[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Biochemistry](https://durgical.com/en/biochemistry/) › Fructose metabolism [Biochemistry](https://durgical.com/en/biochemistry/) · Metabolic pathway # Fructose metabolism The liver turns fructose from fruit, table sugar and syrups into trioses that enter glycolysis without going through PFK-1. Updated 2026-11-01 ## Interactive map **Balance:** ATP -2 nadph -1 NADH +1 ## Key facts - Liver route: fructokinase → fructose 1-phosphate → aldolase B → DHAP + glyceraldehyde → triokinase → glyceraldehyde 3-phosphate. - It bypasses PFK-1, the main control of glycolysis: that is why fructose is metabolised fast and, in excess, favours fat synthesis in the liver. - Polyol pathway (steps 1–2): aldose reductase reduces glucose to sorbitol with NADPH; sorbitol dehydrogenase oxidises it to fructose. This is how seminal fructose is made. - Fructokinase deficiency: essential fructosuria, benign. Aldolase B deficiency: hereditary fructose intolerance, serious. **Also called** Fructolysis and the polyol pathway ## Regulation - Fructokinase has no major physiological brake: fructose enters the liver in proportion to the amount eaten. - Aldose reductase works harder the higher the glucose (its Km is high): in diabetes sorbitol builds up in lens, nerves and retina. ## The steps, one by one - **Step 1 · Aldose reductase** irreversible [Glucose](https://durgical.com/en/biochemistry/molecules/glucose/) + [NADPH](https://durgical.com/en/biochemistry/molecules/nadph/) → Sorbitol + nadp It reduces glucose to sorbitol (polyol pathway); it matters most when glucose is high. **If it fails:** Sorbitol built up in lens and nerves contributes to diabetic cataract and neuropathy. [Cataract →](https://durgical.com/en/conditions/cataract/) - **Step 2 · Sorbitol dehydrogenase** Sorbitol + [NAD⁺](https://durgical.com/en/biochemistry/molecules/nad/) ⇌ [Fructose](https://durgical.com/en/biochemistry/molecules/fructose/) + NADH It oxidises sorbitol to fructose; lens, retina and nerves have little of it, so sorbitol stays there. - **Step 3 · Fructokinase (ketohexokinase)** irreversible [Fructose](https://durgical.com/en/biochemistry/molecules/fructose/) + [ATP](https://durgical.com/en/biochemistry/molecules/atp/) → Fructose 1-phosphate + ADP It phosphorylates fructose on carbon 1 and traps it in liver, kidney and intestine. **If it fails:** Its deficiency is essential fructosuria: fructose spills into the urine without symptoms. [Essential fructosuria →](https://durgical.com/en/conditions/essential-fructosuria/) - **Step 4 · [Aldolase B](https://durgical.com/en/biochemistry/enzymes/aldolase-b/)** Fructose 1-phosphate ⇌ Glyceraldehyde + dihydroxyacetone phosphate It splits fructose 1-phosphate into DHAP, which enters glycolysis directly, and glyceraldehyde. **If it fails:** Its deficiency is hereditary fructose intolerance: fructose 1-phosphate traps phosphate and halts glycogenolysis and gluconeogenesis. [Hereditary fructose intolerance →](https://durgical.com/en/conditions/hereditary-fructose-intolerance/) - **Step 5 · Triokinase** irreversible Glyceraldehyde + [ATP](https://durgical.com/en/biochemistry/molecules/atp/) → glyceraldehyde 3-phosphate + ADP It phosphorylates glyceraldehyde to glyceraldehyde 3-phosphate, which goes on through glycolysis or gluconeogenesis. ## Clinical notes - In hereditary fructose intolerance fructose 1-phosphate accumulates, phosphate is trapped and ATP and blood glucose fall: hypoglycaemia, vomiting and liver and kidney damage. - Sorbitol crosses membranes poorly and draws in water: it contributes to diabetic cataract, neuropathy and retinopathy. - High intake of added fructose is associated with fatty liver, high triglycerides and high uric acid. ## Related conditions - [Hereditary fructose intolerance](https://durgical.com/en/conditions/hereditary-fructose-intolerance/) - [Essential fructosuria](https://durgical.com/en/conditions/essential-fructosuria/) - [Type 2 diabetes mellitus](https://durgical.com/en/conditions/type-2-diabetes-mellitus/) - [Diabetic retinopathy](https://durgical.com/en/conditions/diabetic-retinopathy/) - [Cataract](https://durgical.com/en/conditions/cataract/) ## Related - [Fructose](https://durgical.com/en/biochemistry/molecules/fructose/) - [Glucose](https://durgical.com/en/biochemistry/molecules/glucose/) - [Aldolase B](https://durgical.com/en/biochemistry/enzymes/aldolase-b/) - [Glycolysis](https://durgical.com/en/biochemistry/pathways/glycolysis/) - [Gluconeogenesis](https://durgical.com/en/biochemistry/pathways/gluconeogenesis/) - [NADPH](https://durgical.com/en/biochemistry/molecules/nadph/) - [Galactose metabolism](https://durgical.com/en/biochemistry/pathways/galactose-metabolism/) ## Test yourself Think of the answer, then tap the question to see it. ## Why is fructose metabolised faster than glucose in the liver? It enters glycolysis as trioses, after PFK-1, and escapes its control. ## Which enzyme is missing in essential fructosuria and why is it benign? Fructokinase; fructose is not phosphorylated, nothing toxic builds up and it leaves in the urine. ## Which molecule builds up in hereditary fructose intolerance? Fructose 1-phosphate, because aldolase B is missing; it traps phosphate and lowers ATP. ## Why does sorbitol damage the lens in diabetes? Aldose reductase makes it from high glucose; the lens has almost no sorbitol dehydrogenase and sorbitol draws in water. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/biochemistry/pathways/fructose-metabolism/) Educational information, not medical advice. For any symptom, see a health professional. Sources: [Fructose metabolism](https://reactome.org/PathwayBrowser/#/R-HSA-5652084) (Reactome, 2026); [KHK: Ketohexokinase (human)](https://www.uniprot.org/uniprotkb/P50053/entry) (UniProt, 2026); [ALDOB: Fructose-bisphosphate aldolase B (human)](https://www.uniprot.org/uniprotkb/P05062/entry) (UniProt, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011). [← All of biochemistry](https://durgical.com/en/biochemistry/) ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/biochemistry/pathways/fructose-metabolism/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/pathways/fructose-metabolism/