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Small Intestinal Bacterial Overgrowth (SIBO)


Atenodoro R. Ruiz, Jr.

, MD, The Medical City, Pasig City, Philippines

Last full review/revision Feb 2021| Content last modified Feb 2021
Click here for the Professional Version

Small intestinal bacterial overgrowth 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.

  • People have abdominal discomfort, diarrhea, bloating, and excessive flatulence.

  • The diagnosis is based on symptoms, particularly in people who have had certain types of surgery or who have certain disorders, plus the results of a breath test or an intestinal fluid culture.

  • Antibiotics can eliminate the excess bacteria.

Causes of Bacterial Overgrowth Syndrome

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 (see Gallbladder and Biliary Tract). The loss of bile salts causes difficulty absorbing fats, leading to diarrhea and poor nutrition.

Symptoms of Bacterial Overgrowth Syndrome

The most common symptom of small intestinal bacterial overgrowth is bloating. Other symptoms are abdominal discomfort, diarrhea, excess flatulence, weight loss, or symptoms of nutritional deficiencies. Some people have severe diarrhea or steatorrhea (light-colored, soft, bulky, greasy, and unusually foul-smelling stool).

Diagnosis of Bacterial Overgrowth Syndrome

  • A doctor's evaluation of symptoms

  • Breath tests

  • Intestinal fluid culture

  • Imaging tests

Doctors base the diagnosis of small intestinal bacterial overgrowth on typical symptoms, particularly in people who have had certain types of surgery or have certain disorders.

Doctors may take a fluid sample from a flexible tube inserted into the small intestine in a procedure called endoscopy. Doctors culture the fluid to determine the type and amount of bacteria present.

Or doctors may do a breath test. When bacteria in the intestines break down certain sugars, hydrogen is produced. Doctors do a breath test to help them determine how much hydrogen a person exhales through breathing. After drinking a liquid containing a certain sugar (glucose or lactulose) mixed with water, a person exhales into a collection bag at 1-hour intervals over a period of 4 hours. The bags are then analyzed. If the amount of hydrogen in the breath rises significantly after consuming either of the liquids, the person has small intestinal bacterial overgrowth.

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 that shows up on x-rays (called an upper gastrointestinal [GI] series).

Treatment of Bacterial Overgrowth Syndrome

  • Antibiotics

  • Changes in diet

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.

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