The colonic microbiome is the community of helpful bacteria living mainly in the large bowel, where they play an important role in nutrition, recycling, gas production, immunity and overall gut wellbeing.

When this balance is disturbed, certain sugars, known as FODMAPs, may be abnormally fermented, leading to gut symptoms like bloating and altered bowel habit. Measuring hydrogen and methane in expired breath helps identify this by measuring these gases when challenged with a sequence of the FODMAP sugars.

The breath fermentation pattern following a glucose challenge may also suggest SIBO (small intestinal bacterial overgrowth), where bacteria inhabit the small intestine, which normally contains very few microbes, and can result in abnormal gut reactions.