The small intestine is the main site where your gut absorbs nutrients from the food you eat.  Unlike the large intestine (or colon) there are usually very few bacteria in the small intestine.  In SIBO there is a build-up of bacteria in the small intestine.  These can compete with your gut for the nutrients in your food.  Bacteria can break down or ferment the food resulting in gas production, abdominal cramps and often diarrhea. 

In most cases there is no clear explanation for why a person develops SIBO.  It may occur due to food passing more slowly through the digestive tract resulting in excess bacterial growth. Possible causes of this include pH changes in the small bowel, nerve damage or muscular malfunction in the intestine. 

Risk factors for developing SIBO include previous gastrointestinal surgery (including certain weight-loss operations), diabetes, Crohn’s disease, hypothyroidism, Parkinson’s disease and scleroderma.  Chronic use of medications that slow gut transit (such as certain strong painkillers) or reduce stomach acidity may also contribute.