Most of what leaves the human colon is odorless. Nitrogen, hydrogen, carbon dioxide and methane make up the bulk of intestinal gas, and none of them smell.
The stench of a bad fart comes from trace sulfur gases that account for a tiny fraction of the total volume. That finding traces back to a controlled experiment published in the journal Gut in 1998.
Researchers at the Minneapolis VA Medical Center collected flatus through a rectal tube from 16 healthy volunteers. To boost gas output, the participants first ate pinto beans and lactulose.
Two judges rated each sample for odor while the gas was analyzed for its chemical fingerprint.
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The three gases that carry the smell
Three sulfur compounds dominated the odorous fraction. Hydrogen sulfide was the most abundant at roughly 1.06 micromoles per liter, followed by methanethiol at 0.21 and dimethyl sulfide at 0.08.
Each has its own signature. Hydrogen sulfide smells like rotten eggs, methanethiol like decaying cabbage, and dimethyl sulfide has a sulfurous note often compared to overcooked vegetables.
In the researchers' words, "sulphur-containing gases are the major, but not the only, malodorous components of human flatus." The perceived stench tracked hydrogen sulfide concentration most closely, with a highly significant statistical correlation.
Why the same person's gas can smell different from one day to the next
The mix of these three gases shifts with what you have eaten in the past day or two. High-sulfur foods feed the bacteria that make sulfur gases in the colon.
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That list includes beans, eggs, garlic, onions, red meat, and cruciferous vegetables such as broccoli, cabbage and Brussels sprouts.
"As people often say, what goes in comes out," Christine Lee, MD, a gastroenterologist at the Cleveland Clinic, said in an article on why farts smell on the clinic's website. She points to dietary factors as the main driver of foul-smelling gas.
Cysteine, a sulfur-containing amino acid found in protein-heavy foods and many protein powders, is a favorite fuel for some of these microbes. Certain gut species generate hydrogen sulfide by breaking down cysteine and taurine for energy, according to a 2023 review published by the American Society for Microbiology.
A separate group of bacteria, in the Desulfovibrio genus, produces the same gas by pulling sulfate out of food and drinking water.
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Gut bacteria and transit time do the rest
Two people eating the same meal can still produce very different-smelling gas. The composition of the gut microbiome decides which bacterial species get to work on the leftover food.
A larger population of sulfate-reducing bacteria means more hydrogen sulfide from the same plate.
Transit speed matters too. When stool moves slowly through the colon, bacteria have more time to ferment residue and generate sulfur gases. Cleveland Clinic lists constipation as one of the everyday factors that can make gas smell worse.
When bad-smelling gas is more than diet
Foul gas that clears within a day or two is usually the food, not the body. Persistent changes are worth a check.
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Cleveland Clinic points to infections such as C. difficile, inflammatory bowel disease, colorectal cancer and bowel obstruction as medical causes of a lasting change in gas odor, and recommends seeing a doctor when the change does not resolve on its own.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.