Farts form when gas builds up in your digestive tract and gets pushed out through the rectum. That gas comes from two main sources: air you swallow and gas produced by bacteria in your large intestine as they break down food your body couldn’t digest on its own. A healthy person passes gas about 10 times a day on average, with an upper limit of normal around 20 times daily.
The Two Sources of Intestinal Gas
Every time you eat, drink, or swallow saliva, small amounts of air travel down into your stomach and intestines. This swallowed air is mostly nitrogen and oxygen, the same gases in the atmosphere around you. Some of it gets absorbed into the bloodstream or burped back up, but whatever makes it past the stomach continues through your digestive tract and eventually exits as flatulence.
The second, more significant source is bacterial fermentation in your colon. Your small intestine absorbs most nutrients from food, but certain carbohydrates, fibers, and starches resist digestion. They pass intact into the large intestine, where trillions of bacteria go to work breaking them down. This fermentation process releases gases as a byproduct, much like the fermentation that makes bread rise or beer fizz. The volume of gas you produce depends directly on how much undigested material reaches your colon and which bacteria are living there.
What’s Actually in a Fart
Flatulence is overwhelmingly made up of odorless gases. A meta-analysis of intestinal gas composition found the average breakdown is roughly 65% nitrogen, 14% methane, 10% carbon dioxide, 3% hydrogen, and 2% oxygen. None of these have a smell. The total daily volume ranges from about 500 to 1,500 milliliters, with a typical amount around 700 ml, or roughly three cups of gas spread across the day.
The smell comes from sulfur-containing gases that make up a tiny fraction of the total. Hydrogen sulfide is the primary culprit, the same compound responsible for the rotten-egg smell. It’s present at concentrations thousands of times smaller than the main gases, yet your nose is extraordinarily sensitive to it. Two other sulfur compounds, methanethiol and dimethyl sulfide, contribute to the odor as well. Research has confirmed that the intensity of the smell correlates directly with hydrogen sulfide concentration.
Why Beans and Certain Foods Cause More Gas
Beans are famous for causing flatulence for a specific biological reason. They contain complex sugars called raffinose and stachyose that humans simply cannot digest. Your small intestine lacks the enzyme needed to break these sugars apart. So they arrive in the colon fully intact, where bacteria ferment them enthusiastically and release gas in the process. Multiple species of gut bacteria can break down raffinose, and several of them produce gas as they do it.
Beans aren’t the only culprits. Other high-gas foods share a similar pattern: they contain carbohydrates or fibers that resist digestion in the small intestine.
- Cruciferous vegetables like broccoli, cabbage, and Brussels sprouts contain raffinose and sulfur compounds, contributing both volume and odor.
- Dairy products cause gas in people who don’t produce enough lactase to digest lactose. The undigested milk sugar ferments in the colon.
- Whole grains and legumes are rich in soluble fiber that feeds gas-producing bacteria.
- Onions and garlic contain fructans, another type of carbohydrate that often escapes digestion.
The sulfur-rich foods on that list, particularly cruciferous vegetables, onions, garlic, and eggs, tend to produce smellier gas because they provide the raw materials bacteria need to generate hydrogen sulfide.
Why Some People Produce Methane and Others Don’t
Not everyone’s gas contains methane. Whether you produce it depends on whether a specific microorganism called Methanobrevibacter smithii lives in your gut. This isn’t technically a bacterium but an archaeon, a different type of single-celled organism that thrives in low-oxygen environments. It can make up to 10% of all anaerobic organisms in your intestines when it’s present.
Studies estimate that somewhere between 33% and 70% of adults are methane producers. The variation is wide because gut microbiome composition differs based on diet, geography, and individual biology. If you’re not a methane producer, the hydrogen generated during fermentation gets consumed by other microbes or simply passed as hydrogen gas instead.
How Gas Moves Through Your Body
Once gas forms in the colon, it doesn’t just sit there. Your intestines are constantly contracting in rhythmic waves called peristalsis, the same muscle movements that push food through your digestive tract. These contractions move gas toward the rectum. When enough gas accumulates and pressure builds, the internal anal sphincter relaxes. Your external sphincter, the one under voluntary control, is what allows you to hold it in or let it pass.
The sound comes from vibration of the anal opening as gas passes through. Pitch and volume depend on the speed of the gas, the tightness of the sphincter muscles, and the volume being released. A large volume of gas released quickly through a tight opening produces a louder sound, while a slow, relaxed release is quieter. The composition of the gas itself doesn’t affect the noise.
What Increases Gas Production
Beyond food choices, several everyday habits influence how much gas your body produces. Eating quickly, chewing gum, drinking carbonated beverages, and smoking all increase the amount of air you swallow. Carbonated drinks add carbon dioxide directly into your stomach, some of which travels through the intestines rather than being burped out.
Sudden changes in fiber intake are one of the most common triggers for increased flatulence. If you switch to a high-fiber diet overnight, your colon bacteria suddenly have a surge of fermentable material to work with. This typically settles down over a few weeks as your gut microbiome adjusts to the new diet. Gradually increasing fiber gives bacteria time to adapt and reduces the temporary spike in gas production.
Stress and anxiety can also play a role. When you’re anxious, you tend to swallow more air, and stress hormones can speed up or slow down gut motility, changing how gas moves through your system. Antibiotics can temporarily alter gas production too, since they disrupt the bacterial populations responsible for fermentation.