Flatulence is the medical term for passing gas through the rectum, commonly called “farting” or “passing wind.” It’s a completely normal part of digestion. Healthy adults pass gas up to 25 times a day, though most people barely notice the majority of those episodes.
How Your Body Produces Gas
Gas in your digestive system comes from two main sources: swallowed air and bacterial fermentation in your colon.
Every time you eat, drink, or swallow saliva, small amounts of air travel down into your digestive tract. Certain habits increase how much air you swallow: eating too fast, talking while eating, chewing gum, sucking on hard candy, drinking through straws, smoking, and drinking carbonated beverages. Some of this air comes back up as a burp, but the rest continues through the intestines and eventually exits as flatulence.
The second, larger source of gas is your gut bacteria. When you eat carbohydrates that your small intestine can’t fully digest, they travel to the colon, where trillions of bacteria break them down through fermentation. This process produces hydrogen, carbon dioxide, and sometimes methane as byproducts. These gases accumulate and need somewhere to go.
What Flatus Is Actually Made Of
The gas you pass is mostly odorless. On average, about 65% is nitrogen, 10% is carbon dioxide, and roughly 14% is methane. Hydrogen and oxygen make up smaller portions. None of these gases have a smell.
The odor comes from trace sulfur-containing compounds, primarily hydrogen sulfide (the “rotten egg” smell). Two other sulfur compounds, methanethiol and dimethyl sulfide, also contribute. Research has shown a strong correlation between hydrogen sulfide concentration and how bad flatulence smells. These sulfur gases make up a tiny fraction of the total volume but have an outsized effect on the nose.
Why Some Foods Cause More Gas
Beans have their reputation for a reason. Legumes like soybeans, lentils, and chickpeas are packed with a group of carbohydrates called raffinose family oligosaccharides. Your body lacks the enzyme to break these down in the small intestine, so they arrive in the colon fully intact, where bacteria ferment them and produce hydrogen, methane, and carbon dioxide. Other foods in this category include cruciferous vegetables (broccoli, cabbage, Brussels sprouts), whole grains, and onions.
Dairy products cause gas in people who don’t produce enough of the enzyme that breaks down lactose, the sugar in milk. Fructose, the sugar found naturally in fruits and added to many processed foods, can also trigger gas in people who have difficulty absorbing it. In both cases, the undigested sugar reaches the colon and feeds bacteria that produce gas as a byproduct.
High-fiber foods in general tend to increase flatulence, especially when you add them to your diet quickly. Your gut bacteria adjust over time, so gradually increasing fiber intake usually reduces the gassiness after a few weeks.
When Gas Signals Something Else
Flatulence on its own is rarely a sign of a medical problem. But a sudden change in your gas patterns, especially paired with other symptoms, can point to an underlying condition. Abdominal pain, persistent bloating, diarrhea, constipation, or unexplained weight loss alongside increased gas are worth paying attention to.
Several conditions can cause excessive gas. Irritable bowel syndrome (IBS) often involves bloating and changes in bowel habits that go hand in hand with increased flatulence. Celiac disease, where the body reacts to gluten, can impair nutrient absorption and lead to more fermentation in the colon. Small intestinal bacterial overgrowth, a condition where too many bacteria colonize the small intestine, produces extra gas and can also cause diarrhea and weight loss. Lactose intolerance and fructose intolerance are among the more common and easily managed causes.
Reducing Flatulence
The most effective approach starts with identifying which foods trigger your gas and moderating your intake of them. Keeping a simple food diary for a week or two can reveal patterns you might not notice otherwise. Eating more slowly and avoiding gum, straws, and carbonated drinks cuts down on swallowed air.
Over-the-counter options can help in specific situations. Simethicone, the active ingredient in many anti-gas products, works by reducing the surface tension of gas bubbles in your digestive tract. This causes small bubbles to merge into larger ones that are easier for your body to expel, either as a burp or as flatulence. It doesn’t reduce the total amount of gas your body produces, but it can relieve the bloating and discomfort that comes from many small trapped bubbles.
For bean-heavy meals, enzyme supplements containing alpha-galactosidase can help. These provide the enzyme your body lacks to break down raffinose and related sugars before they reach the colon, reducing the amount of material available for bacteria to ferment. Taking them with your first bite of food gives the best results.
Probiotics may help some people by shifting the balance of gut bacteria toward species that produce less gas, though results vary widely from person to person. Regular physical activity also helps gas move through the digestive tract more efficiently, reducing the sensation of bloating even if the total gas volume stays the same.