What is gas?
Gas is air in the digestive tract—the large, muscular tube that extends from the mouth to the anus, where the movement of muscles, along with the release of hormones and enzymes, allows for the digestion of food. Gas leaves the body when people burp through the mouth or pass gas through the anus.
Gas is primarily composed of carbon dioxide, oxygen, nitrogen, hydrogen, and sometimes methane. Flatus, gas passed through the anus, may also contain small amounts of gasses that contain sulfur. Flatus that contains more sulfur gasses has more odor.
Everyone has gas. However, many people think they burp or pass gas too often and that they have too much gas. Having too much gas is rare.
What causes gas?
Gas in the digestive tract is usually caused by swallowing air and by the breakdown of certain foods in the large intestine by bacteria.
Most foods that contain carbohydrates can cause gas. In contrast, fats and proteins cause little gas. Foods that produce gas in one person may not cause gas in someone else, depending on how well individuals digest carbohydrates and the type of bacteria present in the intestines.*
*U.S. Department of Health and Human Services, National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases, "Gas in the Digestive Tract"