Everybody produces gas, and everybody needs to pass gas. The amount depends on the individual, and there is a wide range of "normal." Passing gas is normal; nevertheless, it can be embarrassing or cause discomfort. A better understanding of what causes intestinal gas can help most people reduce symptoms and find some relief.

Sources of Intestinal Gas

Gas in the digestive tract (the esophagus, stomach, small intestine, and large intestine) comes from two sources:

  • swallowed air and
  • the normal breakdown of certain undigested foods by harmless bacteria that are naturally present in the large intestine.

Swallowed Air – Air swallowing (aerophagia) is a common cause of gas in the stomach. Everyone swallows small amounts of air when eating and drinking. However, eating or drinking rapidly, talking while eating, chewing gum, smoking, or wearing loose dentures can cause some people to take in more air.

Burping, or belching, is the way most swallowed air leaves the stomach. The remaining gas moves into the small intestine where it is partially absorbed. A small amount travels into the large intestine for release through the rectum. (The stomach also releases carbon dioxide when stomach acid and bicarbonate mix, but most of this gas is absorbed into the bloodstream and does not enter the large intestine.)

Bacteria – Gases are produced as a by-product when certain food materials are digested by naturally occurring bacteria in the large intestine, or colon. These bacteria are responsible for digesting materials like complex carbohydrates (sugar, starches, and fiber found in many foods) and cellulose, which are not normally digested in the upper gastrointestinal tract.

The quantity and mixture of gases depend on the types of bacteria in the colon; everyone has a unique assortment of bacteria from the time of birth. These gases include hydrogen, carbon dioxide, and, in some people methane. Trace gases, such as hydrogen sulfide, are responsible for the odor. Foods that produce gas in one person may not cause gas in another.

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