You fart because bacteria in your large intestine produce gas as they break down food your body couldn’t fully digest higher up in the gut. Most people pass gas somewhere around 15 to 20 times per day, and the total volume can range from about 500 to 1,500 milliliters in a 24-hour period. It’s one of the most universal human experiences, and it happens because your digestive system is working exactly as designed.
What Actually Happens Inside Your Gut
Your small intestine handles most of the heavy lifting when it comes to digestion and nutrient absorption. But certain carbohydrates, fibers, and starches resist digestion there and pass intact into the large intestine. That’s where trillions of bacteria get to work, fermenting these leftovers into useful byproducts like short-chain fatty acids (which fuel the cells lining your colon) and, inevitably, gases.
The main gases produced are hydrogen, carbon dioxide, and in some people, methane. These three, along with swallowed nitrogen and small amounts of oxygen, make up the bulk of every fart. One study measuring 24-hour gas output in healthy volunteers found a median hydrogen volume of 361 milliliters per day, with carbon dioxide contributing around 68 milliliters. Only a few of the volunteers produced methane at all, which means the bacterial makeup of your gut determines part of your personal gas profile.
None of these major gases have any smell. The odor comes from trace sulfur compounds that represent a tiny fraction of the total volume.
Why Some Farts Smell Worse Than Others
The smell of flatulence comes almost entirely from three sulfur-containing compounds. Hydrogen sulfide is the dominant one, present at roughly five times the concentration of the next culprit, methanethiol. A third compound, dimethyl sulfide, contributes in smaller amounts. Research published in the journal Gut found that the intensity of flatulence odor correlated directly with hydrogen sulfide concentration.
Your gut bacteria produce these sulfur gases when they break down sulfur-containing amino acids from protein-rich foods like eggs, meat, and cruciferous vegetables such as broccoli and cabbage. So a high-protein meal or a big plate of Brussels sprouts doesn’t necessarily make you produce more gas overall, but it shifts the chemical composition toward smellier output.
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. Your body lacks the enzyme needed to break these sugars down in the small intestine, so they travel whole into the colon where bacteria ferment them into hydrogen, methane, and carbon dioxide. It’s not that beans are unusual in causing gas. It’s that they deliver an especially large dose of fuel for bacterial fermentation in a single serving.
The same principle applies to a broader category of carbohydrates known as FODMAPs, short for fermentable oligosaccharides, disaccharides, monosaccharides, and polyols. These are found in foods like onions, garlic, wheat, apples, pears, and sugar-free sweeteners. FODMAPs are poorly absorbed in the small intestine, which means they pull extra water into the gut through osmotic effects and then get rapidly fermented once they reach the colon. Studies show that eating FODMAPs increases both small bowel water and colonic gas production in healthy people, not just those with sensitive stomachs.
Fiber-rich foods in general, including whole grains, legumes, and many vegetables, tend to increase gas output simply because they provide more undigested material for your gut bacteria to work on. This is actually a sign of a healthy, well-fed microbiome, even if the social consequences are inconvenient.
Lactose Intolerance and Other Triggers
If dairy products reliably give you gas, bloating, or diarrhea, the explanation is straightforward. Lactose intolerance means your body doesn’t produce enough lactase, the enzyme that breaks down the sugar in milk. Without enough lactase, undigested lactose passes into the colon, where bacteria ferment it into carbon dioxide, methane, and hydrogen, along with liquids and fatty acids that can cause diarrhea. Roughly 65 to 70 percent of the global adult population has some degree of reduced lactase production after childhood.
Other food intolerances follow the same basic pattern. Fructose malabsorption, for instance, means excess fructose from fruit, honey, or high-fructose corn syrup reaches the colon undigested and gets fermented. Celiac disease and other conditions that damage the small intestine can also increase gas by allowing more nutrients to escape digestion and reach the colon.
Swallowed Air Plays a Role Too
Not all the gas in your digestive system comes from bacterial fermentation. You swallow small amounts of air every time you eat, drink, chew gum, or smoke. Most of this air, primarily nitrogen and oxygen, gets burped back up. But some of it travels through the digestive tract and exits the other end. Eating quickly, drinking carbonated beverages, or talking while you eat can increase the amount of air you swallow and contribute to more frequent gas.
How Much Is Normal
Passing gas up to 20 times a day falls within the normal range. The actual volume varies enormously between individuals, and day-to-day variation within the same person can be significant depending on what you ate. In one controlled study where healthy volunteers ate their normal diet plus 200 grams of baked beans, daily gas volumes ranged from 476 to 1,491 milliliters, with a median around 705 milliliters.
Excessive flatulence on its own is rarely a sign of a serious medical problem. But if increased gas comes with persistent pain, unintentional weight loss, chronic diarrhea, bloody stools, or ongoing nausea, something more may be going on. Small intestinal bacterial overgrowth, for example, occurs when bacteria that normally live in the colon colonize the small intestine, producing excess gas along with diarrhea and poor nutrient absorption. Conditions like irritable bowel syndrome can also make you more sensitive to normal amounts of intestinal gas, so that a volume that wouldn’t bother someone else causes real discomfort for you.
Reducing Gas Without Eliminating Healthy Foods
If you’re bothered by how much gas you produce, the most effective approach is identifying which specific foods trigger the most fermentation for you. Keeping a simple food diary for a week or two can reveal patterns you might not notice otherwise.
Introducing high-fiber foods gradually rather than all at once gives your gut bacteria time to adjust, which typically reduces gas over a few weeks. Soaking dried beans before cooking and discarding the soaking water removes some of the oligosaccharides responsible for fermentation. Eating more slowly and avoiding carbonated drinks cuts down on swallowed air.
For lactose intolerance, lactase enzyme supplements taken with dairy can reduce symptoms. A low-FODMAP diet, ideally guided by a dietitian, can help identify which fermentable carbohydrates cause the most trouble for your particular gut. The goal isn’t to eliminate all gas-producing foods permanently, since many of them are among the healthiest things you can eat. It’s to find the balance where your digestion works comfortably without sacrificing nutrition.