A description vs a mechanism.
IBS (irritable bowel syndrome) is defined by symptoms — abdominal pain plus altered bowel habits — once other diseases are ruled out. It is a diagnosis of exclusion: it tells you what you feel, not why. SIBO (small intestinal bacterial overgrowth) is a specific state — bacteria overgrowing where the small intestine should stay low in density — that is testable and produces the same symptoms. So they sit on different layers: IBS describes, SIBO explains.
"IBS vs SIBO" is often the wrong frame. The better question is: is there a SIBO underneath this IBS? → What SIBO is
SIBO vs IBS, compared.
| Dimension | IBS | SIBO |
|---|---|---|
| What it is | A functional-symptom syndrome — a label, not a cause | A specific bacterial overgrowth in the small intestine — a mechanism |
| How it's diagnosed | By symptom criteria after excluding other disease (diagnosis of exclusion) | By a breath test for gases produced by overgrown bacteria |
| Identified cause | None by definition — "cause unknown" | Overgrowth, often downstream of diet, motility, or low stomach acid |
| Core symptoms | Bloating, abdominal pain, diarrhea/constipation | The same — plus often foul (sulfur) gas, bad breath, and whole-body symptoms |
| Testing | No confirmatory test | Breath test (though standard panels miss H₂S / methyl mercaptan) |
| Typical advice | More fiber, probiotics, fermented foods, low-FODMAP, antispasmodics | Lower fermentation substrate + targeted antibacterial treatment |
| Diet risk | Fiber/probiotic advice can feed an underlying overgrowth | Removing substrate starves it — the opposite direction |
| Relationship | A substantial share of "IBS" tests positive for SIBO — the labels overlap | |
Much of what's called IBS tests positive for SIBO.
Studies that breath-test people diagnosed with IBS find SIBO in a large fraction — reported anywhere from roughly a third to the majority, depending on the test method and criteria. Because IBS is diagnosed by ruling other things out rather than by finding a cause, an untested overgrowth can live underneath the IBS label indefinitely. The tell is often the gas: a genuinely foul, sulfur smell points toward overgrowth (VSC-producing bacteria) rather than a purely functional pattern.
The two labels point to opposite diets.
This is not a semantic argument — the treatments diverge. Standard IBS advice often means more fiber, probiotics, and fermented foods. If a SIBO is underneath, those are fuel: they can entrench the very bloating and gas they were meant to relieve. A SIBO-oriented approach goes the other way — lower the fermentation substrate and, where appropriate, reduce the overgrowth directly. Following the wrong map is why some "IBS" never improves.
If your "IBS" comes with foul gas and worsens on fiber/probiotics, read the SIBO guide, what the smell of gas means, and the diet that starves overgrowth.
Follow the full essay series.
The English-language writing on SIBO, VSCs, and the gut origins of "functional" symptoms lives on Substack.
Subscribe on Substack