Gas in the digestive tract—the esophagus, stomach, small intestine, and large intestine—comes from two sources:
Aerophagia, or air swallowing, is a common cause of gas in the stomach. Everyone swallows small amounts of air when eating and drinking. However, eating or drinking rapidly, 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—which contains nitrogen, oxygen, and carbon dioxide—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 mixes with the bicarbonate in digestive juices, but most of this gas is absorbed into the bloodstream and does not enter the large intestine.
Breakdown of Undigested Foods
The body does not digest and absorb some carbohydrates—the sugar, starches, and fiber found in many foods—in the small intestine because of a shortage or absence of certain enzymes that aid digestion.
This undigested food then passes from the small intestine into the large intestine, where normal, harmless bacteria break down the food, producing hydrogen, carbon dioxide, and, in about one-third of all people, methane. Eventually 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. Research has not shown 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 destroy the hydrogen that other bacteria produce. The balance of the two types of bacteria may explain why some people have more gas than others.
Most foods that contain carbohydrates can cause gas. By contrast, fats and proteins cause little gas.
The sugars that cause gas are raffinose, lactose, fructose, and sorbitol.
Beans contain large amounts of this complex sugar. Smaller amounts are found in cabbage, brussels sprouts, broccoli, asparagus, other vegetables, and whole grains.
Lactose is the natural sugar in milk. It is also found in milk products, such as cheese and ice cream, and processed foods, such as bread, cereal, and salad dressing. Many people, particularly those of African, Native American, or Asian background, normally have low levels of lactase, the enzyme needed to digest lactose, after childhood. Also, as people age, their enzyme levels decrease. As a result, over time people may experience increasing amounts of gas after eating food containing lactose.
Fructose is naturally present in onions, artichokes, pears, and wheat. It is also used as a sweetener in some soft drinks and fruit drinks.