Fructose absorption: the pathway in the body

This page shows the biochemical pathway behind the laboratory value H₂ after a fructose load: which stations follow one another, which enzymes carry out each step and which cofactors they use. Every statement has a source. The page describes general textbook knowledge and says nothing about any individual person.

In brief

Fructose is a simple sugar found in fruit and table sugar. The small intestine and liver absorb it via transporters and convert it into glucose and other substances; what reaches the large intestine is fermented by bacteria, producing hydrogen.

The pathway step by step

1. Fructose in small bowel → Into the gut cell GLUT5 The transporter GLUT5 lets fructose into the cells of the small intestinal wall. It uses no energy and follows the gradient; its amount adjusts to the fructose supply. Source 1, 6
2. Into the gut cell → Conversion in gut cell Aldolase B · ATP Part of the fructose is already converted inside the gut cell: fructokinase and aldolase B produce glucose, lactate and other small acids. Source 2
3. Conversion in gut cell → Via the portal vein GLUT2 What has not been converted leaves the gut cell via GLUT2 and reaches the liver with the portal blood. Source 1, 2
4. Via the portal vein → In the liver Fructokinase · ATP Fructokinase attaches a phosphate, using ATP. Aldolase B splits the molecule into two fragments of three carbons each, which enter glycolysis and gluconeogenesis. Source 3
5. Fructose in the colon → Short-chain fatty acids Gut bacteria Colonic bacteria ferment the fructose. This produces short-chain fatty acids such as acetate, propionate and butyrate, which the gut lining absorbs. Source 4
6. Fructose in the colon → Hydrogen (H₂) Hydrogenases Fermentation leaves surplus electrons. Bacterial enzymes called hydrogenases pass them to protons, producing hydrogen gas. Source 4
7. Hydrogen (H₂) → Methane Archaea · Carbon dioxide In some people the gut hosts archaea that convert hydrogen and carbon dioxide into methane. Less hydrogen and more methane are then found. Source 4, 5
8. Hydrogen (H₂) → H₂ in the blood Part of the hydrogen passes through the gut wall into the blood. Human cells do not produce hydrogen themselves; it comes exclusively from microorganisms. Source 4, 5
9. H₂ in the blood → H₂ in exhaled air The blood carries the hydrogen to the lungs, where it passes into the breath. This is why exhaled air can be used to follow how much sugar was fermented in the colon. Source 5

Cofactors in this pathway

• Magnesium — Forms the Mg-ATP complex with ATP that fructokinase uses as its phosphate source Source 7, 3In the ORY catalogue as a laboratory value: Magnesium
• ATP (adenosine triphosphate) — Supplies fructokinase with the phosphate group for fructose 1-phosphate Source 3
• Niacin (NAD⁺) — NAD⁺ and NADH carry electrons when the fragments are converted to lactate and glucose Source 2, 3In the ORY catalogue as a laboratory value: NAD⁺ (Nicotinamidadenindinukleotid)
• Iron — Metal centre of bacterial hydrogenases, which form hydrogen during fermentation Source 4

Sources

1. Ferraris RP, Choe JY, Patel CR. Intestinal Absorption of Fructose. Annu Rev Nutr 2018 · PubMed 29751733
2. Jang C, Hui S, Lu W et al. The Small Intestine Converts Dietary Fructose into Glucose and Organic Acids. Cell Metab 2018 · PubMed 29414685
3. Iizuka K. Recent Progress on Fructose Metabolism-Chrebp, Fructolysis, and Polyol Pathway. Nutrients 2023 · PubMed 37049617
4. Carbonero F, Benefiel AC, Gaskins HR. Contributions of the microbial hydrogen economy to colonic homeostasis. Nat Rev Gastroenterol Hepatol 2012 · PubMed 22585131
5. Rezaie A, Buresi M, Lembo A et al. Hydrogen and Methane-Based Breath Testing in Gastrointestinal Disorders: The North American Consensus. Am J Gastroenterol 2017 · PubMed 28323273
6. Douard V, Ferraris RP. Regulation of the fructose transporter GLUT5 in health and disease. Am J Physiol Endocrinol Metab 2008 · PubMed 18398011
7. de Baaij JH, Hoenderop JG, Bindels RJ. Magnesium in man: implications for health and disease. Physiol Rev 2015 · PubMed 25540137

Related pathways

• Gut fermentation and breath gases — short-chain fatty acids
• Lactose digestion — short-chain fatty acids
• Gut microbiome — short-chain fatty acids
• Alanine — Magnesium, NAD⁺ (Nicotinamidadenindinukleotid)
• Citric acid cycle — Magnesium, NAD⁺ (Nicotinamidadenindinukleotid)

As of 2026-09-16. Draft written by Claude to schema v2; sources checked in PubMed; expert review pending
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