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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.

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Medical Editor:

Bhupinder Anand, MD

In this Article

• What is small intestinal bacterial overgrowth (SIBO)?
• What causes small intestinal bacterial overgrowth?
• What are the small intestinal bacterial overgrowth symptoms?
• What is the normal relationship between bacteria and the small intestine?
• What conditions cause increased production of gas?
• How does small intestinal bacterial overgrowth cause symptoms?
• How is small intestinal bacterial overgrowth diagnosed?
• Is there a relationship between small intestinal bacterial overgrowth and irritable bowel syndrome?
• How is small intestinal bacterial overgrowth treated?
• What's new in small intestinal bacterial overgrowth?
• Small Intestinal Bacterial Overgrowth (SIBO) At A Glance
• Patient Comments: Small Intestinal Bacterial Overgrowth - Symptoms
• Patient Comments: Small Intestinal Bacterial Overgrowth - Causes
• Find a local Gastroenterologist in your town
• Small Intestinal Bacterial Overgrowth Index

There are three situations in which abnormally increased amounts of gas are produced in the colon.

1. Malabsorption of sugars and carbohydrates. Reduced digestion or absorption by the small intestine allows increased amounts of sugar and carbohydrate to reach the colon where greater amounts of gas are produced. The most common example of malabsorption leading to increased production of gas is lactose (milk) intolerance. Lactose intolerance is due to a genetic lack of an enzyme in the lining of the small intestine that digests lactose, the sugar in milk. Other causes of malabsorption that can lead to excessive production of gas include: (1) genetically-determined malabsorption of other sugars such as sucrose, sorbitol, and fructose; (2) diseases of the pancreas that result in inadequate production of pancreatic enzymes (that are necessary for digesting sugars and carbohydrates in the small intestine); and (3) diseases of the lining of the small intestine (for example, celiac disease) that reduce the sugar and carbohydrate-digesting enzymes in the lining and reduce absorption of sugars and carbohydrates in the body.


2. Rapid intestinal transit. Normal digestion and absorption of sugars and carbohydrates requires time. If food passes through the small intestine too rapidly, there is not enough time for digestion and absorption to be completed, and more sugar and carbohydrate reach the colon. The best example of rapid intestinal transit is in individuals who have had a large portion of their small intestine removed surgically. There are also a small number of individuals with intact small bowel who, for unexplained reasons, have abnormally rapid transit through the small intestine.


3. Small intestinal bacterial overgrowth (SIBO). In patients with SIBO, large numbers of gas-producing bacteria (normally present in the colon) are present in the small intestine. The abundant bacteria in the small intestine compete with the small intestine the digestion of sugars and carbohydrates, but unlike the small intestine, the bacteria and produce large amounts of gas.