Laktase-Gen (LCT, Laktoseintoleranz-Gentest): the pathway in the body

Laktase-Gen (LCT, Laktoseintoleranz-Gentest) is part of the pathway “Lactose digestion”. This page shows the whole pathway; the station of Laktase-Gen (LCT, Laktoseintoleranz-Gentest) is highlighted.

Where this laboratory value sits: Lactase gene (LCT) — control in neighbouring gene. How much lactase is made is controlled by a region in the neighbouring gene MCM6. In many adults production declines after childhood; in others it stays high. This is called lactase persistence. This determines how much lactose is split. Source 3

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. Unsplit lactose binds water osmotically.

The pathway step by step

Each station states what the compound does there. Three signs: ↑ supplies — builds up or makes available · ↓ depletes — inhibits, consumes or withholds · ↕ both, depending on amount. Behind it stands what the statement rests on: established physiology, observed in studies, or contested. The signs do not grade; they name the direction.

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. Its activity determines how much lactose is used in the small intestine. Source 2↑ supplies Lactase turns a double sugar that cannot be absorbed into two simple sugars that can. Its activity at the brush border determines how much lactose is used in the small intestine. established physiology Source 2, 1
   ⚖ When the balance tips

   too much — If plenty of lactase is present, glucose and galactose form almost completely in the small intestine and go into the blood instead of to the bacteria.

   too little — If little lactase is present at the brush border, fewer simple sugars form; the unsplit rest of the lactose travels on into the colon.

   established physiology · Source 2, 1

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. Glucose serves the cells directly as fuel. Source 1↑ supplies Glucose serves the cells directly as fuel. The liver converts galactose into glucose 1-phosphate via galactokinase, GALT and GALE; in this way it enters the same metabolism. established physiology Source 1, 5
   ⚖ When the balance tips

   too much — If galactose builds up because the three enzymes do not convert it fast enough, part of it is turned into galactitol via a side pathway.

   too little — If little is absorbed because little lactose was split, it supplies hardly any energy; the sugars are instead available to the bacteria in the colon.

   established physiology · Source 5, 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. In this way they break down the water-binding lactose. Source 1↕ both, depending on amount Fermentation breaks down the water-binding lactose and turns it into short-chain fatty acids, which the gut wall absorbs, and gases, which leave the body via the gut and the breath. established physiology Source 1
   ⚖ When the balance tips

   too much — If bacteria ferment a lot of lactose at once, more gas forms than the gut can absorb or release in the same time; the gas stretches the gut.

   too little — If the bacteria ferment little, more lactose remains unsplit and continues to bind water in the gut.

   established physiology · 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. In this way the gut recovers part of the energy. Source 1↑ supplies The fatty acids recover part of the energy from the unsplit lactose: the colon wall absorbs them, and butyrate supplies the gut cells themselves. established physiology Source 1
   ⚖ When the balance tips

   too much — If many fatty acids form, the colon wall absorbs more of them and so recovers more energy from the lactose.

   too little — If few fatty acids form, the gut recovers hardly any energy from the unsplit lactose, and less butyrate is available to the gut cells.

   established physiology · 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. Archaea use it further, for example to form methane. Source 4↑ supplies Hydrogen has no task in human metabolism, but serves archaea and other bacteria as starting material. Because only bacteria form it, it indicates that sugar is being fermented. established physiology Source 4
   ⚖ When the balance tips

   too much — If a lot of hydrogen forms, more of it passes into the blood and appears in the breath; part of it leaves the gut as gas.

   too little — If little hydrogen forms or archaea quickly convert it to methane, its share stays small even though lactose is being fermented.

   established physiology · 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. In this way they consume hydrogen. Source 4↓ depletes Archaea consume hydrogen: from four H₂ and one CO₂ they form one methane. The amount of hydrogen thus decreases, and the same fermentation yields less measurable hydrogen. established physiology Source 4, 7
   ⚖ When the balance tips

   too much — If archaea form a lot of methane, they consume a lot of hydrogen; methane then rises in the breath while hydrogen stays low.

   too little — If few archaea live in the colon, the hydrogen is largely retained and reaches the breath via the blood and lungs.

   established physiology · 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. Another part leaves the gut as gas. Source 4↑ supplies The blood carries the hydrogen from the gut to the lungs. Only this part becomes visible in the breath; the rest leaves the gut as gas or is consumed by microorganisms. established physiology Source 4
   ⚖ When the balance tips

   too much — If more gas forms in the gut, more of it also passes into the blood and reaches the lungs.

   too little — If little hydrogen is formed or it is already consumed in the gut, little of it reaches the blood.

   established physiology · 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. If hydrogen is converted to methane, the rise is smaller. Source 1, 4↓ depletes With exhalation the hydrogen leaves the body. Its course in the breath reflects when and how much lactose is fermented in the colon. established physiology Source 4, 1
   ⚖ When the balance tips

   too much — If a lot of lactose is fermented, hydrogen in the breath rises clearly after the lactose portion; the timing follows the path of the lactose to the bacteria.

   too little — If little lactose is fermented or archaea convert the hydrogen to methane, the rise in the breath stays small; this is why methane is often measured as well.

   established physiology · Source 4, 1

Further stations

• Lactose in food — sugar from milk Lactose is a double sugar made of glucose and galactose. In this form the small intestine cannot absorb it; it first has to be split. Once split, it supplies two sugars as energy. Source 1↑ supplies Lactose supplies two simple sugars as energy, but only if lactase splits it. Unsplit, it stays dissolved in the gut and becomes the starting material for fermentation in the colon. established physiology Source 1
  ⚖ When the balance tips

  too much — If more lactose arrives than the available lactase can split in the time, the rest passes into the colon; how much that is depends on the amount and on lactase activity.

  too little — If little lactose arrives, even low lactase activity splits most of it, and hardly any reaches the colon.

  established physiology · Source 1

• Lactase gene (LCT) — control in neighbouring gene How much lactase is made is controlled by a region in the neighbouring gene MCM6. In many adults production declines after childhood; in others it stays high. This is called lactase persistence. This determines how much lactose is split. Source 3↑ supplies The region in MCM6 acts as a switch for reading the lactase gene: with lactase persistence it stays active, otherwise lactase production is turned down after childhood. established physiology Source 3
  ⚖ When the balance tips

  too much — If gene reading stays high, the small intestine keeps enough lactase into adulthood to split larger amounts of lactose.

  too little — If the gene is read little, lactase activity at the brush border falls, and a larger share of the lactose reaches the colon unchanged.

  established physiology · Source 3, 1

• Lactose in the colon — what remains unsplit Whatever is not split in the small intestine reaches the large intestine. There the dissolved sugar draws water into the gut by osmosis. At the same time it serves the bacteria as food. Source 1↕ both, depending on amount Unsplit lactose acts in two ways: dissolved, it binds water osmotically in the gut lumen, and it serves the bacteria as starting material for fermentation. established physiology Source 1
  ⚖ When the balance tips

  too much — If a lot of lactose builds up in the colon, it draws more water into the gut lumen, and the bacteria form more gas and fatty acids.

  too little — If little lactose reaches the colon, the osmotic pull stays small, and hardly any additional gas forms.

  established physiology · Source 1

Cofactors in this pathway

• Sodium — The transporter SGLT1 carries glucose and galactose into the gut cell together with sodium; sodium pulls sugar along 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; this keeps galactose in the cell Source 5
• UDP-glucose — Partner of GALT, which transfers the uridine group onto galactose 1-phosphate; this yields glucose 1-phosphate Source 5
• NAD⁺ — Tightly bound cofactor of UDP-galactose 4-epimerase (GALE) and so regenerates UDP-glucose 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; without F430 no methane 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

Whole pathway: Lactose digestion

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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