Sorbit-Atemgastest (Sorbitmalabsorption): the pathway in the body

Sorbit-Atemgastest (Sorbitmalabsorption) is part of the pathway “Gut fermentation and breath gases”. This page shows the whole pathway; the station of Sorbit-Atemgastest (Sorbitmalabsorption) is highlighted.

Where this laboratory value sits: Sorbitol — sugar alcohol from fruit. Sorbitol is a sugar alcohol from stone fruit and sweeteners. The small intestine has no dedicated transporter for it; only part of it passes slowly through and is converted to fructose in the liver by sorbitol dehydrogenase. Source 9, 11, 10

In brief

Hydrogen (H₂) and methane (CH₄) are gases formed not by the human body but by microorganisms in the gut. They arise when bacteria break down undigested sugars without oxygen, and reach the exhaled air via the blood and lungs. If H₂ is used up, fermentation keeps going.

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. Starch and sugars → Simple sugars Amylase, lactase Pancreatic amylase breaks down starch. Enzymes on the brush border of the small intestine, including lactase, split the double sugars into simple sugars. Only as simple sugars can they be absorbed. Source 1↑ supplies Only as simple sugars can glucose, galactose and fructose be absorbed. Splitting at the brush border decides whether a sugar is used in the small intestine or travels on. established physiology Source 1
   ⚖ When the balance tips

   too much — If more simple sugars are present in the small intestine than the transporters can absorb, they stay dissolved in the gut and bind water osmotically.

   too little — If a splitting enzyme such as lactase is not very active, fewer simple sugars form; the unsplit double sugar passes into the colon for fermentation.

   established physiology · Source 1, 5

2. Simple sugars → Uptake into blood SGLT1, GLUT5 Transport proteins such as SGLT1 and GLUT5 move the simple sugars across the gut wall into the blood. Whatever is not absorbed here travels on. If the capacity of the transporters is exceeded, sugar stays in the gut. Source 1↑ supplies SGLT1 absorbs glucose and galactose together with sodium, GLUT5 absorbs fructose along the gradient; GLUT2 releases them into the blood. What is absorbed in this way serves the body as fuel and does not reach the bacteria. established physiology Source 1
   ⚖ When the balance tips

   too much — If the capacity of GLUT5 is exceeded, fructose stays in the gut, draws water osmotically and passes into the colon.

   too little — If the transporters absorb little, a larger share of the sugars passes into the colon and is available there to the bacteria for fermentation.

   established physiology · Source 1, 5

3. Undigested sugars → Bacterial fermentation · Gut bacteria Bacteria break down the sugars without oxygen. This happens mainly in the colon; when more bacteria live in the small intestine, fermentation already starts there. This yields fatty acids as fuel and gases as a by-product. Source 5, 4↕ both, depending on amount Fermentation supplies short-chain fatty acids as energy for the gut and the body, and gases as a by-product. How much of each forms depends on the amount of sugar and on the bacteria involved. established physiology Source 5, 2, 6
   ⚖ When the balance tips

   too much — If a lot of fermentation runs at once, more gas collects in the gut; if it already starts in the small intestine, hydrogen and methane rise earlier in the exhaled air.

   too little — If little fermentation takes place, fewer short-chain fatty acids and fewer gases form; more dietary fibre leaves the gut unchanged.

   established physiology · Source 4, 5

4. Bacterial fermentation → Short-chain fatty acids Most fermentation ends in short-chain fatty acids. The gut lining absorbs them; butyrate serves as fuel for the gut cells themselves. The liver uses propionate to make glucose. Source 2↑ supplies Short-chain fatty acids are fuel: butyrate supplies the gut cells, propionate serves the liver for making glucose, acetate enters the circulation. They also act as signalling substances at their own receptors. established physiology Source 2
   ⚖ When the balance tips

   too much — If many short-chain fatty acids form, the gut lining absorbs more of them; the liver and other tissues use the part that the gut cells do not consume.

   too little — If few short-chain fatty acids form, less butyrate is available to the gut cells as fuel, and less acetate and propionate reach the liver.

   established physiology · Source 2

5. Bacterial fermentation → Hydrogen H₂ Hydrogenases Fermentation leaves spare electrons. Enzymes called hydrogenases release them as hydrogen gas – human cells themselves make no H₂. If hydrogen builds up, it slows fermentation. Source 3, 6↕ both, depending on amount For fermentation to continue, hydrogen has to be removed: if it builds up, it slows the hydrogenases. Other microorganisms consume it and so keep fermentation going. established physiology Source 6, 3
   ⚖ When the balance tips

   too much — If hydrogen builds up in the gut, fermentation slows, and a larger share passes into the blood and the exhaled air or leaves the gut as gas.

   too little — If little hydrogen is present, methane producers, acetate producers and sulphate reducers have less starting material; fermentation itself then runs unhindered.

   established physiology · Source 6, 5

6. Hydrogen H₂ → Methane CH₄ Methyl-CoM reductase · Archaea, CO₂ Methane-producing archaea, such as Methanobrevibacter smithii, consume the hydrogen and combine it with CO₂ to form methane. Other bacteria use H₂ to make acetate or sulphide. Using up H₂ keeps fermentation going. Source 3, 6↓ depletes Methane producers consume hydrogen: four H₂ and one CO₂ yield one methane. The amount of gas thus decreases, and fermentation can continue. established physiology Source 3, 6
   ⚖ When the balance tips

   too much — If archaea form a lot of methane, more hydrogen is consumed; methane then rises in the exhaled air while hydrogen turns out lower.

   too little — If few methane producers live in the gut, other microorganisms use the hydrogen, or it remains as H₂ and appears in the exhaled air.

   established physiology · Source 4, 3

7. Hydrogen H₂ → Gases in blood Part of both gases passes through the gut wall into the blood. The rest leaves the body as intestinal gas. The blood carries the gases unchanged to the lungs. Source 5↑ supplies The blood carries the gases unchanged to the lungs, where they are released. How much crosses over depends on how much gas forms in the gut and how long it stays there. established physiology Source 5
   ⚖ When the balance tips

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

   too little — If little gas forms or microorganisms consume it quickly, little of it reaches the blood.

   established physiology · Source 5, 3

8. Methane CH₄ → Gases in blood Part of both gases passes through the gut wall into the blood. The rest leaves the body as intestinal gas. The blood carries the gases unchanged to the lungs. Source 5↑ supplies The blood carries the gases unchanged to the lungs, where they are released. How much crosses over depends on how much gas forms in the gut and how long it stays there. established physiology Source 5
   ⚖ When the balance tips

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

   too little — If little gas forms or microorganisms consume it quickly, little of it reaches the blood.

   established physiology · Source 5, 3

9. Gases in blood → Exhaled air In the lungs, the gases pass into the exhaled air. The breath test measures H₂ and CH₄ at regular intervals after a defined portion of sugar. If hydrogen is converted to methane, less of it appears in the breath. Source 4↓ depletes The lungs remove the gases with every breath. Their content in the exhaled air thus reflects gas formation in the gut in the hours after a portion of sugar. established physiology Source 4, 5
   ⚖ When the balance tips

   too much — If a lot of gas is formed, its content in the exhaled air rises; the timing of the rise depends on how quickly the sugar meets bacteria.

   too little — If little hydrogen forms or it is quickly converted to methane, its content in the exhaled air stays low even though sugar is fermented; this is why the test measures both gases.

   established physiology · Source 4, 5

Further stations

• Starch and sugars — from food Carbohydrates arrive on the plate as long chains (starch) or as double sugars such as milk sugar and table sugar. What the small intestine does not break down later becomes food for the bacteria. Source 1↑ supplies Carbohydrates supply the sugars that the small intestine can absorb once they are broken down into simple sugars. Whatever cannot be broken down there stays in the gut and becomes the starting material for fermentation. established physiology Source 1, 5
  ⚖ When the balance tips

  too much — If more sugar arrives at once than enzymes and transporters can handle in the same time, part of it reaches the colon undigested and is fermented there.

  too little — If few fermentable carbohydrates arrive, the gut bacteria have less starting material, and fewer gases and short-chain fatty acids are formed.

  established physiology · Source 1, 5

• Undigested sugars — reach the colon Dietary fibre and uncleaved sugars reach the colon. This is where most gut bacteria live. They are the starting material for fermentation. Source 5, 2↑ supplies Dietary fibre and undigested sugars are the food of the colonic bacteria. The more of them arrive, the more fermentation products form: short-chain fatty acids and gases. established physiology Source 5, 2
  ⚖ When the balance tips

  too much — If many undigested sugars arrive, they draw water osmotically into the gut, and fermentation forms more gas.

  too little — If few fermentable substances arrive, the bacteria form fewer short-chain fatty acids, and the gut cells receive less butyrate as fuel.

  established physiology · Source 5, 2

• Sorbitol — sugar alcohol from fruit Sorbitol is a sugar alcohol from stone fruit and sweeteners. The small intestine has no dedicated transporter for it; only part of it passes slowly through and is converted to fructose in the liver by sorbitol dehydrogenase. Source 9, 11, 10↑ supplies The unabsorbed part holds water in the gut osmotically and reaches the colon, where bacteria ferment it into gases and short-chain fatty acids. established physiology Source 9, 11
  ⚖ When the balance tips

  too much — If more sorbitol arrives than can be absorbed slowly, more stays in the gut: it binds more water, and more hydrogen forms in the colon.

  too little — If little sorbitol arrives, a larger share of it is absorbed, and hardly any gas forms from it in the colon.

  established physiology · Source 9, 11

Cofactors in this pathway

• Nickel — Metal in coenzyme F430 of methyl-CoM reductase, which archaea use to form methane; without F430 no methane Source 7
• Iron — Metal in the hydrogenases that bacteria use to release electrons as H₂; electrons thus flow away Source 6
• NAD⁺ — Electron carrier of fermentation; bacteria release part of the electrons from NADH as H₂; this frees NAD⁺ again Source 6
• Sodium — Drives the SGLT1 transporter, which carries glucose and galactose into the gut cell; sodium pulls the sugar along Source 1In the ORY catalogue as a laboratory value: Natrium (intrazellulär)
• Calcium — Bound ion of alpha-amylase, which breaks down starch in the small intestine and keeps its shape stable Source 8In the ORY catalogue as a laboratory value: Calcium (intrazellulär)
• Chloride — Bound ion near the active site of alpha-amylase and activates it Source 8

Sources

1. Drozdowski LA, Thomson AB. Intestinal sugar transport. World J Gastroenterol 2006 · PubMed 16586532
2. den Besten G, van Eunen K, Groen AK et al. The role of short-chain fatty acids in the interplay between diet, gut microbiota, and host energy metabolism. J Lipid Res 2013 · PubMed 23821742
3. Smith NW, Shorten PR, Altermann EH et al. Hydrogen cross-feeders of the human gastrointestinal tract. Gut Microbes 2019 · PubMed 30563420
4. 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
5. Di Stefano M, Mengoli C, Bergonzi M et al. Hydrogen breath test and intestinal gas production. Eur Rev Med Pharmacol Sci 2013 · PubMed 24443066
6. Carbonero F, Benefiel AC, Gaskins HR. Contributions of the microbial hydrogen economy to colonic homeostasis. Nat Rev Gastroenterol Hepatol 2012 · PubMed 22585131
7. Thauer RK. Methyl (Alkyl)-Coenzyme M Reductases: Nickel F-430-Containing Enzymes Involved in Anaerobic Methane Formation and in Anaerobic Oxidation of Methane or of Short Chain Alkanes. Biochemistry 2019 · PubMed 30951290
8. Brayer GD, Luo Y, Withers SG. The structure of human pancreatic alpha-amylase at 1.8 A resolution and comparisons with related enzymes. Protein Sci 1995 · PubMed 8528071
9. Lenhart A, Chey WD. A Systematic Review of the Effects of Polyols on Gastrointestinal Health and Irritable Bowel Syndrome. Adv Nutr 2017 · PubMed 28710145
10. El-Kabbani O, Darmanin C, Chung RP. Sorbitol dehydrogenase: structure, function and ligand design. Curr Med Chem 2004 · PubMed 14965227
11. Montalto M, Gallo A, Ojetti V et al. Fructose, trehalose and sorbitol malabsorption. Eur Rev Med Pharmacol Sci 2013 · PubMed 24443064

Whole pathway: Gut fermentation and breath gases

Related pathways

• Fructose absorption — short-chain fatty acids
• Lactose digestion — short-chain fatty acids
• Gut microbiome — short-chain fatty acids
• Digestion and absorption — simple sugars
• Magnesium — Natrium (intrazellulär), Calcium (intrazellulär)

As of 2026-09-16. Draft written by Claude to schema v2; sources checked in PubMed; expert review pending
Legal notice Privacy policy All biomarkers