1. Take the Tummy Trouble Quiz
2. Digestive Disease Myths Slideshow Pictures
3. Ulcerative Colitis Slideshow

Medical Author:

Jay W. Marks, MD

Jay W. Marks, MD

Jay W. Marks, MD, is a board-certified internist and gastroenterologist. He graduated from Yale University School of Medicine and trained in internal medicine and gastroenterology at UCLA/Cedars-Sinai Medical Center in Los Angeles.

View Full Profile

Medical Editor:

William C. Shiel Jr., MD, FACP, FACR

William C. Shiel Jr., MD, FACP, FACR

Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

View Full Profile

In this Article

• Intestinal gas facts
• What causes belching?
• What causes bloating?
• What causes flatulence (gas)?
• What are the causes of intermittent abdominal bloating/distention?
• How is belching, bloating/distention, and flatulence evaluated?
• How is excessive intestinal gas treated?
• Patient Comments: Intestinal Gas - Effective Treatments
• Patient Comments: Intestinal Gas - Bloating Causes
• Patient Comments: Intestinal Gas - Causes
• Intestinal Gas (Belching, Bloating, Flatulence) Index

Flatulence, also known as farting, is the act of passing intestinal gas from the anus. Gas in the gastrointestinal tract has only two sources. It is either swallowed air or is produced by bacteria that normally inhabit the intestines, primarily the colon. Swallowed air is rarely the cause of excessive flatulence.

The usual source of excessive gas is intestinal bacteria. The bacteria produce the gas (primarily hydrogen and/or methane) when they digest foods, primarily sugars and nondigestible polysaccharides (for example, starch, cellulose), that have not been digested during passage through the small intestine. The bacteria also produce carbon dioxide, but the carbon dioxide is so rapidly absorbed from the intestine that very little passes in flatus.

Sugars

Sugars that are commonly poorly digested (maldigested) and malabsorbed are lactose, sorbitol, and fructose.

• Lactose is the sugar in milk. The absence of the enzyme lactase in the lining of the intestines, which is a genetic trait, causes the maldigestion. Lactase is important because it breaks apart the lactose so that it can be absorbed.
  
  
• Sorbitol is a commonly used sweetener in low calorie foods.
  
  
• Fructose, primarily as high fructose corn syrup is a commonly used sweetener in all types of candies and drinks.

Polysaccharides

Starches are another common source of intestinal gas. Starches are polysaccharides that are produced by plants and are composed of long chains of sugars, primarily fructose. Common sources of different types of starch include wheat, oats, potatoes, corn, and rice.

• Rice is the most easily digested starch, and little undigested rice starch reaches the colon and the colonic bacteria. Accordingly, the consumption of rice produces little gas.
• In contrast, some of the starches in wheat, oats, potatoes, and, to a lesser extent, corn, all may reach the colon. These starches, therefore, may result in the production of appreciable amounts of gas.
• The starch in whole grains produces more gas than the starch in refined (purified) grains. Thus, more gas is formed after eating foods made with whole wheat flour than with refined wheat flour. This difference in gas production probably occurs because of the fiber (similar to a complex starch) present in the whole grain flour. Much of this fiber is removed during the processing of whole grains into refined flour.
• Finally, certain fruits and vegetables, for example, cabbage, also contain poorly digested starches that reach the colon and are easily converted by bacteria into gas.
• Most vegetables and fruits contain cellulose, another type of polysaccharide that is not digested at all as it passes through the small intestine. However, unlike sugars and other starches, cellulose is used only very slowly by colonic bacteria. Therefore, the production of gas after the consumption of fruits and vegetables usually is not great unless the fruits and vegetables also contain sugars or polysaccharides other than cellulose.

Small amounts of air are continuously being swallowed and bacteria are constantly producing gas. Contractions of the intestinal muscles normally propel the gas through the intestines and cause the gas to be expelled. Flatulence (passing intestinal gas) prevents gas from accumulating in the intestines.

However, there are two other ways in which gas can escape the intestine.

• First, it can be absorbed across the lining of the intestine into the blood. The gas then travels in the blood and ultimately is excreted in the breath.
• Second, gas can be removed and used by certain types of bacteria within the intestine. In fact, most of the gas that is formed by bacteria in the intestines is removed by other bacteria in the intestines. (Thank goodness!)