What Are Farts Made Of? (And Who Actually Makes Them?)

Mostly not you. Most of the gas in a fart is made by trillions of microbes in your large intestine, which eat what your own body cannot digest and give off gas as their exhaust. So what are farts made of? Mainly hydrogen, nitrogen and carbon dioxide and, in some people, methane, plus tiny traces of sulfur gases that supply the smell.

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Swallowed air, a busy stomach, gassy beans: why the usual answers miss

Ask around and you will hear three explanations. You swallowed too much air. Your stomach is working hard. Beans are just gassy. Each has a crumb of truth, but none of them answers the real question: who made that?

Swallowed air exists, but a standard medical reference says almost none of the gas in flatus comes from it. Beans do not arrive with gas inside them; they carry sugars your own enzymes cannot split, more on that below. And the stomach is not the factory. About 75% of flatus comes from bacterial fermentation in the colon. The whole digestive tract holds less than 200 mL of gas at any moment, yet people pass gas an average of 10 to 20 times a day.

What are farts made of? Lessons from a day of collected gas

In 1991, researchers in Sheffield, England, had ten healthy volunteers collect every bit of their gas for 24 hours. The volunteers ate their normal food plus 200 grams of baked beans. Daily totals ranged from 476 to 1,491 mL, with a median of 705 mL, and the biggest single ingredient was hydrogen.

Then the volunteers spent 48 hours on a liquid formula diet with no fiber. Hydrogen production was almost completely eliminated, and the daily total dropped to a median of 214 mL. Same bodies, same stomachs. The only thing that changed was what reached the far end of the gut.

The gas did not drop to zero. About 200 mL a day remained, which the researchers presumed was mostly nitrogen. Nitrogen is not made in the gut. Most of the nitrogen there diffuses in from the blood, which picked it up from the air you breathe. So some of a fart really is you. The hydrogen, the carbon dioxide that travels with it and the methane have a different author.

How much gas people make varies a lot from person to person. Diet accounts for much of the variation, and gut microbes and the speed of the gut may also play a part.

Why do farts smell when 99% of the gas has no smell?

About 99% of a fart is odorless gas, such as nitrogen, carbon dioxide, hydrogen and methane. The smell comes from trace amounts of other compounds.

In 1998, a team that included Minneapolis gastroenterologist Michael Levitt, who studied intestinal gas for decades, asked 16 healthy volunteers to eat pinto beans and take lactulose, a sugar that increases gas. Two judges rated the odor of each sample. Hydrogen sulfide, the rotten-egg gas, was the main sulfur gas, followed by methanethiol and dimethyl sulfide, and the more hydrogen sulfide a sample held, the worse the judges rated it.

The same study tested a charcoal-lined cushion meant to trap the smell. Eight volunteers wore gas-tight Mylar pants while sulfur gases were released at the anus, and the cushion adsorbed more than 90% of them.

Who is cooking down there? Thirty-eight trillion roommates

For decades, a popular claim said bacteria in your body outnumber your own cells ten to one. That figure traces back to a single back-of-the-envelope estimate from the 1970s. In 2016, a team including Ron Milo at the Weizmann Institute of Science in Israel recounted. For a 70 kg "reference man" aged 20 to 30, they estimated about 38 trillion bacteria and about 30 trillion human cells, a ratio of roughly 1.3 to 1. Nearly all the bacteria live in the colon, and together they weigh about 0.2 kg.

The same team estimates that each bowel movement removes about a third of the colon's bacteria, and the authors point out that the two counts are close enough for each one to flip the ratio in favor of human cells for a while. That is an estimate, not a measurement.

Why do your roommates need to be there at all? Beans are rich in sugars called raffinose and stachyose. Humans lack the enzyme that breaks the bond holding them together, so they pass untouched through the small intestine, which holds far fewer microbes than the colon. In the colon, bacteria ferment them and release hydrogen and carbon dioxide, and in some people, methane. Hydrogen and methane are both flammable, so please do not test that.

The methane has a twist. It is not made by bacteria but by archaea, a separate domain of life. The most common one in the human gut is Methanobrevibacter smithii, which uses hydrogen to turn carbon dioxide into methane. Not everyone carries a big crowd of them. In the Sheffield study, only 3 of the 10 volunteers produced any methane.

Exhaust, fuel, and what happens when the roommates go hungry

Some of the gas never leaves the way you would expect. Hydrogen made by gut microbes can pass through the gut wall into the blood, travel to the lungs and be breathed out. Doctors use this in the hydrogen breath test: you drink a sugar such as lactose or lactulose and breathe into a device, and a rise in breath hydrogen means gut bacteria fermented that sugar.

Gas is only the exhaust. When microbes ferment fiber they also make short-chain fatty acids, including butyrate. In a 1980 experiment on isolated human colon cells, butyrate alone accounted for 73% to 75% of their oxygen use, a rough stand-in for energy use. The author's hypothesis was that fatty acids from anaerobic bacteria are a major energy source for the lining of the colon.

So what if the roommates are not fed? In 2016, a team including Eric Martens at the University of Michigan tested it in mice raised with no microbes and then given 14 kinds of bacteria that normally live in human guts. When the mice ate no fiber, even for a few days, some microbes began to eat the colon's mucus layer, the layer got thinner, and the mice got sicker when infected with a gut germ. Martens summed it up: "if you don't feed them, they can eat you." That work was in mice, and whether the same happens in people is still an open question.

None of this is a diet recommendation, and it is general information, not medical advice. If something hurts, keeps changing or just feels wrong, that is a conversation for a doctor.

The twist: a baby's first meal is for the roommates

After fat and lactose, the third most abundant solid component of human milk is a family of sugars called human milk oligosaccharides. An infant cannot digest them. They reach the intestines intact, where they have several roles, including feeding bacteria such as Bifidobacterium, the main but not the only group with genes to break them down.

So from your first meal, you were eating for two, or really for about 38 trillion. A fart is not a malfunction. It is a receipt showing that somebody down there got fed. The video above has the elevator scene and the rest of the story, and it ends with an answer for the next time someone asks who made that: we did.