H₂ after fructose challenge

A rise in H₂ after fructose shows that part of the fructose was not absorbed in the small intestine; absorption via the GLUT5 transporter has a limited capacity that varies between individuals. Fructose absorption is often examined in irritable bowel.

How fructose is absorbed in the gut

Fruit sugar (fructose) is transported into the gut cells in the small intestine mainly via the transporter GLUT5 and released into the blood via GLUT2; the liver processes it further. GLUT5 works without energy expenditure and has a limited capacity that varies from person to person. Glucose present at the same time improves absorption, while sorbitol inhibits it. This is different from the rare hereditary fructose intolerance, in which an enzyme in the liver is missing.

Low H₂ after fructose – what does it mean?

If hydrogen stays low after the test solution, the fructose was largely absorbed in the small intestine. As with all breath tests, however, the hydrogen can also be consumed by methane-producing archaea; methane then rises instead.

High H₂ after fructose – what happens in the gut?

If the amount of fructose exceeds the absorption capacity, part of it remains in the small intestine and draws in water. In the large intestine, bacteria ferment the fructose into hydrogen, carbon dioxide and short-chain fatty acids; the hydrogen appears in the breath. Gas and fluid stretch the gut, which can be felt as bloating, pressure and cramps, and the stool becomes softer. How pronounced this is depends on the amount, on glucose and sorbitol consumed at the same time and on the sensitivity of the gut.

The interpretation is given in your report.