Biology & Digestion

Why Do We Fart?

Farting is not only swallowed air escaping. Much of the gas comes from gut bacteria fermenting food your body did not fully digest.

The short answer

Farting is the colon releasing gas that has accumulated from two sources: swallowed air that was not released as burps, and gas produced by the trillions of bacteria in your large intestine as they ferment carbohydrates that your digestive enzymes could not break down. Humans produce between 500 and 1,500 milliliters of intestinal gas per day, released in 10 to 25 individual events on average. The gas is mostly odorless. About 99 percent of it is nitrogen, oxygen, carbon dioxide, hydrogen, and methane, none of which smell. The distinctive odor comes from the remaining fraction of sulfur-containing compounds, principally hydrogen sulfide and methanethiol, produced when bacteria ferment sulfur-containing amino acids and fibers. A high-protein or high-sulfur-vegetable diet dramatically increases odor because it provides more substrate for sulfur-producing bacteria.

Scientific illustration of the large intestine showing bacterial fermentation and gas production

Average production

A healthy adult produces 500 to 1,500 milliliters of intestinal gas per day and releases it in 10 to 25 episodes. People who think they have a gas problem often produce amounts within the normal range.

Mostly odorless

99 percent of flatus is odorless gas. Hydrogen sulfide and similar sulfur compounds make up less than 1 percent by volume but are detectable by the nose at concentrations of parts per billion.

Common myth

Beans are not the gassiest food category. Artificial sweeteners like sorbitol and xylitol, certain dairy products in lactose-intolerant individuals, and raw onions and garlic typically produce more intense gas than beans.

Holding it in

Suppressing flatulence does not eliminate the gas. It moves the gas backward through the colon and some is reabsorbed into the bloodstream and eventually exhaled through the lungs. The rest exits later.

Visual answer

Where Intestinal Gas Comes From and How It Is Released

The two sources of gut gas and the process from bacterial fermentation to flatulence.

Swallowed air reaches the colon

Air swallowed during eating and drinking that is not released as burps travels through the stomach and small intestine into the colon. Nitrogen and oxygen from swallowed air make up a significant fraction of total flatus.

Bacteria ferment undigested carbohydrates

Dietary fiber, resistant starch, and certain sugars that digestive enzymes cannot break down reach the colon intact. Resident bacteria ferment these, producing hydrogen, carbon dioxide, and in some people methane.

Sulfur-producing bacteria add the odor

A subset of bacteria produce hydrogen sulfide and methanethiol when metabolizing sulfur-containing compounds from proteins and certain vegetables. These trace gases are responsible for odor.

The external anal sphincter controls release

The internal anal sphincter relaxes when gas pressure builds. The external sphincter is under voluntary control and allows the sensation of gas to be felt and release to be timed socially.

Diagram of the digestive tract showing swallowed air, bacterial fermentation in the colon, gas composition, and anal sphincter release

Gut bacteria

Your Gut Bacteria Are Producing Gas as a Byproduct of Keeping You Healthy

The gas-producing fermentation that causes flatulence is not a digestive failure. It is the gut microbiome doing exactly what it is supposed to do. Fermenting dietary fiber produces short-chain fatty acids like butyrate, which are the primary fuel for colon cells and play important roles in colon health, immune function, and reducing inflammation. The gas is a byproduct of this beneficial process.

Individual gas production varies enormously based on microbiome composition. About 30 to 60 percent of people carry Methanobrevibacter smithii, an archaeon that converts hydrogen and CO2 into methane. People with this microbe produce methane, which adds to their gas volume and causes firmer, slower-moving stools. People without it produce different fermentation byproducts. This is why two people eating the same diet can have dramatically different gas profiles.

Lactase deficiency (lactose intolerance) is one of the clearest examples of diet-microbiome gas interaction. People who lack the enzyme lactase cannot break down lactose in the small intestine. It passes to the colon intact where bacteria ferment it rapidly, producing large volumes of hydrogen and CO2 along with diarrhea and bloating. It affects roughly 65 percent of the global adult population to some degree.

Causes of excess gas

Common Causes of Increased Flatulence

High-fiber diet

Dietary fiber reaches the colon undigested and is fermented by bacteria. Fiber is health-promoting, so increased gas from fiber intake is a healthy tradeoff.

Lactose intolerance

Undigested lactose ferments rapidly in the colon. Causing large amounts of hydrogen and CO2. Usually accompanied by bloating and urgent diarrhea.

Artificial sweeteners

Sorbitol, xylitol, and mannitol are poorly absorbed and heavily fermented by gut bacteria. A single pack of sugar-free gum can produce significant gas in sensitive individuals.

Swallowed air

Eating quickly, drinking through straws, carbonated beverages, and anxiety increase air intake. This air reaches the colon and exits as odorless nitrogen and oxygen.

Tiny note

Human flatulence is a measurable greenhouse gas contributor

Methane produced by gut bacteria in methane-producing individuals is a genuine greenhouse gas with a global warming potential roughly 25 times that of carbon dioxide over 100 years. While individual humans contribute far less methane than livestock, the global total from human flatulence is not trivial. Researchers studying gut methane production have measured methane excretion using sealed whole-body calorimeter chambers, producing data that has fed into global methane budget calculations.

Quick answers

Common questions

Why do some foods cause more gas than others?

Foods high in fermentable carbohydrates produce more bacterial gas. Beans contain oligosaccharides, onions contain fructans, dairy contains lactose. All are resistant to small intestine digestion and become bacterial fuel in the colon.

Is it bad to hold in gas?

Occasionally is fine. Regularly suppressing gas causes the same gas to be reabsorbed or move backward through the colon, sometimes causing bloating and discomfort. There is no evidence that holding gas causes long-term harm.

Why does fasting still produce gas?

Even without food, bacteria continue fermenting residual material in the colon and swallowed air still reaches the large intestine. Gas production slows during fasting but does not stop.

Why does gas sometimes come out silently?

The sound of flatulence depends on anal sphincter tension and gas velocity. Slow release at low pressure produces little or no sound. Rapid release with a tense sphincter produces more vibration and louder sound.

When does excessive gas indicate a health problem?

Gas accompanied by blood in the stool, unintentional weight loss, severe pain, or a significant change from your baseline pattern warrants medical evaluation. Conditions like celiac disease, Crohn's disease, and colorectal cancer can change gas patterns.