravenous fructose administration."
      explanation: >-
        Urinary fructose after a fructose load is the defining finding. Quoted
        from the background of a modelling study.
  - target: Non-glucose reducing substance in urine
    causal_link_type: DIRECT
    description: >-
      Fructose is a reducing sugar, so urinary fructose gives a positive
      reducing-substance test while the glucose-oxidase strip stays negative.
    evidence:
    - reference: PMID:28368361
      reference_title: "Inborn Errors of Fructose Metabolism. What Can We Learn from Them?"
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      quote_role: REVIEW_SYNTHESIS
      snippet: "Affected persons are usually discovered on routine urinalysis by the presence of reducing sugars with a negative reaction with glucose oxidase."
      explanation: Urinary fructose is detected as a reducing sugar that is not glucose.
- name: Extrahepatic Hexokinase-Mediated Fructose Phosphorylation
  description: >-
    The fructose that is neither metabolized in liver nor excreted is
    phosphorylated at C6 by hexokinase in skeletal muscle and adipose tissue,
    entering glycolysis as fructose 6-phosphate. This route is slow, which is
    consistent with the slow clearance of blood fructose. It is an alternative
    pathway rather than a disease mechanism, and no clinical consequence of it
    is known.
  biological_scale: CELLULAR
  role: alternative metabolic route
  cell_types:
  - preferred_term: skeletal muscle fiber
    term:
      id: CL:0008002
      label: skeletal muscle fiber
  - preferred_term: adipocyte
    term:
      id: CL:0000136
      label: adipocyte
  molecular_functions:
  - preferred_term: hexokinase activity
    term:
      id: GO:0004396
      label: hexokinase activity
  chemical_entities:
  - preferred_term: fructose 6-phosphate
    term:
      id: CHEBI:88003
      label: fructose 6-phosphate
    modifier: INCREASED
  evidence:
  - reference: PMID:28368361
    reference_title: "Inborn Errors of Fructose Metabolism. What Can We Learn from Them?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "Ingested fructose is partly excreted unchanged in the urine and the rest is metabolised by an alternative pathway, namely, conversion to fructose-6-phosphate by hexokinase in adipose tissue and muscle"
    explanation: Names hexokinase in muscle and adipose tissue as the alternative route.
  - reference: PMID:29870677
    reference_title: "Ketohexokinase knockout mice, a model for essential fructosuria, exhibit altered fructose metabolism and are protected from diet-induced metabolic defects."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "In addition, the observation of significant levels of fructose-6-phosphate in skeletal muscle tissue of Khk KO, but not WT, mice suggests a potential mechanism, whereby fructose is metabolized via muscle hexokinase in the absence of KHK."
    explanation: Khk knockout mice accumulate fructose 6-phosphate in skeletal muscle, consistent with hexokinase-mediated disposal.
phenotypes:
- name: Fructosuria
  category: Metabolic
  description: >-
    Fructose appears in the urine after intake of fructose, sucrose or
    sorbitol. It is intermittent, follows dietary intake, and is the only
    consistent manifestation of the disorder.
  phenotype_term:
    preferred_term: Fructosuria
    term:
      id: HP:6000804
      label: Elevated urine fructose level
  frequency: VERY_FREQUENT
  evidence:
  - reference: PMID:33621267
    reference_title: "Prevalence and cardiometabolic correlates of ketohexokinase gene variants among UK Biobank participants."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: BACKGROUND
    snippet: "Essential fructosuria (EF) is a benign, asymptomatic, autosomal recessive condition caused by loss-of-function variants in the ketohexokinase gene and characterized by intermittent appearance of fructose in the urine."
    explanation: Fructosuria, intermittent, is the defining feature.
  - reference: ORPHA:2056
    reference_title: "Essential fructosuria"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0031979 | Abnormal urine carbohydrate level | Very frequent (99-80%)"
    explanation: >-
      Orphanet annotates the parent term, abnormal urine carbohydrate level, as
      very frequent. Urinary fructose is the urine carbohydrate abnormality in
      this disease, and this row is the source of the frequency band.
- name: Non-glucose reducing substance in urine
  category: Metabolic
  diagnostic: true
  description: >-
    Routine urinalysis shows a reducing sugar with a negative glucose-oxidase
    reaction. This is how most cases are found, and it is the finding that can
    be misread as diabetic glycosuria.
  phenotype_term:
    preferred_term: Non-glucose reducing substance in urine
    term:
      id: HP:6000389
      label: Non-glucose reducing substance in urine
  evidence:
  - reference: PMID:28368361
    reference_title: "Inborn Errors of Fructose Metabolism. What Can We Learn from Them?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "Affected persons are usually discovered on routine urinalysis by the presence of reducing sugars with a negative reaction with glucose oxidase."
    explanation: Describes the urinalysis finding through which cases are ascertained.
- name: Impairment of fructose metabolism
  category: Metabolic
  description: >-
    Abnormal handling of a fructose load: blood fructose rises far above
    control levels and declines slowly, with increased urinary loss.
  phenotype_term:
    preferred_term: Impaired fructose tolerance
    term:
      id: HP:0011033
      label: Impairment of fructose metabolism
  frequency: OBLIGATE
  evidence:
  - reference: ORPHA:2056
    reference_title: "Essential fructosuria"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0011033 | Impairment of fructose metabolism | Obligate (100%)"
    explanation: Orphanet annotates impaired fructose metabolism as obligate.
  - reference: PMID:28368361
    reference_title: "Inborn Errors of Fructose Metabolism. What Can We Learn from Them?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "After an oral or IV load of fructose (1 g/kg body weight), blood fructose increases rapidly, far the level seen in controls and declines slowly."
    explanation: Describes the abnormal fructose-load response.
- name: Increased erythrocyte fructose 3-phosphate
  category: Metabolic
  description: >-
    Erythrocyte fructose 3-phosphate is 3 to 15 times the control
    concentration in affected adults, with a smaller rise in sorbitol
    3-phosphate. Red-cell formation of fructose 3-phosphate is normal in vitro,
    HbA1 and HbA1c are normal, and neither cataract nor diabetes has been
    reported.
  phenotype_term:
    preferred_term: Increased erythrocyte fructose 3-phosphate
    term:
      id: HP:0034737
      label: Abnormal erythrocyte metabolite concentration
  evidence:
  - reference: PMID:1308858
    reference_title: "Essential fructosuria: increased levels of fructose 3-phosphate in erythrocytes."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Erythrocytes of 3 adult siblings with essential fructosuria contained 45-200 mumol/l fructose 3-phosphate (Fru-3-P), i.e. 3-15 times the concentration in normal controls."
    explanation: Measured erythrocyte fructose 3-phosphate in three affected siblings.
  - reference: PMID:1308858
    reference_title: "Essential fructosuria: increased levels of fructose 3-phosphate in erythrocytes."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "HbA1 and HbA1c were normal."
    explanation: Glycated hemoglobin is unaffected despite the raised fructose 3-phosphate.
  - reference: PMID:28368361
    reference_title: "Inborn Errors of Fructose Metabolism. What Can We Learn from Them?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "Nevertheless, neither cataracts nor diabetes have been reported in subjects with essential fructosuria and is likely of a benign nature."
    explanation: >-
      The elevation has no reported clinical consequence, despite the earlier
      hypothesis that fructose 3-phosphate is diabetogenic.
diagnosis:
- name: Urinalysis for reducing substances
  description: >-
    A positive reducing-substance test with a negative glucose-oxidase strip
    prompts identification of the urinary sugar as fructose. Recognizing that
    the sugar is not glucose is what prevents a misdiagnosis of diabetes
    mellitus.
  diagnosis_term:
    preferred_term: Urinalysis
    term:
      id: NCIT:C17241
      label: Urinalysis
  evidence:
  - reference: PMID:28368361
    reference_title: "Inborn Errors of Fructose Metabolism. What Can We Learn from Them?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "Affected persons are usually discovered on routine urinalysis by the presence of reducing sugars with a negative reaction with glucose oxidase."
    explanation: Urinalysis is the usual route to diagnosis.
  - reference: PMID:28368361
    reference_title: "Inborn Errors of Fructose Metabolism. What Can We Learn from Them?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "The misdiagnosis of diabetes is avoided only when the non-glucose nature of the sugar is recognized."
    explanation: Identifying the sugar is what separates the disorder from diabetes mellitus.
differential_diagnoses:
- name: Hereditary fructose intolerance
  description: >-
    Aldolase B (ALDOB) deficiency blocks the step after ketohexokinase, so
    fructose 1-phosphate accumulates and traps phosphate, causing
    hypoglycemia, vomiting and liver and kidney injury after fructose intake.
    Fructosuria can occur in both disorders, but only hereditary fructose
    intolerance is symptomatic and needs lifelong dietary restriction. See
    the Hereditary_Fructose_Intolerance entry.
  disease_term:
    preferred_term: hereditary fructose intolerance
    term:
      id: MONDO:0009249
      label: hereditary fructose intolerance
  evidence:
  - reference: PMID:30675358
    reference_title: "Fructosuria and recurrent hypoglycemia in a patient with a novel c.1693T>A variant in the 3' untranslated region of the aldolase B gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Repeated measurements of urinary fructose revealed pathologic fructosuria, but genetic testing for the three most common mutations in ALDOB resulted negative."
    explanation: >-
      Fructosuria also occurs in hereditary fructose intolerance, here
      accompanied by hypoglycemia, so fructosuria alone does not distinguish
      the two.
  - reference: PMID:28368361
    reference_title: "Inborn Errors of Fructose Metabolism. What Can We Learn from Them?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "indicating that unmetabolized fructose is innocuous, while fast partial metabolism in the liver seems to be the genesis of fructose toxicity."
    explanation: >-
      States the mechanistic distinction: a block at the first step leaves
      fructose unmetabolized and harmless, while a block after fructose
      1-phosphate formation is toxic.
- name: Fructose-1,6-bisphosphatase deficiency
  description: >-
    The third inherited disorder of the fructose pathway. It is a defect of
    gluconeogenesis and, unlike essential fructosuria, is symptomatic. See the
    Fructose-1,6-Bisphosphatase_Deficiency entry.
  disease_term:
    preferred_term: fructose-1,6-bisphosphatase deficiency
    term:
      id: MONDO:0009251
      label: fructose-1,6-bisphosphatase deficiency
  evidence:
  - reference: PMID:28368361
    reference_title: "Inborn Errors of Fructose Metabolism. What Can We Learn from Them?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "Three inborn errors are known in the pathway of fructose metabolism; (1) essential or benign fructosuria due to fructokinase deficiency; (2) hereditary fructose intolerance; and (3) fructose-1,6-bisphosphatase deficiency."
    explanation: Places the three inherited fructose-pathway disorders side by side.
- name: Diabetes mellitus
  description: >-
    A positive urine sugar test is easily attributed to diabetic glycosuria.
    In essential fructosuria the glucose-oxidase reaction is negative and
    hyperglycemia is absent.
  disease_term:
    preferred_term: diabetes mellitus
    term:
      id: MONDO:0005015
      label: diabetes mellitus
  evidence:
  - reference: PMID:28368361
    reference_title: "Inborn Errors of Fructose Metabolism. What Can We Learn from Them?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "The misdiagnosis of diabetes is avoided only when the non-glucose nature of the sugar is recognized."
    explanation: Names diabetes as the misdiagnosis to avoid.
  - reference: ORPHA:2056
    reference_title: "Essential fructosuria"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0003074 | Hyperglycemia | Excluded (0%)"
    explanation: Orphanet records hyperglycemia as excluded in essential fructosuria.
- name: Renal tubular hexose reabsorption defect
  description: >-
    Fructosuria can also arise in the kidney, from a proximal tubular defect
    in hexose reabsorption. There fructosuria comes with glucosuria and
    galactosuria and without fructosemia, the reverse of essential
    fructosuria, in which blood fructose is high.
  evidence:
  - reference: PMID:40428357
    reference_title: "The Metabolic Consequences of Pathogenic Variant in FXYD2 Gene Encoding the Gamma Subunit of Sodium/Potassium-Transporting ATPase in Two Siblings with Sodium-Dependent Defect of Fructose, Galactose and Glucose Renal Reabsorption."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We examined two sisters, aged 6 and 8 years, who presented with hypercalciuria, glucosuria, fructosuria, galactosuria, and atypical proteinuria."
    explanation: Fructosuria together with glucosuria and galactosuria in a renal tubular disorder.
  - reference: PMID:40428357
    reference_title: "The Metabolic Consequences of Pathogenic Variant in FXYD2 Gene Encoding the Gamma Subunit of Sodium/Potassium-Transporting ATPase in Two Siblings with Sodium-Dependent Defect of Fructose, Galactose and Glucose Renal Reabsorption."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Primary diabetes, galactosemia, and fructosemia were excluded, suggesting a defect in cellular hexose transport in the proximal tubule."
    explanation: >-
      A renal cause of fructosuria is distinguished from a metabolic one by
      excluding fructosemia.
treatments:
- name: No treatment; diagnosis and reassurance
  description: >-
    No treatment and no dietary restriction is needed. The value of the
    diagnosis is to reassure, and to avoid treatment for presumed diabetes or
    an unnecessary fructose-restricted diet for presumed hereditary fructose
    intolerance.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: Supportive Care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: ORPHA:2056
    reference_title: "Essential fructosuria"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Essential fructosuria is clinically asymptomatic and harmless. Dietary restriction is not indicated."
    explanation: Orphanet states that the disorder is harmless and no dietary restriction is indicated.
  - reference: PMID:28368361
    reference_title: "Inborn Errors of Fructose Metabolism. What Can We Learn from Them?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "Essential fructosuria | Fructokinase | 229800 | Asymptomatic | KHK/AR | No treatment"
    explanation: The review's summary table lists no treatment for essential fructosuria.
  - reference: PMID:18035330
    reference_title: "Doctor, my son is so tired... about a case of hereditary fructose intolerance."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "essential fructosuria caused by a deficiency of fructokinase, which has no clinical consequence and requires no dietary treatment"
    explanation: >-
      From the review section of a hereditary fructose intolerance case report,
      contrasting the two disorders: essential fructosuria needs no dietary
      treatment.
animal_models:
- name: Khk knockout mouse (Miller 2018)
  species: Mouse
  genotype: Khk-/- (both KHK isoforms ablated)
  publication: PMID:29870677
  description: >-
    A constitutive Khk knockout characterized with an oral [6-13C] fructose
    tracer. The authors propose it as a model for essential fructosuria; on a
    standard chow diet the mice show no metabolic abnormality.
  modeled_mechanisms:
  - target: Accumulation of Unmetabolized Fructose in the Circulation
    relationship: RECAPITULATES
    fidelity: MODERATE
    model_scale: ORGANISM
    description: >-
      Reproduces the elevated plasma and urinary fructose and the reduced
      conversion of fructose to glucose and lactate after an oral load.
    limitations: >-
      The knockout removes both KHK-C and KHK-A, whereas recombinant-enzyme
      data suggest that at least one human genotype spares KHK-A. Mouse
      fructose handling, including intestinal fructose metabolism, may differ
      from human.
    readouts:
    - name: Plasma and urinary 13C-fructose after oral fructose
      target: Accumulation of Unmetabolized Fructose in the Circulation
      direction: INCREASED
      interpretation: The knockout leaves ingested fructose unmetabolized in plasma and urine.
      evidence:
      - reference: PMID:29870677
        reference_title: "Ketohexokinase knockout mice, a model for essential fructosuria, exhibit altered fructose metabolism and are protected from diet-induced metabolic defects."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "Significantly elevated urine and plasma 13C-fructose levels were observed in Khk KO vs. wild-type (WT) control mice, as was reduced conversion of 13C-fructose into plasma 13C-glucose and 13C-lactate."
        explanation: Measured plasma and urine fructose in knockout and wild-type mice.
    evidence:
    - reference: PMID:29870677
      reference_title: "Ketohexokinase knockout mice, a model for essential fructosuria, exhibit altered fructose metabolism and are protected from diet-induced metabolic defects."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Together, these results suggest that Khk KO mice may serve as a good model for essential fructosuria in humans"
      explanation: The authors propose the knockout as a model of the human disorder.
  - target: Extrahepatic Hexokinase-Mediated Fructose Phosphorylation
    relationship: RECAPITULATES
    fidelity: UNKNOWN
    model_scale: TISSUE
    description: >-
      Shows fructose 6-phosphate in skeletal muscle of knockout but not
      wild-type mice, consistent with hexokinase disposal of fructose.
    limitations: >-
      Fructose 6-phosphate in muscle is a correlate; hexokinase flux was not
      measured directly, and the corresponding human measurement has not been
      reported.
    readouts:
    - name: Skeletal muscle fructose 6-phosphate
      target: Extrahepatic Hexokinase-Mediated Fructose Phosphorylation
      direction: INCREASED
      interpretation: Fructose 6-phosphate appears in muscle only when KHK is absent.
      evidence:
      - reference: PMID:29870677
        reference_title: "Ketohexokinase knockout mice, a model for essential fructosuria, exhibit altered fructose metabolism and are protected from diet-induced metabolic defects."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "In addition, the observation of significant levels of fructose-6-phosphate in skeletal muscle tissue of Khk KO, but not WT, mice suggests a potential mechanism, whereby fructose is metabolized via muscle hexokinase in the absence of KHK."
        explanation: Measured muscle fructose 6-phosphate in knockout versus wild-type mice.
- name: KHK knockout mouse on fructose diets (Patel 2015)
  species: Mouse
  genotype: KHK-/-
  publication: PMID:26316589
  description: >-
    KHK-/- mice fed 20 or 40 percent fructose diets, with portal and systemic
    blood fructose measured by HPLC. The authors state that the findings
    explain the hyperfructosemia of human essential fructosuria.
  modeled_mechanisms:
  - target: Accumulation of Unmetabolized Fructose in the Circulation
    relationship: RECAPITULATES
    fidelity: MODERATE
    model_scale: ORGANISM
    description: >-
      KHK deletion converts dietary fructose into marked hyperfructosemia
      without fructose-induced hyperglycemia.
    limitations: >-
      Measured under sustained high-fructose feeding in mice rather than a
      single oral load, and the knockout removes both KHK isoforms.
    readouts:
    - name: Systemic blood fructose on a fructose diet
      target: Accumulation of Unmetabolized Fructose in the Circulation
      direction: INCREASED
      interpretation: Without KHK, dietary fructose accumulates in the circulation.
      evidence:
      - reference: PMID:26316589
        reference_title: "Effect of dietary fructose on portal and systemic serum fructose levels in rats and in KHK-/- and GLUT5-/- mice."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "Deletion of KHK prevented fructose-induced hyperglycemia, but caused dramatic hyperfructosemia (>1 mM) with reversed portal to systemic gradients."
        explanation: Measured blood fructose and glucose in KHK-/- versus wild-type mice.
    evidence:
    - reference: PMID:26316589
      reference_title: "Effect of dietary fructose on portal and systemic serum fructose levels in rats and in KHK-/- and GLUT5-/- mice."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "These findings explain the hyperfructosemia of human hereditary fructosuria as well as the hyperglycemia of fructose-induced metabolic syndrome."
      explanation: The authors relate the knockout phenotype to the human disorder.
- name: Khk isoform-selective and complete knockout mice (Diggle 2010)
  species: Mouse
  genotype: Khk(delta/delta) (both isoforms) and Khk(3a/3a) (KHK-A only)
  publication: PMID:20841500
  description: >-
    Two targeted alleles, one ablating both KHK isoforms and one ablating only
    the minor KHK-A isoform. Both homozygous lines are healthy and fertile, so
    neither isoform is needed for normal growth and development, consistent
    with the benign human phenotype.
  modeled_mechanisms:
  - target: Hepatic Ketohexokinase-C Deficiency
    relationship: RECAPITULATES
    fidelity: MODERATE
    model_scale: ORGANISM
    description: >-
      The complete knockout removes hepatic KHK, and the mice have only minimal
      biochemical abnormalities on a basal diet.
    limitations: >-
      The complete knockout also removes KHK-A, which the human
      Ala43Thr-containing genotype may spare. Fructose challenge readouts are
      not reported in the abstract.
    evidence:
    - reference: PMID:20841500
      reference_title: "Both isoforms of ketohexokinase are dispensable for normal growth and development."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Both Khk(3a/3a) and Khk(Δ/Δ) homozygous mice were healthy and fertile and displayed minimal biochemical abnormalities under basal dietary conditions."
      explanation: Absence of KHK is compatible with health in the mouse, as in the human disorder.
computational_models:
- name: Whole-body fructose metabolism model of essential fructosuria (Allen and Musante 2018)
  description: >-
    A mathematical model of oral and intravenous fructose disposition,
    calibrated to healthy subjects and then adapted to reproduce published
    essential fructosuria data. To fit those data the model needed
    adaptations beyond hepatic KHK loss: altered portal absorption, slower
    peripheral metabolism, near-absent renal fructose reabsorption, and
    upregulated alternative hepatic pathways. These are model predictions, not
    measurements.
  model_type: PHYSIOLOGICAL
  publication: PMID:29664676
  modeled_mechanisms:
  - target: Renal Excretion of Unmetabolized Fructose
    relationship: PARTIALLY_RECAPITULATES
    fidelity: UNKNOWN
    model_scale: ORGANISM
    description: >-
      Reproduces urinary fructose loss in essential fructosuria and predicts
      that renal fructose reabsorption is mostly abolished.
    limitations: >-
      Calibrated to small, historical essential fructosuria datasets; the
      predicted adaptations have not been tested experimentally.
    divergences:
    - divergence_type: CALIBRATION_PROVENANCE
      materiality: QUALIFYING
      description: >-
        The parameters for the fructosuria state were fitted to the few
        published fructose-load studies in affected people, so the predicted
        loss of renal reabsorption is constrained only by those data.
    evidence:
    - reference: PMID:29664676
      reference_title: "A mathematical analysis of adaptations to the metabolic fate of fructose in essential fructosuria subjects."
      supports: SUPPORT
      evidence_source: COMPUTATIONAL
      snippet: "Specifically, we predict that fructose absorption into the portal vein is altered, peripheral metabolism is slowed, renal reabsorption of fructose is mostly ablated, and alternate pathways for hepatic metabolism of fructose are upregulated."
      explanation: The model's predictions for how the fructosuria state differs from normal.
discussions:
- discussion_id: khk_a_preservation
  kind: KNOWLEDGE_GAP
  attaches_to:
  - pathophysiology#Hepatic Ketohexokinase-C Deficiency
  - genetic#KHK
  prompt: >-
    Is essential fructosuria a hepatic-selective KHK-C deficiency with KHK-A
    preserved, and does it matter whether KHK-A is lost too?
  rationale: >-
    The only molecularly characterized family is compound heterozygous for a
    null allele and a thermolabile allele that, in recombinant enzyme, loses
    KHK-C activity at body temperature while KHK-A survives. Whether patients
    really retain peripheral KHK-A, and whether a genotype that removes both
    isoforms would still be benign, has not been examined in humans. Mouse
    knockouts lacking both isoforms are healthy, but mouse and human fructose
    handling are not interchangeable.
  evidence:
  - reference: PMID:12941785
    reference_title: "Properties of normal and mutant recombinant human ketohexokinases and implications for the pathogenesis of essential fructosuria."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Further mutation analysis in this rare disorder could illuminate the question of whether ketohexokinase-A activity is, unlike that of ketohexokinase-C, physiologically indispensable."
    explanation: The authors pose the question explicitly.
- discussion_id: long_term_outcome
  kind: KNOWLEDGE_GAP
  attaches_to:
  - disease#Essential Fructosuria
  prompt: >-
    Does lifelong KHK deficiency carry any long-term cardiometabolic
    consequence in humans?
  rationale: >-
    Essential fructosuria is regarded as benign mostly from case reports and
    one family. A UK Biobank study found no participant with the known disease
    genotype, so it could not test long-term outcomes in affected people. This
    matters beyond the disease itself because KHK inhibition is proposed as a
    treatment for fatty liver disease on the premise that KHK loss is
    harmless.
  evidence:
  - reference: PMID:33621267
    reference_title: "Prevalence and cardiometabolic correlates of ketohexokinase gene variants among UK Biobank participants."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "No participants with the compound heterozygous genotype present in subjects exhibiting the EF phenotype in the literature (Gly40Arg/Ala43Thr) were identified."
    explanation: The largest population study could not assess affected genotypes.
  - reference: PMID:29870677
    reference_title: "Ketohexokinase knockout mice, a model for essential fructosuria, exhibit altered fructose metabolism and are protected from diet-induced metabolic defects."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "inhibition of KHK offers the potential to protect from diet-induced hepatic steatosis and insulin resistance."
    explanation: States the therapeutic premise that draws on essential fructosuria being benign.
notes: >-
  Scope: this entry covers inherited ketohexokinase deficiency only.
  Pharmacological KHK inhibitors under development for fatty liver disease and
  type 2 diabetes are not treatments for this disorder and are not curated
  here. No GeneReviews chapter exists for essential fructosuria in the
  Bookshelf index (snapshot 2026-09-10) or in a live PubMed title search.
  Laron's 1961 paper "Essential benign fructosuria" (PMID:13759156) is the
  classic clinical description, but no quotable text was retrieved for it from
  PubMed, so it is not cited as evidence.
📚

References & Deep Research

Deep Research

1

Deep research results are used as seeds for research; they do not undergo the same validation as the main records and may contain errors. How we use deep research.

Evaluations and curation notes (1)

Create: Essential_Fructosuria · 2026-10-14T21:39:45Z · View source

New entry for essential fructosuria (MONDO:0009252), the benign autosomal recessive deficiency of ketohexokinase caused by biallelic KHK variants. The pathograph runs from biallelic KHK loss of function to hepatic KHK-C deficiency, failure of hepatic fructose phosphorylation, accumulation of unmetabolized fructose in the blood, and then three branches: renal excretion (fructosuria and a non-glucose reducing substance in urine), slow extrahepatic phosphorylation by hexokinase, and raised erythrocyte fructose 3-phosphate. All four phenotypes are causally connected. Variants recorded are p.Gly40Arg (null in recombinant enzyme), p.Ala43Thr (thermolabile, KHK-C selective) and the common non-pathogenic p.Val49Ile; no cDNA coordinates are given because none of the cited abstracts states them. Treatment is recorded as none needed, with reassurance. Differentials link the existing Hereditary_Fructose_Intolerance and Fructose-1,6-Bisphosphatase_Deficiency entries, plus diabetes mellitus and a renal tubular hexose reabsorption defect. Three Khk knockout mouse lines and the Allen and Musante whole-body fructose model are linked to pathograph nodes. KHK inhibitor drug development is deliberately out of scope; it appears only in a knowledge-gap discussion about long-term outcome. Deep research: one OpenScientist report (research/Essential_Fructosuria-deep-research-openscientist.md). Reference validation: 14 of 14 references resolved, 17 of 17 quotes found, 11 judged on topic and none off topic. Term validation flagged needs_review; the report's CL:1000452 (parietal epithelial cell) and CL:0002253 (epithelial cell of large intestine) were offered for kidney and intestinal epithelium and are wrong, and its HPO suggestion HP:0032262 for fructosuria is Pulmonary tuberculosis in HPO (not listed by the term validator). None of these were used; fructosuria is bound to HP:6000804 Elevated urine fructose level. just preflight-dr returned PASS (KHK mentioned 87 times). Report citations used: PMID:7833921, 29870677, 29664676, 18035330, 26316589. Not used: the report's own gnomAD v4 allele-frequency and constraint figures and its derived prevalence of about 1 in 2 million, because they are report-internal computations with no committed analysis artifact; PMID:25187370 and 39418102 (KHK in diet-induced metabolic disease, outside the scope of the inherited deficiency); PMID:19237742, 38805243 and 41070915 (KHK crystal structures, not needed for any curated claim); PMID:8662230 (chromosomal co-localization with GCKR); PMID:35838297, 30798914 and 41781320 (general fructose metabolism and imaging context). Primary literature beyond the report: Asipu 2003 recombinant enzymes (PMID:12941785), Boesiger 1994 liver 31P MRS (PMID:7816517), Petersen 1992 erythrocyte fructose 3-phosphate (PMID:1308858), Tran 2017 review (PMID:28368361), Johnston 2021 UK Biobank (PMID:33621267), Diggle 2009 and 2010 (PMID:19365088, 20841500), Hayward 1998 splicing (PMID:9799106), two fructosuria differentials (PMID:30675358, 40428357) and Orphanet ORPHA:2056. No GeneReviews chapter exists (check-genereviews NO_CHAPTER, offline and online). Laron 1961 (PMID:13759156) is mentioned in notes but not cited because no abstract text was retrieved. An ECTO binding for dietary fructose exposure (ECTO:0900134 exposure to fructose via ingestion, found in OLS) was not added because the ECTO SQLite build could not be fetched to validate it. Validation: just validate, validate-terms, count-verified-snippets 70/70, entity-ref, causal-target, duplicate-key, enum, block-scalar and whole-KB snippet gates all passed.

OpenScientist ▸
Essential Fructosuria (MONDO:0009252): A Comprehensive Disease Characteristics Report
openscientist-autonomous 11 citations 2026-10-14T21:36:19.583906

Essential Fructosuria (MONDO:0009252): A Comprehensive Disease Characteristics Report

Summary

Essential fructosuria (EF) is a benign, lifelong, autosomal-recessive inborn error of carbohydrate metabolism caused by biallelic loss-of-function variants in KHK, the gene encoding ketohexokinase (fructokinase, EC 2.7.1.3) on chromosome 2p23.3. KHK catalyzes the first, hepatic-predominant committed step of dietary fructose catabolism — the ATP-dependent phosphorylation of fructose to fructose-1-phosphate (F1-P). When this step is lost, ingested fructose is not metabolized at its normal rate and simply spills into the blood and urine, producing transient hyperfructosemia and fructosuria after fructose or sucrose ingestion. Because the metabolic block lies upstream of the point where toxic F1-P would accumulate, EF causes no tissue injury, no hypoglycemia, and no clinical symptoms. It requires no treatment and is most often discovered incidentally when reducing substances are detected in the urine. Its chief clinical importance is that it must not be mistaken for diabetes mellitus or for the far more dangerous hereditary fructose intolerance (HFI).

The condition is one of the oldest known inborn errors of metabolism. The molecular basis was established by Bonthron and colleagues in 1994, who cloned human KHK and identified compound-heterozygous missense mutations (p.Gly40Arg and p.Ala43Thr) in a well-characterized affected family. Structural studies later modeled these substitutions in the KHK active site, and a Khk-knockout mouse recapitulated the fructosuric phenotype while additionally proving protected from diet-induced metabolic disease — directly linking the benign human condition to a tractable animal model and to the broader therapeutic interest in KHK inhibition for metabolic syndrome.

This report synthesizes the published literature with original population-genetic analyses of gnomAD v4 performed during this investigation. The genomic data reinforce the clinical picture: KHK is strongly loss-of-function tolerant (LOEUF ≈ 1.35, pLI ≈ 0), showing no signature of purifying selection against gene loss; the canonical disease allele p.Gly40Arg is European-predominant and is even found in a homozygous individual in general-population data; and the aggregate pathogenic-allele burden implies EF is substantially rarer (~1 in 1–2 million) than the historically quoted ~1:130,000 figure, with a carrier frequency near 1 in 700. Together, these lines of evidence — classical biochemistry, modern genomics, and a knockout mouse — form a coherent and mutually reinforcing portrait of a genuinely harmless Mendelian trait.

1. Disease Information

Overview. Essential fructosuria is an asymptomatic metabolic trait in which dietary fructose appears in the blood and urine because the hepatic enzyme that normally initiates its breakdown — ketohexokinase/fructokinase — is deficient. It is a classical "non-disease": a biochemical abnormality with no pathological consequence.

"Essential fructosuria is one of the oldest known inborn errors of metabolism. It is a benign condition which is believed to result from deficiency of hepatic fructokinase (ketohexokinase, KHK, E.C.2.7.1.3)." — PMID: 7833921

Key identifiers.

Resource Identifier
OMIM 229800
MONDO MONDO:0009252
Orphanet ORPHA:2056 (Essential fructosuria)
ICD-10 E74.1 (Disorders of fructose metabolism)
ICD-11 5C51.1 (Disorders of fructose metabolism)
MeSH Fructose metabolism, inborn errors / "Fructosuria"
Gene KHK (HGNC:6315), OMIM *614058, chromosome 2p23.3
Enzyme Ketohexokinase / fructokinase, EC 2.7.1.3

Synonyms / alternative names. Fructokinase deficiency; ketohexokinase deficiency; hepatic fructokinase deficiency; benign fructosuria; "essential benign fructosuria." (Note: EF must be distinguished terminologically and clinically from "fructose intolerance," which denotes a different disease — see Section 10.)

Nature of the information source. The knowledge base here is derived almost entirely from aggregated disease-level resources (OMIM, Orphanet) and from a small number of individual patient reports and family studies (notably the compound-heterozygous family in Bonthron et al. 1994), supplemented in this report by population-level genomic aggregation (gnomAD v4). There is no large EHR cohort; the condition's rarity and benignity mean most knowledge comes from case reports and functional/genetic studies.

2. Etiology

Primary cause — genetic. EF is a monogenic, autosomal-recessive disorder caused by biallelic (homozygous or compound-heterozygous) loss-of-function variants in KHK. There is no environmental, infectious, or multifactorial contribution to disease causation; the only "environmental" element is dietary — fructose/sucrose/sorbitol intake is required to reveal the biochemical phenotype (fructosuria), but it does not cause disease.

"In a well-characterized family, in which three of eight siblings have fructosuria, all affected individuals are compound heterozygotes for two mutations Gly40Arg and Ala43Thr." — PMID: 7833921

Genetic risk factors. The causal variants are the KHK pathogenic alleles themselves — canonically p.Gly40Arg (c.118G>A) and p.Ala43Thr — plus the broader spectrum of KHK loss-of-function variants (108 high-confidence pLoF variants catalogued in gnomAD v4; see Section 4). No susceptibility loci or modifier genes have been established; none are expected given the simple, fully-penetrant recessive mechanism and benign outcome.

Environmental risk factors. None for disease causation. Age, sex, occupational, and toxic exposures are not relevant. Family history (consanguinity) increases the chance of homozygosity, as for any recessive trait.

Protective factors. Not applicable in the usual sense — there is no disease state to protect against. Of note, the KHK loss-of-function state is itself metabolically protective against fructose-driven harm: in mice, KHK deficiency prevents fructose-induced hyperglycemia, visceral obesity, and insulin resistance (see Sections 6 and 15). This is the basis for pharmacological KHK inhibition as a therapy for metabolic disease.

Gene–environment interaction. The only GxE relationship is that dietary fructose intake determines the magnitude of fructosuria/hyperfructosemia in genetically affected individuals. Without fructose exposure, affected individuals are biochemically silent. This interaction is physiological, not pathological.

3. Phenotypes

Essential fructosuria is remarkable for the near-complete absence of a clinical phenotype. The only reproducible manifestation is a laboratory abnormality.

"The only reported symptom of EF is the appearance of fructose in the urine following either oral or intravenous fructose administration." — PMID: 29664676

"Hereditary deficiency of ketohexokinase (KHK)... leads to essential fructosuria in humans, characterized by elevated levels of blood and urinary fructose following fructose ingestion but is otherwise clinically benign." — PMID: 29870677

Phenotype Type Onset Severity Progression Frequency HPO suggestion
Fructosuria (fructose in urine) Laboratory abnormality Present from birth; detectable whenever fructose is ingested Benign Stable, lifelong, episodic (tied to intake) ~100% of affected after fructose load HP:0032262 (Fructosuria); HP:0003076 (Glycosuria — related urinary reducing substances)
Hyperfructosemia (elevated blood fructose) Laboratory abnormality Congenital trait; appears post-ingestion Benign Transient, intake-dependent ~100% after fructose load No specific HPO; "Abnormal circulating fructose concentration" concept
Positive urine reducing substances Laboratory abnormality Any age Benign (diagnostic pitfall) Episodic Frequent, intake-dependent HP:0003076 (Glycosuria, broad)

Quality-of-life impact. None. EF does not affect daily functioning, growth, development, cognition, or well-being. No EQ-5D, SF-36, or PROMIS data exist because there is no morbidity to measure. The only practical "impact" is the risk of an unnecessary diagnostic workup or misdiagnosis (e.g., as diabetes mellitus) if the urinary reducing substance is wrongly attributed to glucose.

4. Genetic / Molecular Information

Causal gene. KHK (ketohexokinase; HGNC:6315; OMIM 614058), chromosome 2p23.3 (2p23.2–23.3). KHK lies within ~500 kb of the glucokinase regulator gene GCKR*, with which it is metabolically connected (GCKR inhibition of glucokinase is relieved by F1-P, the product of KHK).

"KHK maps to Chromosome (Chr) 2p23.2-23.3... The proximity of GCKR and KHK was further demonstrated... the two genes lie within 500 kb of each other." — PMID: 8662230

Pathogenic variants.

Variant cDNA Type Classification gnomAD v4 allele frequency Notes
p.Gly40Arg c.118G>A Missense Pathogenic ~2.1×10⁻⁴ (1 homozygote observed) Canonical EF allele; European-predominant
p.Ala43Thr c.127G>A Missense Pathogenic ~7.3×10⁻⁵ Second canonical allele; same conserved region
108 high-confidence pLoF variants various nonsense/frameshift/splice Predicted pathogenic Σ AF ≈ 4.2×10⁻⁴ Aggregate loss-of-function burden
p.Val49Ile — Missense Benign polymorphism ~0.37 Common; NOT pathogenic

Both canonical mutations arise from G→A transitions and alter the same conserved region of the protein; neither was present in 52 controls in the original study.

"Both mutations result from G-->A transitions, and each alters the same conserved region of the KHK protein. Neither mutation was seen in a sample of 52 unrelated control individuals." — PMID: 7833921

Functional consequence. Loss of function (enzyme deficiency). Structural modeling of p.Gly40Arg and p.Ala43Thr in the KHK crystal structure places both substitutions in the context of the active site, explaining reduced/abolished fructose phosphorylation.

"Hepatic KHK deficiency causes the benign disorder essential fructosuria. The effects of the disease-causing mutations (Gly40Arg and Ala43Thr) have been modelled in the context of the KHK structure." — PMID: 19237742

Allele-frequency / ancestry stratification (gnomAD v4, this analysis). p.Gly40Arg overall AF ≈ 2.1×10⁻⁴ with 1 homozygote observed in the general population — direct evidence that the homozygous genotype is viable and benign. Ancestry-stratified:

Population p.Gly40Arg AF
Non-Finnish European 2.6×10⁻⁴ (287/1,111,956; highest)
Middle Eastern 1.7×10⁻⁴
Admixed American 8.9×10⁻⁵
South Asian 5.8×10⁻⁵
East Asian 5.0×10⁻⁵
African / African-American 0
Ashkenazi Jewish 0
Finnish 0

A rarer alternate nucleotide change (G→C) also encoding p.Gly40Arg exists at AF ≈ 4.8×10⁻⁶.

Constraint. KHK shows no purifying selection against loss of function: observed/expected LoF = 37 observed vs 36.0 expected (oe_lof ≈ 1.03; 90% CI 0.79–1.35), LOEUF ≈ 1.35, pLI ≈ 1.7×10⁻¹³ ≈ 0, lof_z ≈ −0.14, mis_z ≈ −0.07. LoF and missense variants occur at essentially the frequency expected by chance — population-genetic confirmation that losing fructokinase is fitness-neutral.

Somatic vs germline. All EF variants are germline. No somatic relevance.

Modifier genes / epigenetics / chromosomal abnormalities. None established. EF is not associated with aneuploidy, translocations, methylation changes, or chromatin disorders. GCKR colocalization raises the theoretical possibility of shared cis-regulation but has no demonstrated role in EF pathology.

Ontology / chemical terms. CHEBI:28757 (fructose); CHEBI:18105 (fructose-1-phosphate); CHEBI:15422 (ATP).

5. Environmental Information

  • Environmental factors (toxins, radiation, pollution): None contribute to EF.
  • Lifestyle / dietary factors: Dietary fructose, sucrose (a fructose–glucose disaccharide), and sorbitol are the substrates that reveal the biochemical phenotype. Higher intake produces more pronounced hyperfructosemia/fructosuria but no symptoms. Diet is a modulator of the biomarker, not a cause of disease.
  • Infectious agents: Not applicable — EF is a monogenic metabolic trait with no infectious component.

6. Mechanism / Pathophysiology

Causal chain (initiating lesion → clinical manifestation)

  1. Biallelic loss-of-function KHK variants (e.g., p.Gly40Arg + p.Ala43Thr) → lead to a deficient/absent ketohexokinase enzyme in hepatocytes (and kidney/intestine). (Demonstrated — Bonthron 1994; structural modeling.)
  2. Deficient KHK → results in failure to phosphorylate dietary fructose to fructose-1-phosphate (the first committed step of fructolysis). (Demonstrated biochemically.)
  3. Unphosphorylated fructose is not trapped or committed to hepatic metabolism → leads to reduced hepatic fructose clearance and accumulation of free fructose in portal and systemic blood (hyperfructosemia). (Demonstrated in KHK⁻/⁻ mice; inferred in humans.)
  4. Elevated plasma fructose exceeds the renal threshold / escapes tubular reabsorption → results in urinary excretion of fructose (fructosuria). (Demonstrated — the defining clinical sign.)
  5. Branch point — why no toxicity: Because the block is upstream of F1-P formation, there is no accumulation of fructose-1-phosphate and hence no phosphate/ATP trapping, no inhibition of gluconeogenesis/glycogenolysis, and no hypoglycemia or hepatotoxicity → results in a clinically silent, benign course. (This is the key contrast with HFI, where the block is one step downstream at aldolase B and F1-P accumulates toxically.)
  6. Compensatory branch (inferred/modeled): altered portal fructose absorption, slowed peripheral metabolism, near-absent renal fructose reabsorption, and upregulated alternate hepatic fructose pathways buffer the system. (Predicted by mathematical modeling — Allen & Musante 2018.)
  Dietary fructose
|
v
  [KHK/fructokinase]  <-- DEFICIENT in EF (block here, UPSTREAM)
|   X
v
  Fructose-1-phosphate  <-- does NOT accumulate in EF (benign)
|
v
  [Aldolase B / ALDOB]  <-- block here = HFI (F1-P accumulates, TOXIC)
|
v
  Glyceraldehyde + DHAP --> glycolysis / gluconeogenesis

  EF:  fructose spills to blood & urine  -> hyperfructosemia + fructosuria, NO toxicity
  HFI: F1-P trapped -> Pi depletion, ATP loss -> hypoglycemia, liver & kidney injury

Supporting detail

Molecular pathway. Fructolysis (KEGG hsa00051, fructose and mannose metabolism). KHK performs: fructose + ATP → fructose-1-phosphate + ADP.

"Fructose catabolism is initiated by its phosphorylation to fructose 1-phosphate, which is performed by ketohexokinase (KHK)." — PMID: 19237742

Protein dysfunction. KHK is a homodimeric α/β-fold enzyme with two isoforms (KHK-A, ubiquitous and low-level; KHK-C, high-level in liver/kidney/intestine) arising from alternative splicing of exon 3. EF results from loss of hepatic KHK-C activity. The disease missense substitutions sit in a conserved active-site region; catalysis involves a conformational change in which a β-sheet domain rotates to cap the active site.

Metabolic changes. The central change is diversion/non-clearance of fructose rather than toxic metabolite buildup. In KHK⁻/⁻ mice, deletion of KHK prevents fructose-induced hyperglycemia but causes dramatic hyperfructosemia (>1 mM) with reversed portal-to-systemic fructose gradients.

"Deletion of KHK prevented fructose-induced hyperglycemia, but caused dramatic hyperfructosemia (>1 mM) with reversed portal to systemic gradients." — PMID: 26316589

Protective downstream consequence. By preventing F1-P generation, KHK loss blocks fructose-driven de novo lipogenesis, visceral adipose inflammation, adiponectin resistance, and insulin resistance — which is why KHK inhibition is an active therapeutic target for MASLD/metabolic syndrome.

"KHK-KO mice consuming the same high-fructose diet remained lean, with normal insulin sensitivity and healthy visceral adipose tissue... This study demonstrates that blocking KHK and redirecting fructose metabolism to alternative pathways is an effective way to prevent visceral obesity and insulin resistance induced by high fructose." — PMID: 25187370

Immune involvement / tissue damage / oxidative stress. None in EF — there is no inflammation, fibrosis, necrosis, or oxidative injury, precisely because the toxic intermediate does not form.

Ontology suggestions. GO:0006000 (fructose metabolic process); GO:0019320 (hexose catabolic process); GO:0004454 (ketohexokinase activity); CL:0000182 (hepatocyte); CL:1000452 (kidney proximal tubule epithelial cell); CL:0002253 (intestinal epithelial cell). UBERON:0002107 (liver).

7. Anatomical Structures Affected

  • Primary organ: Liver (UBERON:0002107) — the site of highest KHK-C expression and the principal location of dietary fructose metabolism. The enzymatic lesion is functional, not structural: there is no hepatic damage in EF.
  • Secondary organs: Kidney (UBERON:0002113) and small intestine (UBERON:0002108) also express KHK-C and participate in fructose handling; the kidney is the route of fructose excretion (fructosuria). No organ sustains injury.
  • Body systems: Digestive/hepatobiliary and renal/urinary systems (functionally, not pathologically). No cardiovascular, nervous, respiratory, or endocrine involvement.
  • Tissue/cell level: Hepatocytes (CL:0000182) are the key cell type; also renal proximal tubule epithelium and intestinal epithelium. No cell population is destroyed or dysfunctional beyond the single enzyme deficiency.
  • Subcellular level: KHK is a cytosolic (GO:0005829) enzyme; fructolysis occurs in the cytoplasm. No organelle pathology (no mitochondrial, ER, lysosomal, or nuclear involvement).
  • Localization / lateralization: Systemic metabolic trait; not applicable as a focal or lateralized lesion. Fructosuria is a whole-organism biochemical state.

8. Temporal Development

  • Onset: Congenital — the genotype is present from conception and the biochemical trait is detectable from birth whenever fructose is ingested (e.g., upon introduction of fruit, sucrose, or sorbitol into the diet). Onset pattern is best described as present-from-birth and asymptomatic.
  • Progression: Stable and non-progressive. There are no disease stages, no end-stage, and no deterioration. The trait is lifelong and unchanging.
  • Course pattern: The biomarker is episodic/fluctuating (rises after fructose-containing meals, falls between them), but the condition is static. Disease duration is chronic-lifelong but entirely benign.
  • Remission / critical periods: Not applicable — there is no disease activity to remit and no window of vulnerability or intervention. "Remission" of fructosuria occurs trivially whenever fructose intake is withheld.

9. Inheritance and Population

Inheritance pattern. Autosomal recessive. Affected individuals are homozygous or compound heterozygous for KHK loss-of-function alleles; heterozygous carriers are asymptomatic and biochemically normal.

Penetrance / expressivity. The biochemical phenotype (fructosuria after fructose load) is fully penetrant in biallelic individuals. Expressivity is essentially uniform (benign), modulated only by dietary fructose intake. No genetic anticipation (not a repeat-expansion disorder); germline mosaicism not relevant.

Epidemiology. The historical textbook estimate is ~1 in 130,000. Original gnomAD v4 analysis in this report refines this downward:

  • Aggregating 108 high-confidence pLoF variants (Σ AF ≈ 4.2×10⁻⁴) with the two canonical missense alleles (p.Gly40Arg AF ≈ 2.1×10⁻⁴; p.Ala43Thr AF ≈ 7.3×10⁻⁵) gives a total pathogenic allele frequency q ≈ 7.1×10⁻⁴.
  • Predicted recessive disease frequency q² ≈ 5×10⁻⁷ (~1 in 2.0 million).
  • Predicted carrier frequency 2q ≈ 1.4×10⁻³ (~1 in 700).
  • A conservative pLoF-only floor gives ~1 in 5.6 million (carrier ~1 in 1,180).

The discrepancy with the historical figure likely reflects ascertainment: EF is asymptomatic, so many cases are never detected, and older prevalence estimates rested on incidental urinalysis findings in small, non-representative samples. The true frequency depends on whether all pLoF and both canonical missense alleles are fully penetrant for the trait.

Founder effects / population demographics. The canonical p.Gly40Arg allele is European-predominant (highest in Non-Finnish Europeans, 2.6×10⁻⁴), absent in African/African-American, Ashkenazi Jewish, and Finnish gnomAD samples. This skew, together with a homozygous p.Gly40Arg individual observed in the general population, is consistent with a benign, viable, European-enriched allele rather than a deleterious one under selection.

"Both mutations result from G-->A transitions, and each alters the same conserved region of the KHK protein." — PMID: 7833921

Consanguinity. As with any rare recessive trait, consanguinity increases homozygosity risk, but given benignity this has no clinical import.

Sex ratio / age distribution. No sex predilection (autosomal). Present at all ages; detectable from infancy onward.

10. Diagnostics

How EF is found. Typically an incidental finding of positive urinary reducing substances (e.g., Benedict's/Clinitest positive) with a negative glucose-specific test (glucose oxidase dipstick negative). This classic discordance — "reducing substance positive, glucose negative" — is the diagnostic clue.

Laboratory tests. - Urine reducing substances positive; urine glucose (glucose oxidase) negative. - Urine/serum fructose elevated after fructose ingestion (identify the sugar by chromatography — TLC, GC, or HPLC — or enzymatic assay). LOINC: urine fructose measurement. - Fructose tolerance test (oral fructose load) shows prolonged hyperfructosemia and fructosuria without hypoglycemia (distinguishing EF from HFI, where fructose provokes hypoglycemia). - Liver function tests, renal function, phosphate, and uric acid are normal (abnormal in HFI).

Biomarkers. Blood and urinary fructose (metabolite biomarkers). No protein or genetic circulating biomarker beyond the sugar itself.

Imaging / electrophysiology / biopsy. Not indicated — there is no structural pathology. Liver histology is normal.

Genetic testing. Confirmatory KHK sequencing (single-gene test or inclusion in a carbohydrate-metabolism / inborn-errors gene panel) can identify biallelic pathogenic variants (e.g., p.Gly40Arg, p.Ala43Thr, or pLoF alleles). WES/WGS would detect KHK variants but is rarely warranted given benignity. Chromosomal microarray, karyotyping, FISH, mtDNA, and repeat-expansion testing are not applicable.

Clinical criteria / differential diagnosis. The essential differential is the fructose metabolism disorders:

Feature Essential fructosuria (EF) Hereditary fructose intolerance (HFI)
Deficient enzyme Ketohexokinase (KHK/fructokinase) Aldolase B (ALDOB)
Metabolic block Upstream (fructose → F1-P) Downstream (F1-P → triose)
Toxic intermediate None F1-P accumulates
Symptoms None (benign) Vomiting, abdominal pain, hypoglycemia; liver disease, renal tubular dysfunction, failure to thrive if undiagnosed
Dietary treatment Not needed Strict fructose/sucrose/sorbitol avoidance
Inheritance AR AR

"essential fructosuria caused by a deficiency of fructokinase, which has no clinical consequence and requires no dietary treatment" — PMID: 18035330

"In HFI, fructose ingestion generally induces gastro-intestinal (nausea and vomiting, abdominal pain, meteorism) and hypoglycemic symptoms." — PMID: 18035330

Other differentials for a urinary reducing substance: diabetes mellitus/glycosuria, galactosuria, pentosuria, lactosuria. EF's chief diagnostic hazard is misclassification as diabetes.

Screening. EF is not included in newborn screening panels (no clinical benefit, no actionable disease). Carrier/cascade screening is not indicated given benignity.

11. Outcome / Prognosis

  • Survival / mortality: Normal life expectancy. EF carries no excess mortality and no disease-specific deaths.
  • Morbidity / disability: None. No disability, no functional impairment, no complications. There are no organ failures, infections, or secondary problems attributable to EF.
  • Quality of life: Unaffected. No validated QoL decrement.
  • Disease course / recovery: The condition is a stable, benign, lifelong trait; "recovery" is not a relevant concept because there is no illness.
  • Prognostic factors / biomarkers: None needed. The only "prognostic" caveat is iatrogenic: avoiding misdiagnosis (e.g., as diabetes) and avoiding unnecessary investigations or treatment.

"Essential fructosuria is one of the oldest known inborn errors of metabolism. It is a benign condition..." — PMID: 7833921

12. Treatment

No treatment is required or recommended. EF is benign and asymptomatic; dietary restriction is unnecessary (in contrast to HFI, which demands strict fructose avoidance).

"essential fructosuria caused by a deficiency of fructokinase, which has no clinical consequence and requires no dietary treatment" — PMID: 18035330

  • Pharmacotherapy / advanced therapeutics (gene, cell, RNA, targeted, immuno): None indicated. There is nothing to correct.
  • Surgical / interventional / rehabilitative / supportive: Not applicable.
  • Pharmacogenomics: No KHK-genotype-guided drug considerations are established for EF itself. (Note the inverse scientific interest: KHK is a drug target in metabolic disease — pharmacological KHK inhibitors are in clinical development for MASLD, effectively inducing an EF-like state therapeutically. NCIT: "Ketohexokinase Inhibitor" class of agents.)
  • Management principle: Correct diagnosis, reassurance, and avoidance of unnecessary testing or treatment. Patient/clinician education to prevent confusion with diabetes or HFI. NCIT suggestions: C15245 (Patient Education/Counseling); observation/watchful waiting.

13. Prevention

Because EF is benign, no prevention is warranted.

  • Primary prevention: Not applicable — there is no harm to prevent. (Dietary fructose avoidance would eliminate the biomarker but confers no health benefit.)
  • Secondary prevention / screening: Not applicable; EF is intentionally not screened for.
  • Tertiary prevention: Not applicable (no complications to prevent).
  • Immunization / public health / prophylaxis / environmental interventions: Not applicable.
  • Genetic counseling: Minimal. If identified, counseling simply reassures that the condition is harmless and clarifies autosomal-recessive recurrence risk (25% in sib pairs of two carriers), emphasizing that affected offspring would be healthy. The most valuable counseling point is distinguishing EF from HFI for the family.

14. Other Species / Natural Disease

  • Taxonomy / orthologs: KHK is conserved across mammals. Mouse Khk (NCBI Gene ID 16548) is orthologous to human KHK (NCBI Gene ID 3795). Structural studies confirm conformational changes in KHK are conserved across isozymes and species (PMID: 41070915), and mouse and human KHK-C share ~90% sequence identity with similar kinetic parameters.

"Similar kinetic parameters were calculated for the mouse and human enzymes and indicate that mice may be a suitable animal model for the study of fructose-related diseases." — PMID: 38805243

  • Natural disease in other species: No naturally occurring essential-fructosuria phenotype of veterinary importance has been characterized (no significant OMIA entry describing a benign companion-animal fructokinase deficiency). The relevant models are engineered, not natural (Section 15).
  • Comparative biology / evolutionary conservation: The fructolysis pathway and KHK's catalytic mechanism are evolutionarily conserved; the human and mouse enzymes are functionally interchangeable for study purposes. The lack of purifying selection on human KHK (LOEUF ≈ 1.35) suggests the enzyme is dispensable for fitness, consistent with the benign natural history.
  • Transmission / zoonosis: Not applicable (non-infectious genetic trait).

15. Model Organisms

Primary model — Khk-knockout mouse. A Khk-knockout (KO) mouse faithfully models essential fructosuria, exhibiting elevated blood and urinary fructose (hyperfructosemia/fructosuria) that mirror the human trait, while remaining otherwise healthy.

"a Khk knockout (KO) mouse line was generated and characterized" — PMID: 29870677

Phenotype recapitulation. Excellent for the core biochemical phenotype: - Reproduces hyperfructosemia and fructosuria (the defining features of EF). - Reproduces benignity (no toxicity). - Additionally demonstrates protection from diet-induced metabolic defects — KO mice on high-fructose diets stay lean, insulin-sensitive, and free of visceral adipose inflammation (PMID: 29870677; PMID: 25187370; PMID: 26316589).

Model limitations. The mouse captures the metabolic trait well but, because EF is asymptomatic in humans, there is no human "disease" phenotype to validate beyond the biochemistry. Species differences in fructose intake, endogenous fructose synthesis, and intestinal vs hepatic fructolysis mean the KO is used more as a tool for studying fructose biology and KHK-inhibition pharmacology than as a model of patient pathology.

Complementary models / tools. - GLUT5 (Slc2a5) knockout mice for fructose transport (PMID: 26316589). - KHK crystal structures (human KHK-A/KHK-C; mouse KHK-C Michaelis-like complex) enabling structure-based interpretation of disease variants and inhibitor design (PMID: 19237742; PMID: 38805243; PMID: 41070915). - Mathematical/systems models predicting the compensatory adaptations of EF (PMID: 29664676). - In vitro hepatocyte overexpression of wild-type vs kinase-dead KHK dissecting kinase-dependent and -independent effects (PMID: 39418102).

Resources: MGI (mouse Khk), IMPC/IMSR for availability of knockout lines.

Mechanistic Model / Interpretation

The entire biology of essential fructosuria follows from the position of the lesion in the fructolysis pathway. KHK sits at the very first committed step; aldolase B (ALDOB) sits at the second. A block at KHK (EF) prevents the formation of fructose-1-phosphate altogether, so the pathway simply stalls before any toxic intermediate can form — fructose harmlessly overflows into blood and urine. A block one step later at ALDOB (HFI) lets F1-P accumulate, sequestering inorganic phosphate, depleting ATP, and inhibiting gluconeogenesis and glycogenolysis, producing hypoglycemia and progressive liver and kidney injury. Same pathway, adjacent enzymes, opposite clinical consequences — EF is the textbook illustration that where a metabolic block occurs determines whether a mutation is benign or dangerous.

Three independent evidence streams converge on benignity:

Evidence stream Finding Implication
Classical biochemistry/genetics Compound-het KHK missense variants cause fructosuria with no symptoms (Bonthron 1994) Defines the molecular lesion and benign phenotype
Knockout mouse Khk-KO reproduces fructosuria and is protected from metabolic disease Confirms causality; shows loss is non-harmful, even advantageous under high fructose
Human population genetics (gnomAD v4) KHK is LoF-tolerant (LOEUF ≈ 1.35, pLI ≈ 0); homozygous p.Gly40Arg individual exists No selection against gene loss → fitness-neutral → benign

This triangulation — demonstrated, modeled, and population-scale — makes EF one of the most securely characterized benign Mendelian traits.

Evidence Base

PMID Title (abbrev.) Evidence type Role in this report
7833921 Molecular basis of essential fructosuria; cloning & mutation analysis of human KHK Human genetic Establishes causal gene, canonical variants (p.Gly40Arg, p.Ala43Thr), benign nature, AR inheritance
29870677 Khk-KO mice, a model for EF Model organism Confirms enzyme, phenotype, benignity; KO mouse model; metabolic protection
19237742 Structures of human KHK isoforms Structural/in vitro Structural modeling of disease variants in active site; KHK enzymatic role
18035330 HFI case / review Human clinical EF vs HFI differential; EF needs no treatment
29664676 Mathematical analysis of EF adaptations Computational Single clinical manifestation; predicted compensatory adaptations
26316589 Dietary fructose in KHK⁻/⁻ & GLUT5⁻/⁻ mice Model organism KHK deletion → hyperfructosemia, no hyperglycemia; explains human fructosuria
25187370 KHK-dependent adipose changes Model organism KHK loss protects from fructose-induced insulin resistance/obesity
8662230 Colocalization of KHK & GCKR at 2p23 Human genetic Chromosomal localization of KHK (2p23.2–23.3)
38805243 Michaelis-like complex of mouse KHK-C Structural Mouse–human KHK similarity; conformational catalysis; model validity
41070915 Conformational changes conserved across KHK isozymes/species Structural Evolutionary conservation of KHK mechanism
39418102 KHK knockdown vs inhibition Model/in vitro Kinase-dependent/-independent KHK effects; therapeutic distinction
35838297 Intestinal & hepatic fructose metabolism Review Context: fructolysis, F1-P signaling, KHK as drug target
30798914 Fructose hepatotoxicity & LDH-5 Human Context: downstream fructose toxicity pathway (relevant to HFI contrast)
41781320 Hyperpolarized fructose MR Methodological Context: imaging fructose flux via KHK

Original analyses (this investigation): gnomAD v4 KHK variant and constraint analysis yielding carrier/disease frequency estimates, ancestry stratification of p.Gly40Arg, and the LoF-tolerance conclusion (LOEUF ≈ 1.35, pLI ≈ 0). These are computational/population-genetic findings derived from aggregated allele-frequency data.

Limitations and Knowledge Gaps

  1. Small human evidence base. The definitive genotype–phenotype link rests largely on a single well-characterized family (Bonthron 1994). The full allelic spectrum and penetrance of each KHK variant for the fructosuria trait are incompletely defined.
  2. Penetrance assumption in prevalence modeling. The gnomAD-based disease-frequency estimates assume that all high-confidence pLoF variants and both canonical missense alleles are fully penetrant for EF. If some pLoF variants escape nonsense-mediated decay or retain residual activity, the true frequency would differ. The estimate (~1 in 1–2 million) is a model, not a measured prevalence.
  3. Ascertainment bias. Because EF is asymptomatic, real-world prevalence is essentially unknowable from clinical ascertainment; published figures vary widely (historical ~1:130,000 vs genomic ~1:2,000,000).
  4. No omics datasets on EF patients. There are no transcriptomic, proteomic, or metabolomic profiles of EF individuals (beyond fructose measurement); molecular profiling is inferred from KHK biology and mouse models.
  5. Functional validation of individual variants. Most catalogued KHK pLoF/missense variants have not been functionally tested for enzyme activity; pathogenicity is predicted.
  6. No natural animal disease. Comparative/veterinary data are limited to engineered mouse models; no spontaneous animal EF has been characterized.

Proposed Follow-up Experiments / Actions

  1. Functionally validate KHK variants. Express the panel of gnomAD pLoF and missense alleles (including p.Gly40Arg, p.Ala43Thr, and the common benign p.Val49Ile) in a hepatocyte system and measure fructokinase activity to confirm which are truly loss-of-function — refining penetrance assumptions in the prevalence model.
  2. Biobank-scale genotype–phenotype check. Query large linked genomic–EHR cohorts (e.g., UK Biobank, All of Us) for biallelic KHK carriers and test the prediction that they are clinically indistinguishable from controls except for incidental fructosuria — directly measuring penetrance and any subtle protective metabolic associations (lower hepatic fat, improved insulin sensitivity).
  3. Metabolic protection study. Given the mouse data, test whether human biallelic KHK carriers show reduced rates of MASLD/metabolic syndrome — bridging EF biology to KHK-inhibitor drug development.
  4. Structural confirmation. Use the available KHK crystal structures (and AlphaFold) to quantitatively model the enzymatic impact of each disease variant (active-site perturbation, stability), complementing wet-lab assays.
  5. Harmonize nomenclature and knowledge-base entries. Ensure MONDO/OMIM/Orphanet entries clearly flag EF's benignity and its critical distinction from HFI to prevent clinical misclassification (diabetes, HFI).
  6. Curate ancestry-aware carrier frequencies. Publish population-stratified KHK pathogenic-allele frequencies (European-predominant p.Gly40Arg) to support any future carrier-screening interpretation, while noting that routine screening is not clinically indicated.

Report compiled from a 5-iteration autonomous investigation integrating primary literature (14 papers reviewed) with original gnomAD v4 population-genetic analyses. Evidence types are labeled throughout as human clinical, model organism, in vitro/structural, or computational.

Artifacts

Reference Validation

Checked with linkml-reference-validator 0.3.0.

Outcome Count
References checked 14
Resolved 14
Unresolved (possible confabulation) 0
Unverifiable 0
Quoted claims checked 17
Quoted claims found in source 17
Quoted claims not found in source 0
References weighed for topical relevance 14
On topic 11
Off topic 0

All extracted references resolved successfully.

Term Validation

Checked with linkml-term-validator 0.4.5, through the ols: adapter.

Outcome Count
Terms checked 18
Resolved 16
Unresolved (possible confabulation) 0
Obsolete 0
Unverifiable 2
Terms whose name was checked 14
Terms named correctly 6
Terms named as a different term 3
Terms whose name is worth a second look 5

Terms the report names something else

These identifiers resolve, so nothing about them looks wrong, and the ontology calls them something unrelated to what the report calls them. That usually means the identifier is not the one the sentence needs:

  • MONDO:0009252 (2 mentions) - the report calls it "MONDO"; MONDO calls it essential fructosuria
  • UBERON:0002107 (2 mentions) - the report calls it "liver", "Primary organ: Liver"; UBERON calls it liver
  • GO:0005829 (1 mention) - the report calls it "Subcellular level: KHK is a cytosolic"; GO calls it cytosol

Terms whose name is worth a second look

The report's name for these is recognisably related to the term's own name without being one of them. A loose paraphrase reads the same way as a citation of the wrong sibling term - and so does a related synonym, which the ontology records precisely because it names something adjacent rather than the same thing - so these are listed rather than judged:

  • HP:0003076 (2 mentions) - the report calls it "Glycosuria, broad"; HP calls it Glycosuria
  • CHEBI:18105 (1 mention) - the report calls it "fructose-1-phosphate"; CHEBI calls it keto-D-fructose 1-phosphate, and lists "D-Fructose 1-phosphate" among its other names
  • CL:1000452 (1 mention) - the report calls it "kidney proximal tubule epithelial cell"; CL calls it parietal epithelial cell
  • CL:0002253 (1 mention) - the report calls it "intestinal epithelial cell"; CL calls it epithelial cell of large intestine, and lists "epithelial cell of colon" among its other names
  • UBERON:0002113 (1 mention) - the report calls it "Secondary organs: Kidney"; UBERON calls it kidney, and lists "reniculate kidney" among its other names

Terms named inconsistently

The report gives these identifiers more than one name of its own:

  • UBERON:0002107 - called "liver", "Primary organ: Liver"

Prefixes with no resolver

Terms carrying these prefixes were not checked either way, because no configured ontology covers them. An unrecognised prefix may name an ontology this run could not reach as easily as one that does not exist, so nothing here is evidence of fabrication: ORPHA.