Gas in the digestive tract is usually caused by swallowing air and by the breakdown of certain foods in the large intestine by bacteria. Everyone swallows a small amount of air when eating and drinking.
The amount of air swallowed increases when people:
- eat or drink too fast
- chew gum
- suck on hard candy
- drink carbonated or "fizzy” drinks
- wear loose-fitting dentures
Burping allows some gas to leave 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 anus.
The stomach and small intestine do not fully digest some carbohydrates—sugars, starches, and fiber found in many foods. This undigested food passes through the small intestine to the large intestine. Once there, undigested carbohydrates are broken down by bacteria in the large intestine, which release hydrogen and carbon dioxide in the process. Other types of bacteria in the large intestine take in hydrogen gas and create methane gas or hydrogen sulfide, the most common sulfur gas in flatus.
Studies have detected methane in the breath of 30 to 62 percent of healthy adults. A larger percentage of adults may produce methane in the intestines, but the levels may be too low to be detected. Research suggests that people with conditions that cause constipation are more likely to produce detectable amounts of methane. More research is needed to find out the reasons for differences in methane production and to explore the relationship between methane and other health problems.
Some of the gas produced in the intestines is absorbed by the bloodstream and carried to the lungs, where it is released in the breath.
Normally, few bacteria live in the small intestine. Small intestinal bacterial overgrowth is an increase in the number of bacteria or a change in the type of bacteria in the small intestine. These bacteria can produce excess gas and may also cause diarrhea and weight loss. Small intestinal bacterial overgrowth is usually related to diseases or disorders that damage the digestive system or affect how it works, such as Crohn’s disease—an inflammatory bowel disease that causes inflammation, or swelling, and irritation of any part of the gastrointestinal (GI) tract—or diabetes.
Some foods that cause gas include:
- vegetables such as broccoli, cauliflower, cabbage, brussels sprouts, onions, mushrooms, artichokes, and asparagus
- fruits such as pears, apples, and peaches
- whole grains such as whole wheat and bran
- sodas; fruit drinks, especially apple juice and pear juice; and other drinks that contain high-fructose corn syrup, a sweetener made from corn
- milk and milk products such as cheese, ice cream, and yogurt
- packaged foods—such as bread, cereal, and salad dressing—that contain small amounts of lactose, a sugar found in milk and foods made with milk
- sugar-free candies and gums
(Source: National Institute of Diabetes and Digestive Kidney Diseases, National Institutes of Health)