What causes gas?
Gas in the digestive tract (that is, the esophagus, stomach, small intestine, and large intestine) comes from the following two sources:
- Normal breakdown of certain undigested foods by harmless bacteria naturally present in the large intestine (colon).
- 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, 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 body does not digest and absorb all carbohydrates (the sugar, starches and fiber found in many foods) in the small intestine because of a shortage or absence of certain enzymes.This undigested food then passes from the small intestine into the large intestine. In the large intestine, harmless and normal 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.
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 symptomatic gas in one person may not cause symptoms 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. Furthermore, most people who have symptomatic gas do not have more gas than other people, but rather more sensitivity to symptoms caused by this gas.
Which foods cause gas?
Most foods that contain carbohydrates can cause gas. In contrast, fats, and proteins cause little gas.
The sugars that cause gas are raffinose, stachyose, verbascose, lactose, fructose, and sorbitol:
- Raffinose, stachyose, verbascoce are indigestible oligosaccharides present in large amounts in legumes, especially beans. Smaller amounts of this complex sugar 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, have low levels of the enzyme lactase needed to digest lactose. 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.
- Sorbitol is a sugar alcohol found naturally in fruits, including apples, pears, peaches, prunes, and some liquid forms of medications. Excess amounts can lead to diarrhea. It is also used as an artificial sweetener in many dietetic foods and sugar-free candies and gums.
Most starches, including potatoes, corn, noodles and wheat, produce gas. They are broken down in the large intestine. Rice is the only starch that does not cause gas.
Many foods contain soluble and insoluble fiber. Soluble fiber dissolves easily in water and takes on a soft, gel-like texture in the intestines. Found in oat bran, beans, peas and most fruits, soluble fiber is not broken down until it reaches the large intestine, where digestion causes gas.Insoluble fiber, on the other hand, passes essentially unchanged through the intestines and produces little gas. Wheat bran and some vegetables contain this kind of fiber.