Farting comes from two sources: air you swallow and gas produced by bacteria in your large intestine. A healthy person passes gas up to 25 times a day, releasing anywhere from 200 to 2,000 milliliters of it. Most of that gas is completely odorless, and the process is a normal byproduct of digestion.
Swallowed Air
Every time you eat, drink, or swallow saliva, a small amount of air travels down into your digestive tract. Most of it gets burped back up, but some passes through to your intestines and eventually exits the other end. This air is mostly nitrogen and oxygen, the same gases in the room around you, so it doesn’t smell.
Certain habits increase the amount of air you swallow. Eating too fast, talking while you eat, chewing gum, sucking on hard candy, drinking through a straw, smoking, and drinking carbonated beverages all push extra air into your gut. If you’ve ever felt unusually bloated after gulping down a soda, the carbonation released carbon dioxide directly into your stomach on top of the air you swallowed while drinking.
Bacterial Fermentation in Your Gut
The bigger source of gas is your colon. Your small intestine absorbs most nutrients from food, but some carbohydrates pass through undigested because your body simply doesn’t make the enzymes to break them down. The human genome codes for fewer than 20 enzymes capable of handling complex carbohydrates. That leaves a lot of work for the trillions of bacteria living in your large intestine.
When those bacteria feast on undigested carbohydrates, they produce hydrogen, carbon dioxide, and in about a third of people, methane. Together with swallowed nitrogen and oxygen, these five gases make up more than 99% of what comes out. None of them have any smell. The volume of gas you produce depends largely on what you eat and which bacterial species dominate your gut, which varies significantly from person to person.
Foods That Produce the Most Gas
The biggest gas-producing foods contain short-chain carbohydrates that resist digestion in the small intestine. Researchers group these under the acronym FODMAPs, and they show up in a wide range of everyday foods:
- Fructans and galacto-oligosaccharides: found in wheat, rye, onions, garlic, and legumes like beans and lentils. These are among the most potent gas producers because bacteria ferment them readily.
- Lactose: the sugar in milk, soft cheeses, and yogurt. If you lack enough of the enzyme that breaks it down, it ferments in the colon instead.
- Fructose: found in honey, apples, and high-fructose corn syrup. When you consume more fructose than your small intestine can absorb, the excess feeds gut bacteria.
- Polyols: sorbitol and mannitol, which occur naturally in some fruits and vegetables and are added to sugar-free products as sweeteners.
Fiber-rich foods like broccoli, cabbage, and whole grains are healthy but also famously gassy for the same reason. Your enzymes can’t fully break down the complex carbohydrates they contain, so your gut bacteria do the job instead, producing gas as a byproduct.
Why Sugar-Free Products Cause Gas
Sugar alcohols like sorbitol, xylitol, mannitol, and erythritol are used to sweeten “sugar-free” gum, candy, protein bars, and other processed foods. Your body can’t fully digest them, and the unabsorbed portion ferments in your colon. Symptoms tend to hit fast.
Research comparing sugar alcohols found that xylitol caused bloating, gas, upset stomach, and diarrhea, while erythritol was milder, mainly increasing nausea and gas at large doses. The FDA requires products containing added sorbitol or mannitol to carry a warning that “excessive consumption can cause a laxative effect.” Studies suggest 10 to 15 grams a day is generally safe, but many processed foods contain well above that in a single serving.
Why Some Farts Smell Terrible
The overwhelming majority of gas you pass has no odor at all. The smell comes from trace amounts of sulfur-containing compounds, particularly hydrogen sulfide, the same gas responsible for the rotten-egg smell. It makes up less than 1% of flatus but is potent enough that even tiny quantities are noticeable.
Certain gut bacteria produce hydrogen sulfide by breaking down sulfur-containing amino acids found in protein-rich foods like eggs, meat, cheese, and cruciferous vegetables such as broccoli and cauliflower. Another group of bacteria, mainly from the Desulfovibrio genus, generates hydrogen sulfide by processing sulfate, a compound found naturally in various foods and drinking water. So a meal heavy in protein or cruciferous vegetables tends to produce gas that smells far worse than one built around plain starches.
Lactose Intolerance and Enzyme Gaps
Lactose intolerance is one of the most common reasons people experience more gas than they’d expect. The enzyme lactase, produced in the lining of the small intestine, normally breaks lactose into simpler sugars your body can absorb. When you don’t produce enough lactase, undigested lactose travels to the colon, where bacteria ferment it rapidly. Symptoms including gas, bloating, cramping, and diarrhea typically appear 30 minutes to 2 hours after eating dairy.
Lactose intolerance affects a large percentage of the global population, particularly people of East Asian, West African, and Southern European descent. The degree of intolerance varies. Some people can handle a splash of milk in coffee without issue but react to a full glass. Others react to even small amounts.
How to Reduce Gas
The simplest changes target swallowed air. Eating more slowly, chewing thoroughly, skipping straws, and cutting back on carbonated drinks can make a noticeable difference within days. Avoiding gum and hard candy removes another common source.
For gas caused by fermentation, the approach depends on the trigger. If beans and lentils are the culprit, an enzyme supplement containing alpha-galactosidase (sold as Beano and similar products) can help. It breaks down the complex carbohydrates in those foods before they reach your colon, reducing the raw material bacteria have to ferment. For bloating and the sensation of trapped gas, simethicone (the active ingredient in Gas-X) works differently. It doesn’t reduce gas production but helps small gas bubbles merge into larger ones that are easier to pass.
Keeping a food diary for a week or two can help you spot patterns. If dairy seems to be the problem, lactase enzyme tablets taken before meals can help. If sugar-free products are the issue, checking labels for sorbitol, xylitol, and mannitol and cutting back is often enough to bring relief.
When Gas Signals Something Else
Passing gas frequently is normal. But a sudden, persistent increase in gas, especially paired with other symptoms, can point to a digestive condition worth investigating. Red flags include bloody stools, unexplained weight loss, a lasting change in bowel habits (new constipation or diarrhea), ongoing nausea or vomiting, and prolonged abdominal or chest pain. These symptoms can indicate conditions ranging from food intolerances to inflammatory bowel disease or celiac disease, all of which involve impaired digestion that sends more unabsorbed material to the colon for bacterial fermentation.