What is Small Intestinal Bacterial Overgrowth?

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  • Written By: H. Colledge
  • Edited By: Heather Bailey
  • Last Modified Date: 13 September 2019
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Small intestinal bacterial overgrowth, or SIBO, is a disorder in which the levels of bacteria in the small bowel become abnormally high. Symptoms such as bloating, abdominal pain and diarrhea may be experienced. In severe cases, the disorder may interfere with the absorption of food from the gastrointestinal tract, possibly leading to fatigue, anemia and vitamin or mineral deficiencies. Small intestinal bacterial overgrowth can occur following bowel surgery, in association with diseases which affect the immune system, and it can disrupt the normal movement of the gut. Treatment of SIBO involves the use of antibiotics to correct the balance of bacteria in the gut.

The small intestine, also called the small bowel, is a narrow section of the gut of around 21 feet (about 6.4 meters) in length. It attaches the stomach to the large intestine, or large bowel, which is shorter and wider than the small intestine. Food is digested in the small intestine and is absorbed through the intestinal wall into the blood stream. While the large bowel normally contains a lot of bacteria, the small bowel typically contains only low levels of microbes. SIBO is sometimes known alternatively as SBBO, or small bowel bacterial overgrowth.


Conditions which decrease the muscular contraction of the intestinal walls, which normally serves to move food along, and push bacteria out of the small bowel into the large intestine may cause bacterial overgrowth. Complications associated with bowel surgery and disorders such as Crohn's disease, where the intestine is inflamed, can sometimes affect movement of food and bacteria through the gut, leading to SIBO. Diabetes can also cause small intestinal bacterial overgrowth, as it sometimes damages nerves involved in gut contractions. Hypothyroidism can slow down gut movement, and diseases such as AIDS, which suppress the immune system, may lead to excess bacteria in the small bowel. Although the two conditions have very similar symptoms, and some doctors suspect they may be linked, there is no evidence that SIBO causes the condition known as irritable bowel syndrome.

Treating small intestinal bacterial overgrowth generally involves the use of antibiotic drugs. The aims are to restore the normal bacterial population of the small bowel and reduce symptoms. Any underlying conditions, such as hypothyroidism, are also treated. Surgery may be required to remove any gut abnormalities, such as pouches, where bacteria might have been multiplying. In more severe cases, complications such as dehydration from diarrhea, and dietary deficiencies, may need to be managed by giving nutrition and fluids into a vein.


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