Farts happen because your digestive system produces gas in two main ways: you swallow air throughout the day, and bacteria in your large intestine create gas as they break down food. The average healthy adult passes gas about 10 times per day, with up to 20 times still falling within the normal range.
Two Sources of Intestinal Gas
The first source is simply swallowed air. Every time you eat, drink, or swallow saliva, a small amount of air enters your stomach. Most of it comes back up as a burp, but whatever doesn’t leave your stomach travels into your intestines and eventually exits the other end. Certain habits increase the amount of air you swallow: eating too fast, talking while eating, chewing gum, sucking on hard candy, using straws, drinking carbonated beverages, and smoking. Loose-fitting dentures can also cause you to gulp extra air throughout the day.
The second source, and the one responsible for most flatulence, is bacterial fermentation in your large intestine. Your stomach and small intestine can’t fully digest every type of carbohydrate you eat. Certain sugars, starches, and fibers pass through largely intact until they reach the colon, where trillions of bacteria go to work breaking them down. Gas is a byproduct of that process.
What’s Actually in a Fart
Flatulence is a mix of five gases in highly variable proportions. Nitrogen makes up anywhere from 11 to 92 percent, depending on how much air you’ve swallowed. Carbon dioxide ranges from 3 to 54 percent. Hydrogen can account for anywhere from 0 to 86 percent, and methane from 0 to 54 percent. Oxygen rounds things out at 0 to 11 percent. Those wide ranges explain why farts can feel and sound so different from person to person, and even from hour to hour.
None of those five gases have any smell. The odor comes from trace sulfur compounds produced when gut bacteria break down sulfur-containing foods like eggs, meat, cruciferous vegetables, garlic, and onions. A study published in the journal Gut identified hydrogen sulfide as the primary culprit, present at concentrations roughly five times higher than the next sulfur compound, methanethiol. The researchers found that odor intensity correlated directly with hydrogen sulfide levels. So a particularly smelly fart simply means your gut bacteria produced more sulfur compounds during digestion.
Why Some Foods Cause More Gas
The key factor is how much undigested carbohydrate reaches your colon. Foods high in certain short-chain carbohydrates, often grouped under the term FODMAPs, are especially potent gas producers. These include beans, lentils, onions, garlic, wheat, certain fruits like apples and pears, and dairy products (for people who don’t fully digest lactose). Broccoli, cabbage, and Brussels sprouts are also well-known offenders because they contain both fermentable carbohydrates and sulfur compounds.
Research comparing high and low FODMAP diets shows dramatic differences in gas production. In one study, healthy volunteers on a high FODMAP diet produced roughly four times the amount of hydrogen gas over a full day compared to a low FODMAP diet. Even among healthy people with no digestive disorders, the high FODMAP diet led to noticeably increased flatulence. For people with irritable bowel syndrome, the difference was even more pronounced, with the high FODMAP diet also triggering bloating, pain, and fatigue.
Fiber-rich foods deserve a special mention. Soluble fiber from oats, beans, and fruits is highly fermentable and tends to increase gas production. Insoluble fiber from whole grains and vegetable skins passes through more intact and generally produces less gas. If you’ve recently increased your fiber intake and noticed more flatulence, that’s a predictable response. Your gut bacteria are adjusting to the new supply of fermentable material, and gas production typically settles down after a few weeks.
Why Bacterial Fermentation Is Actually Useful
Gas is an inconvenient side effect of a process that’s genuinely good for you. When your gut bacteria ferment undigested carbohydrates, they don’t just produce gas. They also produce short-chain fatty acids, particularly one called butyrate, that serve as the primary fuel source for the cells lining your colon. These fatty acids help maintain the intestinal barrier, regulate inflammation, and support overall gut health. In other words, the same fermentation that makes you gassy also keeps your digestive tract functioning properly.
What Affects How Much Gas You Produce
Your individual gut microbiome plays a major role. Everyone hosts a slightly different community of bacteria, which is why the same meal can leave one person bloated and another feeling fine. People whose microbiome includes more methane-producing organisms, for example, tend to have different gas patterns than those whose bacteria primarily produce hydrogen.
Beyond diet and microbiome composition, several other factors influence gas production. Eating quickly means you swallow more air. Stress and anxiety can alter gut motility, trapping gas longer and making it feel more uncomfortable. Antibiotics temporarily disrupt your bacterial population, which can change fermentation patterns. Hormonal shifts during the menstrual cycle also affect gut motility, which is why some women notice increased bloating and gas at certain points in their cycle.
When Gas Signals Something Else
Flatulence on its own, even if it feels excessive, is rarely a sign of a medical problem. But a sudden, persistent increase in gas paired with other symptoms can point to an underlying condition. Celiac disease, irritable bowel syndrome, and bacterial overgrowth in the small intestine all cause increased gas production because they interfere with normal carbohydrate digestion, sending more undigested food to the colon for bacteria to ferment.
Symptoms worth paying attention to include bloody stools, unexplained weight loss, persistent diarrhea or constipation, ongoing nausea or vomiting, and gas pain severe enough to interfere with daily life. Prolonged abdominal pain or chest pain alongside gas warrants prompt medical attention, since chest pain in particular can mimic other serious conditions.