A fart forms when gas builds up in your large intestine, mostly from bacteria breaking down food your body couldn’t digest on its own. Some of it also comes from air you swallow throughout the day. The gas accumulates until pressure triggers its release through the rectum. The whole process is a normal part of digestion, and the average person passes gas about 15 times a day.
Where the Gas Actually Comes From
There are two main sources of intestinal gas. The first is swallowed air. Every time you eat, drink, chew gum, or even talk, small amounts of air travel down into your digestive tract. Most of this gets burped back up, but some makes it all the way through to your intestines. This swallowed air is mostly nitrogen and oxygen, which is why those two gases can make up a significant portion of what you eventually pass.
The second, and usually larger, source is fermentation in the colon. Your small intestine handles most digestion, but certain carbohydrates resist breakdown there because your body simply doesn’t produce the right enzymes to split them apart. When these undigested leftovers reach your large intestine, trillions of resident bacteria go to work on them. This bacterial fermentation produces hydrogen, carbon dioxide, and in some people, methane. It also generates short-chain fatty acids that your colon absorbs for energy, so the process isn’t purely about making gas. It’s a genuine part of how your body extracts nutrition from food.
Why Certain Foods Cause More Gas
Beans have their reputation for a reason. Legumes are packed with complex sugars called oligosaccharides, specifically raffinose and stachyose, that human enzymes cannot break down or absorb. Soybeans, for instance, contain roughly 14 to 41 grams of stachyose per kilogram of seed depending on the variety. All of that passes intact into the colon, where bacteria ferment it aggressively, producing a surge of hydrogen, carbon dioxide, and methane.
But beans aren’t the only culprits. Any food containing carbohydrates your body can’t fully digest will feed those colonic bacteria. Cruciferous vegetables like broccoli, cauliflower, and cabbage contain sulfur compounds alongside their fiber. Dairy products cause gas in people who lack the enzyme to break down lactose. Whole grains, onions, garlic, and many fruits contain fermentable fibers and sugars. Even artificial sweeteners like sorbitol and xylitol pass undigested into the colon. The common thread is always the same: if your small intestine can’t handle it, your colonic bacteria will, and gas is the byproduct.
What’s Actually in a Fart
Five odorless gases account for about 99% of every fart: nitrogen, oxygen, carbon dioxide, hydrogen, and methane. The proportions vary wildly from person to person. Nitrogen can range from 11% to 92% of the mix, hydrogen from nearly zero to 86%, carbon dioxide from 3% to 54%, and methane from zero to 54%. Not everyone even produces methane. Only about a third of adults have the specific type of microbe (called a methanogen) needed to generate it. Oxygen, carried in from swallowed air, generally stays below 11%.
This huge variability explains why gas differs so much between people, and even from one day to the next in the same person. What you ate, how much air you swallowed, and the specific mix of bacteria in your gut all shift the recipe.
Why Some Farts Smell Terrible
If 99% of the gas is odorless, the smell comes from the remaining 1% or less. The main offenders are sulfur-containing compounds produced when bacteria break down sulfur-rich foods like eggs, meat, garlic, and cruciferous vegetables. These compounds are extraordinarily potent. Sulfides as a group have some of the lowest detection thresholds of any volatile chemical, meaning your nose can pick them up at concentrations measured in parts per billion. A tiny trace is all it takes.
This is why a fart’s volume and its smell are completely independent. You can pass a large, loud fart that’s almost entirely odorless hydrogen and nitrogen, or a small, silent one loaded with sulfur compounds that clears a room. Diet is the biggest factor. A meal heavy in protein and sulfur-containing vegetables will produce far more pungent gas than one centered on plain starches.
How Your Body Decides to Release Gas
Gas doesn’t just leak out randomly. Your rectum and anal sphincter have a surprisingly sophisticated system for managing what leaves your body and when. As gas accumulates in the colon, muscle contractions push it toward the rectum. When it arrives, a reflex called the rectoanal inhibitory reflex briefly relaxes the internal sphincter, allowing the sensitive lining of the anal canal to “sample” what’s there. This is how your body distinguishes between gas and stool, letting you pass one without the other in most situations.
The external sphincter, the one you consciously control, gives you the ability to hold gas in or time its release. When you do let go, the gas passes through the anal opening, and the sound depends on the volume of gas, the speed of its exit, and the tightness of the sphincter. More tension and higher speed produce a higher-pitched sound. A relaxed sphincter lets gas pass quietly.
How Much Gas Is Normal
Healthy adults produce between 400 and 2,000 milliliters of intestinal gas per day, according to the British Society of Gastroenterology. That’s roughly one to five soda cans’ worth. Fifteen passages a day is the commonly cited average, but the normal range extends up to about 40 times daily without indicating a problem. People who eat high-fiber diets will naturally sit at the higher end.
Gas becomes a concern only when it’s accompanied by other symptoms: persistent bloating, cramping, changes in bowel habits, or unintentional weight loss. On its own, frequent farting usually just reflects a fiber-rich diet and a healthy, active population of gut bacteria doing exactly what they’re supposed to do.