Where Does Gas Come From - Excessive Gas: Digestive Disorders

Content provided by the Faculty of the Harvard Medical School

Where does gas come from?

There are only two ways for gas to originate. Either you swallow it (aerophagia), or it's manufactured in the gut (producing flatus).

Aerophagia

Aerophagia — excessive swallowing of air — produces belching or burping. The upper gastrointestinal gas that erupts from the mouth comes from swallowed air that forces itself back up. There are actually two kinds of belching. One is a fairly innocuous condition that stops on its own about an hour after eating and is caused by air swallowed while eating. The other is more persistent and is caused by constant air swallowing, a nervous habit most people don't even know they have.

Technically called eructation, belching occurs when the upper esophageal sphincter relaxes and lets the pressurized gas that has been swallowed escape from the mouth. A study has found that healthy young people belch an average of 11 times in 20 hours, excluding mealtimes.

Flatus

Also known as flatulence, this term describes gas that escapes from the rectum. The gas is mostly the by-product of the fermentation of undigested food by bacteria in the colon. It contains carbon dioxide, hydrogen, and, in some people, methane. Tiny amounts of volatile chemicals produced by bacterial metabolism of residual fats and proteins are responsible for the distinctive foul odor of flatus.

Gas that escapes from the far end of the GI tract may be natural, but nowhere is it an accepted part of society. The average human intestine holds 100–200 milliliters of gas, but researchers have found that in 24 hours, production of flatus averages 2 liters. This gas originates in the intestine, and its quantity and composition depend largely on the foods you eat. Studies using hydrogen breath testing have found that up to one-fifth of the complex carbohydrates eaten by average, healthy individuals is turned into gas.

Last updated:	August 21, 2007