H₂ after lactose challenge

A rise in H₂ after taking lactose points to incomplete lactose digestion. How this is to be read is set out in your findings report. Undigested lactose is fermented in the large intestine → bloating, cramps, diarrhoea.

What happens to lactose in the gut

Milk sugar (lactose) is split in the small intestine by the enzyme lactase, which sits on the surface of the small intestinal cells. The building blocks glucose and galactose are absorbed into the blood. In most adults worldwide, lactase activity declines after early childhood in a genetically controlled way; whether it persists depends mainly on a variant in the genetic material in front of the lactase gene. In addition, lactase can be temporarily lacking when the small intestinal lining is damaged, for example after gastrointestinal infections, in coeliac disease or with bacterial overgrowth of the small intestine.

Low H₂ after lactose – what does it mean?

If hydrogen stays low after the test solution, the lactose was largely split and absorbed in the small intestine. In some people, however, methane-producing archaea immediately convert the hydrogen into methane; methane then rises instead of hydrogen, even though lactose has reached the large intestine.

High H₂ after lactose – what happens in the gut?

Unsplit lactose remains in the small intestine and draws water there osmotically. In the large intestine, bacteria ferment it into hydrogen, carbon dioxide and short-chain fatty acids; the hydrogen passes into the blood and appears in the breath. Gas and additional fluid stretch the gut and speed up transit. How noticeable this is depends on the amount of lactose, the residual lactase activity, the composition of the colonic flora and the sensitivity of the gut.