Hydrogen sulfide SIBO (ISO): sulfur burps and the third pattern

If your gas smells of rotten eggs, or a doctor has mentioned hydrogen sulfide, you have met the newest and least-charted of the three SIBO-type patterns: hydrogen sulfide SIBO (ISO). It sits alongside hydrogen-dominant SIBO and methane SIBO (IMO). This guide explains what it is, why older tests missed it, what it tends to feel like, the sulfur-food question, and how unsettled the science still is. It is educational, not medical advice, and honesty about the gaps is part of the point.

What ISO is

ISO involves too much hydrogen sulfide (H2S) being made in the gut. H2S is the gas that smells of rotten eggs, and gut microbes produce it two ways: by reducing sulfate, and by breaking down the sulfur in protein-rich foods. In small amounts hydrogen sulfide is a normal signaling molecule with useful roles in the gut, the problem is overproduction. Exactly which microbes are responsible is still being worked out: Desulfovibrio is the classic example, but recent work suggests it is not the whole story, so the honest phrasing is "various sulfur-handling gut microbes."

Why older breath tests missed it

For years, breath tests measured only two gases, hydrogen and methane, and simply could not see hydrogen sulfide. Worse, a hydrogen sulfide pattern could hide behind a falsely reassuring result: the microbes that make H2S consume hydrogen to do it, so the hydrogen line can read flat even while sulfide is high. Researchers have proposed that exactly this "flatline" can signal H2S. A newer 3-gas test now adds a hydrogen sulfide reading, which is why someone with clear sulfur symptoms but a "normal" 2-gas test in the past may not have had the full picture.

What it tends to feel like

The signature most tied to hydrogen sulfide is the smell: sulfur or rotten-egg gas and burps. Beyond that, be careful with the common claim that ISO "means diarrhea." Breath-test research has linked higher hydrogen sulfide with a looser-stool, diarrhea-leaning pattern, but the link is modest, and in practice bloating and even constipation show up plenty in people with a sulfur pattern. So the honest read is this: the rotten-egg smell is the most telling clue, while the bowel-habit pattern is a weak tendency, not a defining feature.

How it differs from the other two patterns

Lined up against the others: hydrogen-dominant leans toward diarrhea; methane SIBO (IMO), driven by methane-making archaea, leans toward constipation and has the best-established mechanism of the three; ISO adds the sulfur smell and a softer association with looser stools. The biggest practical difference with ISO is simply how much less settled it is. The other two patterns have agreed breath-test thresholds. ISO does not yet.

What a hydrogen sulfide result means (and how unsettled it is)

This is the most important honesty in this guide. The hydrogen and methane cutoffs come from a published consensus. The hydrogen sulfide cutoff does not. A commercial 3-gas test flags hydrogen sulfide at around 3 ppm, but that is a manufacturer's number, and there is no agreed standard yet; researchers have even used different cutoffs in their own studies. So a hydrogen sulfide reading is best treated as an early, evolving signal that your doctor weighs with everything else, not a hard line to cross. Our breath test guide explains the 3-gas test in more detail.

The sulfur-food question

Because hydrogen sulfide comes partly from sulfur in food, sulfur-rich foods can supply the raw material. The strongest dietary driver is the sulfur in protein: higher meat intake raises gut sulfide, and the sulfur amino acid cysteine is a bigger driver than inorganic sulfate. Other sources include eggs, dairy, and sulfite preservatives. Cruciferous vegetables and alliums such as garlic and onion carry sulfur too, though the evidence that they specifically raise gut hydrogen sulfide is weaker, and their bigger day-to-day gut effect is usually their FODMAP content.

Two cautions before you cut anything out. A low-sulfur diet as a fix is thinly evidenced so far, and fiber actually tends to lower hydrogen sulfide, so bluntly stripping out nutritious foods can backfire. Some people with a sulfur pattern do report feeling better when they moderate high-sulfur foods, but that is an individual experiment to run thoughtfully, ideally with a dietitian, not a proven protocol. Our food database flags high-sulfur foods with a per-type note so you can spot them at a glance.

What comes next

If a standard 2-gas test came back clear but your symptoms persist, especially sulfur-smelling gas, a 3-gas test that also measures hydrogen sulfide may give your doctor more to work with. That is a conversation to have with a clinician, not a self-diagnosis. The science here is moving quickly, and the most useful thing you can do meanwhile is keep a clear record of your symptoms and what seems to feed them.

ISO is real but early. The biology of gut hydrogen sulfide is well established; the clinical pattern, its threshold, and its options are still being worked out. That makes an honest record of your own symptoms, and a good clinician, more valuable than any single number on a printout.