Why gases matter
When bacteria in the small intestine ferment carbohydrates, they produce gases. A breath test measures these gases over two to three hours after drinking a sugar solution such as lactulose. The pattern and the timing of the rise tell us which type of overgrowth is most likely.
Hydrogen-dominant SIBO
Hydrogen is produced by bacteria. A hydrogen-dominant pattern is often associated with loose stools, urgency, bloating shortly after meals and abdominal discomfort.
Methane overgrowth (IMO)
Methane is not produced by bacteria but by archaea, mainly Methanobrevibacter smithii. Because these microbes can also live in the large intestine, we now speak of intestinal methanogen overgrowth (IMO). Methane slows gut transit, which is why IMO is strongly linked to constipation, heaviness and persistent bloating.
Hydrogen sulfide (ISO)
A third gas, hydrogen sulfide, is linked to intestinal sulfide overproduction (ISO). Typical signs include diarrhoea, a 'rotten egg' smell and sometimes sensitivity to sulfur-rich foods. It requires a test that specifically measures this gas.
Why the distinction steers your pathway
- Different microbes respond to different antimicrobial strategies.
- Methane-dominant cases often need extra attention to motility and constipation.
- Mixed patterns are common and require a combined plan.
- Retesting after the intervention phase shows whether the gas pattern has normalised.
This article is for general information and does not replace medical advice. Medical decisions, medication and prescriptions always remain with your physician.
Not sure which type you have?
A breath test shows which gas pattern dominates.