Farting comes from two sources: air you swallow and gas produced by bacteria in your large intestine. The average person passes gas about 15 times a day, though anywhere from a handful to 40 times falls within the normal range. Most of what comes out is completely odorless, and the smell, when it happens, traces back to a tiny fraction of sulfur-containing compounds.
The Two Ways Gas Gets Into Your Gut
Every time you chew, breathe, or talk, small amounts of air enter your stomach. Most of this gets burped back up, but some travels onward into the intestines and eventually exits as flatulence. Certain habits increase how much air you swallow: eating too fast, talking while eating, chewing gum, sucking on hard candy, using straws, drinking carbonated beverages, and smoking. This swallowed air is mostly nitrogen and oxygen, both odorless.
The second, more significant source is bacterial fermentation in your colon. Food components your body can’t digest on its own, particularly certain carbohydrates and fibers, pass through your stomach and small intestine intact. When they reach the colon, trillions of gut bacteria break them down for energy. Gas is the byproduct of that process. The more undigested material reaching your colon, the more gas your bacteria produce.
What Flatulence Is Actually Made Of
Five odorless gases account for 99% of every fart: nitrogen, oxygen, carbon dioxide, hydrogen, and methane. The proportions vary wildly from person to person. Nitrogen can make up anywhere from 11% to 92% of a single episode, hydrogen from 0% to 86%, and methane from 0% to 54%. Some people produce almost no methane at all, while others produce significant amounts, depending entirely on which species of bacteria dominate their gut.
The remaining 1% is what you actually smell. The main culprit is hydrogen sulfide, a colorless gas with a distinct rotten-egg odor. Certain gut bacteria generate it by breaking down sulfur-containing amino acids found in protein-rich foods like eggs, meat, and cruciferous vegetables. Other bacteria in the gut produce hydrogen sulfide by processing sulfate, a compound found naturally in various foods and drinking water.
Foods That Make You Gassier
The biggest gas producers are foods high in fermentable carbohydrates, sometimes grouped under the term FODMAPs. These are sugars and fibers that your small intestine either can’t break down or absorbs poorly, leaving them for colon bacteria to feast on. The main categories include:
- Legumes and pulses (beans, lentils, chickpeas), which are high in a sugar called GOS that humans lack the enzyme to digest
- Cruciferous vegetables (broccoli, cabbage, cauliflower, asparagus), rich in fructans and mannitol
- Whole grains, which contain fructans
- Certain fruits (apples, pears, watermelon), high in sorbitol or excess fructose
- Dairy products, for people who don’t produce enough of the enzyme that breaks down lactose
- Nuts, which contain GOS and fructans
Insoluble fiber deserves special mention. Found in whole grains, beans, and vegetables like broccoli and cabbage, it passes through your stomach completely undigested and heads straight to the large intestine. There, gut bacteria digest it through fermentation, producing gas as a direct byproduct. This is why adding more fiber to your diet often increases flatulence, at least temporarily, before your gut microbiome adjusts.
Sugar-Free Products and Sweeteners
Sugar alcohols like sorbitol, xylitol, and erythritol are common in sugar-free gum, mints, and candy. They’re poorly absorbed in the small intestine, which means they travel to the colon where bacteria ferment them, sometimes aggressively. For some people, even a small amount of sorbitol causes bloating, cramps, and excess gas.
Research from UC Davis found that the gut’s ability to handle sorbitol depends on having enough of a specific group of bacteria called Clostridia. Antibiotics and high-fat diets can reduce Clostridia populations by changing oxygen levels inside the gut, leaving sorbitol undigested and causing significant gastrointestinal symptoms. Sorbitol also occurs naturally in apricots, apples, pears, and avocados, so even “whole food” diets aren’t immune to this effect.
Lactose Intolerance and Other Intolerances
If your body doesn’t produce enough lactase, the enzyme that breaks down the sugar in milk, lactose passes through your small intestine undigested. When it reaches your colon, bacteria interact with it and produce gas, along with bloating, cramping, and sometimes diarrhea. Symptoms typically begin within a few hours of eating or drinking dairy. This is one of the most common causes of unexplained gassiness, since many people develop reduced lactase production gradually during adulthood without realizing it.
Fructose malabsorption works similarly. When your small intestine can’t fully absorb fructose, the excess reaches the colon and gets fermented. Foods and beverages sweetened with high-fructose corn syrup, as well as fruits like apples and pears, are common triggers.
When Excess Gas Signals Something Else
Occasionally, a noticeable increase in gas points to a condition worth investigating. Small intestinal bacterial overgrowth, or SIBO, occurs when bacteria that normally live in the colon proliferate in the small intestine. These bacteria start fermenting carbohydrates earlier in the digestive process, producing more gas than usual. SIBO shares many symptoms with irritable bowel syndrome (IBS), including bloating, abdominal pain, and changes in bowel habits. The key difference is that SIBO can be confirmed with a breath test that measures hydrogen and methane levels, and it responds to targeted treatment.
IBS itself is a functional disorder, diagnosed when symptoms persist but no structural or bacterial cause can be identified. People with IBS often have heightened sensitivity to normal amounts of intestinal gas, meaning the same volume of gas that wouldn’t bother someone else causes noticeable discomfort. Celiac disease, inflammatory bowel conditions, and even certain medications can also shift gas production upward by changing how food is digested or altering the bacterial landscape in the gut.
Why Some Farts Smell Worse Than Others
The smell comes down to sulfur. High-protein meals, especially those rich in eggs, red meat, garlic, onions, and cruciferous vegetables, give gut bacteria more sulfur-containing compounds to break down. Specific bacterial species, including members of the Desulfovibrio genus, are particularly efficient at producing hydrogen sulfide. People whose gut microbiomes contain more of these sulfur-metabolizing bacteria tend to produce smellier gas regardless of diet.
Volume and smell don’t necessarily go together. A large, fiber-fueled fart may be entirely odorless because the gases produced (hydrogen, carbon dioxide, methane) have no scent. A small one after a protein-heavy meal might clear a room because it’s concentrated with hydrogen sulfide. The composition of your gut bacteria, your recent meals, and how quickly food moves through your digestive tract all influence the ratio of odorless to odorous gas on any given day.