Gas in the Digestive Tract
On this page:
What causes gas?
Which foods cause gas?
What are some symptoms and problems of gas?
What diagnostic tests are used?
How is gas treated?
Points to Remember
The digestive tract
Everyone has gas and eliminates it by burping or passing it through the rectum. However, many people think they have too much gas when they really have normal amounts. Most people produce about 1 to 4 pints a day and pass gas about 14 times a day.
Gas is made primarily of odorless vapors--carbon dioxide, oxygen, nitrogen, hydrogen, and sometimes methane. The unpleasant odor of flatulence comes from bacteria in the large intestine that release small amounts of gases that contain sulfur.
Although having gas is common, it can be uncomfortable and embarrassing. Understanding causes, ways to reduce symptoms, and treatment will help most people find relief.
What causes gas?
Gas in the digestive tract (that is, the esophagus, stomach, small intestine, and large intestine) comes from 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 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 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.
Which foods cause gas?
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 the enzyme lactase 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.
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.
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.
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.
What are some symptoms and problems of gas?
The most common symptoms of gas are flatulence, abdominal bloating, abdominal pain, and belching. However, not everyone experiences these symptoms. The determining factors probably are how much gas the body produces, how many fatty acids the body absorbs, and a person's sensitivity to gas in the large intestine.
An occasional belch during or after meals is normal and releases gas when the stomach is full of food. However, people who belch frequently may be swallowing too much air and releasing it before the air enters the stomach.
Sometimes a person with chronic belching may have an upper GI disorder, such as peptic ulcer disease, gastroesophageal reflux disease (GERD), or gastroparesis.
Occasionally, some people believe that swallowing air and releasing it will relieve the discomfort of these disorders, and this person may intentionally or unintentionally develop a habit of belching to relieve discomfort.
Gas-bloat syndrome may occur after fundoplication surgery to correct GERD. The surgery creates a one-way valve between the esophagus and stomach that allows food and gas to enter the stomach but often prevents normal belching and the ability to vomit. It occurs in about 10 percent of people who have this surgery but may improve with time.
Another common complaint is passage of too much gas through the rectum (flatulence). However, most people do not realize that passing gas 14 to 23 times a day is normal. Too much gas may be the result of carbohydrate malabsorption.
Many people believe that too much gas causes abdominal bloating. However, people who complain of bloating from gas often have normal amounts and distribution of gas. They actually may be unusually aware of gas in the digestive tract.
Doctors believe that bloating is usually the result of an intestinal disorder, such as irritable bowel syndrome (IBS). The cause of IBS is unknown, but may involve abnormal movements and contractions of intestinal muscles and increased pain sensitivity in the intestine. These disorders may give a sensation of bloating because of increased sensitivity to gas.
Any disease that causes intestinal inflammation or obstruction, such as Crohn's disease or colon cancer, may also cause abdominal bloating. In addition, people who have had many operations, adhesions (scar tissue), or internal hernias may experience bloating or pain. Finally, eating a lot of fatty food can delay stomach emptying and cause bloating and discomfort, but not necessarily too much gas.
Abdominal Pain and Discomfort
Some people have pain when gas is present in the intestine. When pain is on the left side of the colon, it can be confused with heart disease. When the pain is on the right side of the colon, it may mimic gallstones or appendicitis.
What diagnostic tests are used?
Because gas symptoms may be caused by a serious disorder, those causes should be ruled out. The doctor usually begins with a review of dietary habits and symptoms. The doctor may ask the patient to keep a diary of foods and beverages consumed for a specific time period.
If lactase deficiency is the suspected cause of gas, the doctor may suggest avoiding milk products for a period of time. A blood or breath test may be used to diagnose lactose intolerance.
In addition, to determine if someone produces too much gas in the colon or is unusually sensitive to the passage of normal gas volumes, the doctor may ask patients to count the number of times they pass gas during the day and include this information in a diary.
Careful review of diet and the amount of gas passed may help relate specific foods to symptoms and determine the severity of the problem.
Because the symptoms that people may have are so variable, the physician may order other types of diagnostic tests in addition to a physical exam, depending on the patient's symptoms and other factors.
How is gas treated?
Experience has shown that the most common ways to reduce the discomfort of gas are changing diet, taking medicines, and reducing the amount of air swallowed.
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.
Many nonprescription, over-the-counter medicines are available to help reduce symptoms, including antacids with simethicone. 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. Dosage varies depending on the form of medication and the patient's age.
The enzyme lactase, which aids with lactose digestion, is available in caplet and chewable tablet form without a prescription (Lactaid and Lactrase). 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, an 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 and tablet form. Five drops are added per serving or 1 tablet is swallowed just before eating to break down the gas-producing sugars. Beano has no effect on gas caused by lactose or fiber.
Doctors may prescribe medicines to help reduce symptoms, especially for people with a disorder such as IBS.
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.
Although gas may be uncomfortable and embarrassing, it is not life-threatening. Understanding causes, ways to reduce symptoms, and treatment will help most people find some relief.
Points to remember
Everyone has gas in the digestive tract.
People often believe normal passage of gas to be excessive.
Gas comes from two main sources: swallowed air and normal breakdown of certain foods by harmless bacteria naturally present in the large intestine.
Many foods with carbohydrates can cause gas. Fats and proteins cause little gas.
Foods that may cause gas include
vegetables, such as broccoli, cabbage, brussels sprouts, onions, artichokes, and asparagus
fruits, such as pears, apples, and peaches
whole grains, such as whole wheat and bran
soft drinks and fruit drinks
milk and milk products, such as cheese and ice cream, and packaged foods prepared with lactose, such as bread, cereal, and salad dressing
foods containing sorbitol, such as dietetic foods and sugarfree candies and gums
The most common symptoms of gas are belching, flatulence, bloating, and abdominal pain. However, some of these symptoms are often caused by an intestinal disorder, such as irritable bowel syndrome, rather than too much gas.
The most common ways to reduce the discomfort of gas are changing diet, taking nonprescription medicines, and reducing the amount of air swallowed.
Digestive enzymes, such as lactase supplements, actually help digest carbohydrates and may allow people to eat foods that normally cause gas.
The U.S. Government does not endorse or favor any specific commercial product or company. Trade, proprietary, or company names appearing in this document are used only because they are considered necessary in the context of the information provided. If a product is not mentioned, this does not mean or imply that the product is unsatisfactory.
National Digestive Diseases Information Clearinghouse
2 Information Way
Bethesda, MD 20892-3570
The National Digestive Diseases Information Clearinghouse (NDDIC) is a service of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). The NIDDK is part of the National Institutes of Health under the U.S. Department of Health and Human Services. Established in 1980, the clearinghouse provides information about digestive diseases to people with digestive disorders and to their families, health care professionals, and the public. NDDIC answers inquiries, develops and distributes publications, and works closely with professional and patient organizations and Government agencies to coordinate resources about digestive diseases.
Publications produced by the clearinghouse are carefully reviewed by both NIDDK scientists and outside experts.
This e-text is not copyrighted. The clearinghouse encourages users of this e-pub to duplicate and distribute as many copies as desired.
NIH Publication No. 04-883