Small bowel bacterial overgrowth is a condition in which very large numbers of bacteria grow in the small intestine.
Most of the time, unlike the large intestine, the small intestine does not have a large number of bacteria. Excess bacteria in the small intestine may use up the nutrients needed by the body. As a result, a person may become malnourished.
The breakdown of nutrients by the excess bacteria can also damage the lining of the small intestine. This can make it even harder for the body to absorb nutrients.
Conditions that can lead to overgrowth of bacteria in the small intestine include:
- Complications of diseases or surgery that create pouches or blockages in the small intestine. Crohn disease is one of these conditions.
- Diseases that lead to decreased movement of the small bowel, such as diabetes and scleroderma.
- Immunodeficiency, such as AIDS or immunoglobulin deficiency.
- Short bowel syndrome caused by surgical removal of the small intestine.
- Small bowel diverticulosis, in which small, or, at times, large sacs occur in the wall of the intestine. These sacs allow too many bacteria to grow.
- Surgical procedures that create a loop of small intestine where excess bacteria can grow. An example is a Billroth II type of stomach removal (gastrectomy).
- Some cases of irritable bowel syndrome (IBS).
The most common symptoms are:
- Abdominal fullness
- Abdominal pain and cramps
- Diarrhea (most often watery)
Other symptoms may include:
- Fatty stool
- Weight loss
Exams and Tests
Your health care provider will perform a physical exam and ask about your medical history. Tests may include:
- Blood chemistry tests (such as albumin level)
- Complete blood count (CBC)
- Fecal fat test
- Small intestine endoscopy
- Vitamin levels in the blood
- Small intestine biopsy or culture
- Special breath tests
The goal is to treat the cause of the bacterial overgrowth. Treatment may include:
- Medicines that speed intestinal movement
- Intravenous (IV) fluids
- Nutrition given through a vein (total parenteral nutrition or TPN) in a malnourished person
A lactose-free diet can be helpful.
Severe cases lead to malnutrition. Other possible complications include:
- Excess bleeding or other problems due to vitamin deficiency
- Inflammation of the intestine
Overgrowth - intestinal bacteria; Bacterial overgrowth - intestine; Small intestinal bacterial overgrowth; SIBO
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Manolakis CS, Rutland TJ, Di Palma JA. Small intestinal bacterial overgrowth. In: McNally PR, ed. GI/Liver Secrets Plus. 5th ed. Philadelphia, PA: Elsevier Saunders; 2015:chap 44.
Rej A, Chew TS, Sanders DS. Gastroenterology. In: Penman ID, Ralston SH, Strachan MWJ, Hobson RP, eds. Davidson's Principles and Practice of Medicine. 24th ed. Philadelphia, PA: Elsevier; 2023:chap 23.
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Review Date 5/4/2022
Updated by: Michael M. Phillips, MD, Emeritus Professor of Medicine, The George Washington University School of Medicine, Washington, DC. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.