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Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

What Is It?

Flatulence is the passage of intestinal gas (flatus) through the rectum. Passing gas is normal, and every human being does it at least 14 times a day, consciously or unconsciously. Sometimes flatulence happens more often than expected, and this can become an embarrassing problem. Extreme flatulence can even interfere with a person's ability to work and socialize comfortably with other people.

Most cases of flatulence are related to factors that can be controlled. This is because intestinal gas usually comes from two sources swallowed air or the work of intestinal bacteria on undigested food.

Swallowing air is one cause of flatulence. Although much of this swallowed air is belched upward through the mouth, a small amount passes into the intestines and out through the rectum. People swallow air in many different ways, particularly by:

  • Unconsciously gulping air as they talk, especially when they are upset, excited or nervous

  • Eating or drinking in a hurry

  • Chewing gum

  • Smoking

  • Drinking carbonated beverages

Bacteria in the intestines also can produce gas when they process foods that pass into the colon without being digested higher up in the digestive tract. Some common examples of foods that tend to cause gas include:

  • Foods rich in fiber These include fruits, beans, peas and oat bran.

  • Foods containing fructose Fructose is a simple sugar that occurs naturally in many fruits, especially figs, dates, prunes, pears and grapes. It's also found in smaller amounts in onions, asparagus, artichokes and wheat. Fructose sometimes is added as a sweetener to soft drinks, fruit drinks, and some cookies and cakes.

  • Vegetables containing raffinose Raffinose is a complex sugar found in many cruciferous vegetables (cabbage, Brussels sprouts, broccoli, cauliflower) and in beans. Beans also contain stachyose, another form of poorly digested sugar.

  • Sorbitol Sorbitol is used to sweeten many sugar-free gums and candies, and it also may be added as an inert ingredient to medicines.

  • Dairy products containing lactose, a sugar found in milk People who are lactose intolerant find it difficult to digest milk, cheese, ice cream and other dairy products. These people have unusually low levels of lactase, an enzyme needed to digest lactose. Lactose intolerance is especially common among African-Americans, Native Americans and Asians. It can appear as you grow older, even if you have not had trouble digesting milk as a child or younger adult.

Less commonly, flatulence can be a side effect of certain medications, especially cholestyramine (Questran), used to treat high cholesterol, or the diet drug orlistat (Xenical). It also can be a symptom of irritable bowel syndrome or giardiasis (a parasitic infection).

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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