Humans fart because bacteria in the large intestine ferment food that your body couldn’t fully digest higher up in the gut. This fermentation produces gas as a byproduct, and that gas has to go somewhere. A healthy adult passes gas roughly 10 to 20 times a day, releasing a total volume between 476 and 1,491 milliliters, with a median around 705 ml.
What Actually Happens Inside Your Gut
Your small intestine handles most digestion, but it can’t break down everything. An estimated 20 to 60 grams of dietary carbohydrates escape digestion each day and arrive in the colon intact. The main culprits are resistant starches, plant cell wall fibers, and certain complex sugars your enzymes simply can’t touch.
Once these undigested carbohydrates reach the colon, trillions of anaerobic bacteria go to work on them. The bacteria ferment these leftovers, producing short-chain fatty acids (which your body actually uses for energy and gut health) along with gases: hydrogen, carbon dioxide, and sometimes methane. This is the same basic process that makes bread dough rise or beer fizz, just happening inside you. The gas accumulates until pressure moves it toward the exit.
What’s in a Fart
Five odorless gases make up 99% of flatulence: nitrogen, oxygen, carbon dioxide, hydrogen, and methane. The proportions vary wildly between people. Nitrogen can range from 11% to 92% of the mix, hydrogen from 0% to 86%, and methane from 0% to 54%. Much of the nitrogen and oxygen comes from swallowed air, while the hydrogen, carbon dioxide, and methane are produced by bacterial fermentation.
Methane production depends on having the right gut microbes, and only about half of all people harbor the slow-growing methane-producing organisms needed to generate it. Whether you’re a methane producer or not is mostly a matter of which bacteria colonized your gut early in life.
Why Some Farts Smell
The smell comes from trace sulfur compounds that make up less than 1% of the gas. The primary offender is hydrogen sulfide (the classic rotten-egg smell), present at concentrations averaging about 1.06 micromoles per liter. Methanethiol and dimethyl sulfide contribute smaller amounts. In a study published in the journal Gut, the intensity of the odor correlated directly with hydrogen sulfide concentration. More sulfur-containing foods in your diet, like eggs, meat, garlic, and cruciferous vegetables, means more raw material for bacteria to produce these compounds.
Why Certain Foods Cause More Gas
Beans have their reputation for a reason. They’re rich in a group of sugars called raffinose family oligosaccharides. Humans lack the enzyme (alpha-galactosidase) needed to break these sugars down, so they pass through the stomach and small intestine completely undigested. When colonic bacteria ferment them, they produce hydrogen, methane, and carbon dioxide in generous quantities.
The same principle applies to a broader category of poorly absorbed carbohydrates known as FODMAPs, found in a wide range of foods: wheat, onions, apples, stone fruits, milk, and anything sweetened with sugar alcohols like sorbitol or mannitol. These molecules do two things when they reach your lower gut. They pull extra water into the intestine through osmosis, and they ferment rapidly. The combination of water and gas distends the intestinal walls, which is why high-FODMAP meals can cause both flatulence and that uncomfortable bloated feeling.
Dairy is another common trigger. If you don’t produce enough lactase, the enzyme that breaks down milk sugar, lactose travels intact to your colon and ferments. This process generates enough hydrogen that doctors use it as a diagnostic tool: a breath test measuring hydrogen levels above 20 parts per million after drinking a lactose solution indicates lactose malabsorption.
Swallowed Air Plays a Role Too
Not all gas in your gut comes from fermentation. You swallow small amounts of air every time you eat, drink, chew gum, or talk. Carbonated beverages add carbon dioxide directly. Most swallowed air gets burped back up, but some makes it past the stomach and into the intestines, where it joins the fermentation gases on their way out. Eating quickly or drinking through a straw increases the amount of air you swallow.
When Gas Gets Trapped
Sometimes gas doesn’t move through smoothly. Your colon has several sharp bends, and the one near the spleen (called the splenic flexure) is particularly tight. Gas traveling from the right side of your colon to the left has to negotiate this curve, and when too much gas accumulates there, it can cause sharp pain in your upper left abdomen. This is sometimes called splenic flexure syndrome, and it can mimic heart or stomach problems. Changing positions, walking, or gently massaging the area usually helps the gas move along.
What Excessive Gas Can Signal
Flatulence by itself is normal, but when it comes with persistent bloating, cramping, or chronic watery diarrhea, it can point to something more specific. Small intestinal bacterial overgrowth (SIBO) is one possibility, a condition where bacteria that normally live in the colon migrate into the small intestine, where they ferment food prematurely. Diagnosis involves a breath test: a rise of more than 20 parts per million in hydrogen within 90 minutes, or methane levels above 10 parts per million, supports the diagnosis.
Food intolerances, particularly to lactose, fructose, or gluten, can also increase gas production substantially. The pattern matters more than the frequency. If you notice that gas is consistently accompanied by pain, changes in stool, or weight loss, those are signs worth investigating rather than just tolerating.
Why You Can’t (and Shouldn’t) Stop It
Gas production is a sign that your gut bacteria are doing their job. The same fermentation that produces flatulence also generates short-chain fatty acids that nourish the cells lining your colon and help regulate appetite hormones. A fiber-rich diet, which is consistently linked to lower rates of colon cancer and heart disease, will also produce more gas. The tradeoff is worth it.
You can reduce the volume by soaking beans before cooking, introducing high-fiber foods gradually, limiting carbonated drinks, and eating more slowly. But eliminating gas entirely would mean eliminating the microbial activity your gut depends on.