Everyone passes gas. But when the odor turns distinctly rotten-egg and stays that way, day after day, without an obvious dietary explanation, that persistent smell may be more than a social nuisance. New research on inflammatory bowel disease patients suggests the odor profile of flatulence can track with what is happening inside the gut.
A cross-sectional study published in December 2024 in the Journal of Clinical Medicine compared 233 patients with inflammatory bowel disease, or IBD, to 96 healthy controls.
The Hannover Medical School researchers, led by Lea Pueschel, found a clear tendency toward more malodorous flatulence and feces in the IBD group. The pattern also shifted with disease activity, with significant differences between patients in remission and those in an active flare.
The chemistry behind the smell
Most gas produced during digestion is odorless. The rotten-egg note comes from a small share of sulfur-containing compounds, chiefly hydrogen sulfide. As gastroenterologist Christine Lee of Cleveland Clinic explained in a 2022 patient guide, digestion can produce "hydrogen sulfide, which can add a rotten-egg odor to any air biscuit you cook up."
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Diet supplies the raw material. Sulfur-rich foods like eggs, garlic, onions, cruciferous vegetables and high-protein meals give gut microbes more building blocks for smelly compounds.
As gut microbiome researcher Maximilienne Toetie Allaart of the University of Tübingen wrote in The Conversation in April 2025, sulfur is typically found in the amino acids cysteine and methionine, which are parts of proteins. That is why protein-heavy meals often make gas smellier the next day.
When the microbes take over
The other side of the equation is which microbes are doing the work. Sulfate-reducing bacteria, particularly the Desulfovibrio genus, produce hydrogen sulfide as a metabolic end product. A 2019 review in the Journal of Clinical Medicine noted that Desulfovibrio is the dominant sulfate-reducing organism in the human intestine, and that overgrowth of these bacteria has been repeatedly detected in people with IBD, irritable bowel syndrome and colorectal cancer.
At high levels, hydrogen sulfide is not just smelly. The 2019 review described how it can damage the intestinal lining and interfere with the fuel supply of colon cells. That does not mean every whiff of sulfur is dangerous, but it helps explain why researchers keep circling back to the odor as a potential window on gut ecology.
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What to actually watch for
The 2024 Hannover study relied on patients' own reports of odor, not lab measurements, and the authors flagged that as a limit. Still, the pattern was consistent enough to matter. Malodor perception rose with disease activity and correlated with markers such as C-reactive protein and fecal calprotectin, both of which reflect inflammation.
For someone without an IBD diagnosis, the practical signal is a change. A persistent rotten-egg smell that appears without a shift in diet, especially if it comes with bloating, cramping, diarrhea or unintended weight loss, is worth mentioning to a doctor. Small intestinal bacterial overgrowth, lactose intolerance and inflammatory bowel disease can all produce that combination, and each is treatable once identified.
The unglamorous baseline
Most smelly gas is boring. A big steak dinner, a plate of broccoli, a lactose-heavy dessert or a course of antibiotics can all shift the odor profile for a few days without anything being wrong. Cleveland Clinic's guidance is straightforward. Pay attention when the smell is unusually foul and does not go away. If bad gas continues for an extended period, Dr. Lee advised, talk to your healthcare provider.
The gut microbiome is one of the harder systems in the body to monitor at home. Odor is a rough instrument, but the research suggests it is not a meaningless one.
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Sudden, persistent changes are the ones worth noticing, and other early warning signs that show up in unexpected places tend to work the same way. The body often flags trouble in mundane forms long before it gets loud.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.