Lactose digestion: the pathway in the body

This page shows the biochemical pathway behind the laboratory value H2 after lactose challenge: 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

Lactose is milk sugar, a double sugar made of glucose and galactose. The enzyme lactase separates it in the small intestine; whatever reaches the colon unseparated is fermented by bacteria, among other things into hydrogen.

The pathway step by step

1. Lactose in food → Glucose and galactose Lactase (LPH) The enzyme lactase-phlorizin hydrolase sits on the outer brush border of the small intestinal cells and separates the two sugars from each other. Source 2
2. Glucose and galactose → Absorption into blood SGLT1, GLUT2 · sodium Transporters carry both sugars into the intestinal cell and on into the blood. In the liver, galactose is converted into glucose-1-phosphate by three enzymes. Source 1
3. Lactose in the colon → Bacteria ferment Gut bacteria Bacteria of the gut flora absorb the lactose and ferment it. This produces gases and short-chain fatty acids. Source 1
4. Bacteria ferment → Short-chain fatty acids Besides the gases, short-chain fatty acids are formed. The cells of the colon wall absorb them and use butyrate as fuel. Source 1
5. Bacteria ferment → Hydrogen (H2) Hydrogen is one of the fermentation gases. Human cells do not produce it; it comes exclusively from bacterial metabolism. Source 4
6. Hydrogen (H2) → Methane (CH4) Archaea · hydrogen In some people, the large intestine is home to archaea that make methane from hydrogen and carbon dioxide. The share of hydrogen in the gas is then lower. Source 4
7. Hydrogen (H2) → Into blood and lungs Part of the hydrogen passes from the gut into the blood and is released into the exhaled air in the lungs. Source 4
8. Into blood and lungs → Measurable in breath This is why hydrogen can be measured in the breath after a portion of lactose. A rise shows that lactose has reached the large intestine. Source 1, 4

Cofactors in this pathway

• Sodium — The transporter SGLT1 carries glucose and galactose into the gut cell together with sodium Source 1In the ORY catalogue as a laboratory value: Natrium (intrazellulär)
• ATP — Phosphate donor of galactokinase, the first enzyme in the liver's galactose conversion Source 5
• UDP-glucose — Partner of GALT, which transfers the uridine group onto galactose 1-phosphate Source 5
• NAD⁺ — Tightly bound cofactor of UDP-galactose 4-epimerase (GALE) Source 6In the ORY catalogue as a laboratory value: NAD⁺ (Nicotinamidadenindinukleotid)
• Nickel — Central atom in coenzyme F430 of the archaea that form methane from hydrogen Source 7, 4

Sources

1. Misselwitz B, Butter M, Verbeke K et al. Update on lactose malabsorption and intolerance: pathogenesis, diagnosis and clinical management. Gut 2019 · PubMed 31427404
2. Naim HY. Molecular and cellular aspects and regulation of intestinal lactase-phlorizin hydrolase. Histol Histopathol 2001 · PubMed 11332711
3. Swallow DM. Genetics of lactase persistence and lactose intolerance. Annu Rev Genet 2003 · PubMed 14616060
4. Di Stefano M, Mengoli C, Bergonzi M et al. Hydrogen breath test and intestinal gas production. Eur Rev Med Pharmacol Sci 2013 · PubMed 24443066
5. Timson DJ. The molecular basis of galactosemia - Past, present and future. Gene 2016 · PubMed 26143117
6. Beerens K, Soetaert W, Desmet T. UDP-hexose 4-epimerases: a view on structure, mechanism and substrate specificity. Carbohydr Res 2015 · PubMed 26162744
7. Thauer RK, Bonacker LG. Biosynthesis of coenzyme F430, a nickel porphinoid involved in methanogenesis. Ciba Found Symp 1994 · PubMed 7842854

Related pathways

• Fructose absorption — short-chain fatty acids
• Gut fermentation and breath gases — short-chain fatty acids
• Gut microbiome — short-chain fatty acids
• GABA, glutamate and glutamine — Natrium (intrazellulär), NAD⁺ (Nicotinamidadenindinukleotid)

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