Gas in the digestive tract (that is, the esophagus, stomach, small intestine, and large intestine) comes from two sources:
Swallowed air
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 stomach also releases carbon dioxide when stomach acid and bicarbonate mix,
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.
This undigested food then passes from the small intestine into the large intestine, where 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.
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.
Sugars
The sugars that cause gas are: raffinose, lactose, fructose, and sorbitol.
Raffinose
Beans contain large amounts of this complex sugar. Smaller amounts are found in
cabbage, brussels sprouts, broccoli, asparagus, other vegetables, and whole
grains.
Lactose
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
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
Sorbitol is a sugar found naturally in fruits, including apples, pears, peaches,
and prunes. It is also used as an artificial sweetener in many dietetic foods
and sugarfree candies and gums.
Starches
Most starches, including potatoes, corn, noodles, and wheat, produce gas as they are broken down in the large intestine. Rice is the only starch that does not cause gas.
Fiber
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.
The most common ways to reduce the discomfort of gas are changing diet, taking medicines, and reducing the amount of air swallowed.
Diet
Doctors may tell people to eat fewer foods that cause gas. However, for some people this may mean cutting out healthy foods, such as fruits and vegetables, whole grains, and milk products.
Doctors may also suggest limiting high-fat foods to reduce bloating and discomfort. This helps the stomach empty faster, allowing gases to move into the small intestine.
Unfortunately, the amount of gas caused by certain foods varies from person to person. Effective dietary changes depend on learning through trial and error how much of the offending foods one can handle.
Nonprescription medicines
Many nonprescription, over-the-counter medicines are available to help reduce symptoms, including antacids with simethicone and activated charcoal. Digestive enzymes, such as lactase supplements, actually help digest carbohydrates and may allow people to eat foods that normally cause gas.
Antacids, such as Mylanta II, Maalox II and Di-Gel, contain simethicone, a foaming agent that joins gas bubbles in the stomach so that gas is more easily belched away. However, these medicines have no effect on intestinal gas. The recommended dose is 2 to 4 tablespoons of the simethicone preparation taken 1/2 to 2 hours after meals.
Activated charcoal tablets (Charcocaps) may provide relief from gas in the colon. Studies have shown that when taken before and after a meal, intestinal gas is greatly reduced. The usual dose is 2 to 4 tablets taken just before eating and 1 hour after meals.
The enzyme lactase, which aids with lactose digestion, is available in liquid and tablet form without a prescription (Lactaid, Lactrase, and Dairy Ease). Adding a few drops of liquid lactase to milk before drinking it or chewing lactase tablets just before eating helps digest foods that contain lactose. Also, lactose-reduced milk and other products are available at many grocery stores (Lactaid and Dairy Ease).
Beano, a newer over-the-counter digestive aid, contains the sugar-digesting enzyme that the body lacks to digest the sugar in beans and many vegetables. The enzyme comes in liquid form. Three to 10 drops are added per serving just before eating to break down the gas-producing sugars. Beano has no effect on gas caused by lactose or fiber.
Prescription medicines
Doctors may prescribe medicines to help reduce symptoms, especially for people with a motility disorder, such as IBS. Promotility or prokinetic drugs, such as metoclopramide (Reglan) and cisapride (Propulsid), may move gas through the digestive tract quickly.
Reducing swallowed air
For those who have chronic belching, doctors may suggest ways to reduce the amount of air swallowed. Recommendations are to avoid chewing gum and to avoid eating hard candy. Eating at a slow pace and checking with a dentist to make sure dentures fit properly should also help.