Bacterial overgrowth syndrome is a disorder in which poor movement of intestinal contents allows certain normal intestinal bacteria to grow excessively, causing diarrhea and poor absorption of nutrients (malabsorption).

• Some conditions and disorders slow or stop the movement of contents through the intestines.
• Some people have no symptoms, whereas others have abdominal discomfort, diarrhea, bloating, and excessive flatulence.
• The diagnosis is based on symptoms that occur after people have had certain types of surgery.
• Antibiotics can eliminate the excess bacteria.

The normal steady movement of intestinal contents (peristalsis) is important to help maintain a proper balance of bacteria in the small intestine. Conditions in which intestinal contents slow or pool in one place allow excess bacteria to grow. Such conditions include certain types of surgery on the stomach, intestines, or both. Disorders such as diabetes, systemic sclerosis, and amyloidosis also can slow peristalsis, causing bacterial overgrowth.

The excess bacteria consume nutrients, including carbohydrates and vitamin B₁₂, leading to lower calorie intake and vitamin B₁₂ deficiency. The bacteria also split bile salts, which are secreted by the liver to aid digestion. The loss of bile salts causes difficulty absorbing fats, leading to diarrhea and poor nutrition.

The most frequent symptoms are abdominal discomfort, diarrhea, bloating, and excess flatulence. Some people have few symptoms or only weight loss or nutritional deficiencies. Others have severe diarrhea or steatorrhea (light-colored, soft, bulky, greasy, and unusually foul-smelling stool).

Diagnosis and Treatment

Doctors base the diagnosis on typical symptoms that occur in people who have had certain types of surgery. Sometimes they take a fluid sample from the small intestine by means of a thin plastic tube passed through the nose. Some doctors instead perform breath tests, such as the ¹⁴C-xylose breath test. In this test, the person drinks a liquid containing a special, faintly radioactive marker (carbon-14) attached to a sugar (xylose). If the xylose is broken down by the excess bacteria, the carbon-14 can be detected in the person's breath. Sometimes people have abnormalities of their internal structures that make them susceptible to bacterial overgrowth. To identify these abnormalities, doctors take x-rays of the stomach and small intestines after a person drinks a liquid dye (called an upper gastrointestinal [GI] series).

Most people get better with antibiotics given by mouth for 10 to 14 days. Because the excess bacteria grow faster with carbohydrates, people should eat a diet higher in fat and lower in carbohydrates and fiber. Doctors prescribe supplements to correct any nutritional deficiencies.

Last full review/revision January 2013 by Atenodoro R. Ruiz, Jr., MD