What causes gas in the digestive tract?

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Lawrence S. Friedman, MD
Gastroenterology
The air we breathe is made up mostly of nitrogen (N2) and oxygen (O2), the gas the human body needs to sustain life. After being swallowed, air enters the gastrointestinal (GI)  tract. As it moves along, its makeup changes as oxygen passes into the blood and nitrogen is removed from the blood. Another intestinal gas is carbon dioxide (CO2), a byproduct of a chemical reaction with acid in the stomach. Hydrogen (H2) is released in the colon when undigested carbohydrates undergo bacterial fermentation.

Bacteria in the gut produce foul-smelling gases when they ferment undigested foods that have not been absorbed in the small intestine. These foods consist mostly of carbohydrates, sugars, and fats. The carbohydrates found in high-fiber foods, such as beans, broccoli, cauliflower, and Brussels sprouts, are the worst culprits. These foods release gases such as methane and hydrogen sulfide, which smells like rotten eggs. The worst odor is related to strong-smelling sulfurs that make up just 1% of flatus (gas that escapes from the rectum).
 
Methane is detected in about one-third of adults. Studies show that Americans and Europeans are more likely to produce methane than Asians are, possibly because of diet. Women also produce more than men do. Genes may play a role in methane production, as the trait is passed along in families.

Additional carbon dioxide is produced in the colon as the byproduct of bacterial fermentation of unabsorbed sugars and starches. Eating beans will substantially increase CO2 production, as will taking sodium bicarbonate for heartburn. Thus, it doesn't make sense to use bicarbonate-containing seltzers for gas.

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

Swallowed Air: Aerophagia, or air swallowing, is a common cause of gas in the stomach. We all take in some air while eating or drinking, but eating or drinking rapidly, chewing gum, smoking, or wearing loose dentures can cause some people to take in more air.

Breakdown of Undigested Foods: The body lacks certain enzymes or does not have enough of them to digest and absorb some carbohydrates-the sugar, starches, and fiber found in many foods-in the small intestine. This undigested food then passes from the small intestine into the large intestine, where bacteria break it down, producing hydrogen, carbon dioxide, and, in one-third or more of all people, methane. These gases exit through the rectum.

People who make methane do not necessarily pass more gas or have unique symptoms. A person who produces methane will have stools that consistently float in water. It is not known why some people produce methane and others do not.

Foods that produce gas in one person may not cause gas in another. Some common bacteria in the large intestine can take in the hydrogen that other bacteria produce and produce methane or hydrogen sulfide. The balance between the two types of bacteria may explain why some people produce more gas than others do.re gas than others do.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.