Think of the gut as keeping a ledger of gas, with gas coming in, gas being used up and gas going out. Gas has two main sources, air that is swallowed and gas made when bacteria in the large intestine break down food, and the bacteria are the bigger producers of what leaves through the bottom. Tiny helpful bacteria live in the large intestine, and they break down food your body could not digest. They ferment it, which means they break down food that was not digested earlier and give off gas as a byproduct. The books do not balance simply, though, because gas made by gut bacteria leaves by one of three routes: other microbes use it up and turn it into non-gas products, it is absorbed into the blood and breathed out, or it comes out the bottom as a fart. This is part of why a person passes less gas than the bacteria make.

The raw material for all that fermenting is carbohydrate, the sugars, starches and fiber in food. Some carbohydrates are not digested or absorbed in the small intestine, so they move on to the large intestine, where bacteria break them down and make gas. Most foods with carbohydrates can cause gas, but fats and proteins cause little gas. Foods differ a great deal in how much of their carbohydrate goes unabsorbed: about 20 percent in baked beans, 7 to 10 percent in wheat, oats, potatoes and corn, and only 1 percent in white rice. The unabsorbed part is what bacteria get to ferment. So a much bigger share of the carbohydrate in baked beans reaches the bacteria than the share in wheat, oats, potatoes or corn, and a far bigger share than the 1 percent in white rice. Most starches produce gas as they are broken down in the large intestine, and rice is the one starch that does not.

Beans and other legumes are full of sugars called oligosaccharides that people cannot digest, and they have a high share of carbohydrates that people cannot digest, which is why they are so linked with gas. In a test, flour from soybeans bred to have less of these sugars produced less gas than flour from ordinary soybeans. Raffinose is one of these complex sugars, and it is found not only in beans but in cabbage, Brussels sprouts, broccoli, asparagus, other vegetables and whole grains. Foods with fiber, like beans and cabbage, make more gas because the body cannot fully digest fiber, so bacteria in the large intestine do the job. Almost anyone who eats large amounts of vegetables or fruits gets some extra gas.

Fiber is not one thing. Soluble fiber dissolves in water and becomes soft and gel-like, and it is found in oat bran, beans, peas and most fruits. Insoluble fiber, such as the fiber in wheat bran, passes through the intestines almost unchanged and makes little gas. How much fiber matters shows up in one study, in which a diet with no fiber cut the gas a person passed in a day from about 700 milliliters to about 200 milliliters. That is a drop to less than a third of the starting amount from a single change in what was eaten. Other sugars count too: milk sugar is called lactose, and fruit sugar is called fructose, and both can cause gas in some people. Certain carbohydrates called FODMAPs are poorly absorbed and rapidly fermented by bacteria in the intestines, which makes more gas.

Time is another variable. The longer food stays in the large intestine, the more time bacteria have to ferment it, so more gas can build up. Fat slows digestion, which gives food more time to ferment in the intestines. Sugar alcohols such as sorbitol, mannitol and xylitol, used in some sugar-free candies and gum, are not digested well by people, so they can make gas as well. People also swallow extra air when they eat or drink too fast, talk while eating, chew gum, suck on hard candies or drink carbonated drinks. Swallowing air and giving bacteria more time to ferment are two separate ways a person ends up with more gas.

Even with the same food, people differ, and one reason is the microbes themselves. The large intestine holds a large number of bacteria, plus some fungi and viruses, that together make up the gut microbiome. People can differ in how much gas they make partly because they have more or different bacteria living in their digestive tract. About one in three people make methane gas in the large intestine, along with hydrogen and carbon dioxide. There is great variability in how much and how often people pass gas, and what counts as a normal amount of farting is different for every person. Some people pass gas more than others, and that is normal.

Gas is only one thing the bacteria make. When bacteria ferment food, they also release short-chain fatty acids and many other compounds, and some of the gases are by-products or in-between steps of that chemistry. Bacteria called sulfate reducers make hydrogen sulfide in tiny amounts, and the sulfate they use comes mostly from sulfur-containing amino acids, which are the building blocks of protein. Hydrogen sulfide is the waste of microbes that help digest what the body cannot, and it is one reason gas can smell like rotten eggs. Smelly gas also has other causes: compounds made when meat is digested, and whether stool is in the rectum at the time. Gases that smell make up less than 1 percent of the gas in the gut. The other gases, which make up the rest, have no odor at all.

Which foods turn up on lists of smelly gas? Foods high in sulfur, like proteins and cruciferous vegetables such as cauliflower, cabbage and broccoli, are usually behind foul-smelling gas. Asparagus, beans, cabbage, coffee, dairy products, eggs, onions and prunes are foods linked to smelly gas. Some foods, such as cabbage and meat, can make gas smell stronger. The smell tracks one gas: how strong a fart smells goes along with how much hydrogen sulfide is in it, a gas that contains sulfur. Gas can also burn, because the gas made in the large intestine has hydrogen and methane in it.

How do scientists get numbers for something so private? In one study of how the colon works, they collected every fart from 16 people over 4 hours and ran each one through a gas chromatograph, a machine that sorts a gas mix into its parts and measures each. The mix of gases varied a great deal from one passage to the next in the same person, and varied enormously from person to person. That is why no single recipe describes a typical fart. Other labs count how often people pass gas and test what it contains, and a mix rich in nitrogen points to swallowed air, while more carbon dioxide, hydrogen and methane point to bacteria working on food. A scientist who studies flatulence is called a flatologist.

Here are the totals. On average people pass about 600 to 700 milliliters of gas a day, which sits inside the usual range of half a liter to one and a half liters. As of November 2026, studies suggest people pass gas an average of 8 to 14 times a day, and one hospital says 13 to 21, and experts consider passing gas up to 25 times a day to be normal. There is great variability in how much and how often people pass gas, so a normal day for one person is different from a normal day for another. Passing gas is rarely a sign of a medical issue, and people pass gas more often around mealtimes and just after eating.

Words to know

• ferment: break food down
• oligosaccharides: hard-to-digest sugars
• soluble: dissolves in water
• microbiome: gut microbe community
• flatologist: gas scientist