Hydrogen H₂ (breath gas)

H₂ is produced by bacterial fermentation of carbohydrates in the gut – normally mainly in the large intestine, and with overgrowth also in the small intestine. A rise in H₂ after lactulose is looked at with questions about small intestinal colonisation. Measured over 90 minutes.

How hydrogen is produced in the gut

Human cells do not produce hydrogen – it is formed exclusively when bacteria ferment carbohydrates without oxygen. Part of the gas passes through the gut wall into the blood, is transported to the lungs and exhaled; this is why it can be measured in the breath. The larger part is consumed by other microbes: methane-producing archaea turn it into methane, sulphate-reducing bacteria into hydrogen sulphide and acetogenic bacteria into acetate; the rest leaves the body as intestinal gas. Lactulose is not absorbed in the small intestine and normally only reaches the bacteria in the large intestine. If hydrogen rises earlier, fermentation is already taking place in the small intestine.

Low H₂ – what happens in the gut?

If hydrogen stays low, little was fermented, or the hydrogen formed was consumed by other microbes before it could enter the blood – for example by methane-producing archaea, in which case methane rises instead, or by sulphate-reducing bacteria, whose hydrogen sulphide is not captured in this test. Recent antibiotics, laxatives or a colonoscopy can also temporarily reduce hydrogen production.

High H₂ – what happens in the gut?

A lot of hydrogen means a lot of bacterial fermentation. The gas stretches the gut wall, which can be felt as a bloated abdomen, pressure and cramping pain. At the same time, fermentation acids and other dissolved substances are produced that draw water into the gut and make the stool softer; gut transit tends to speed up. If fermentation already takes place in the small intestine, the bacteria there compete with the body for nutrients.

The interpretation is given in your report.