What causes small intestinal bacterial overgrowth?

The gastrointestinal tract is a continuous muscular tube through which digesting food is transported on its way to the colon. The coordinated activity of the muscles of the stomach and small intestine propels the food from the stomach, through the small intestine, and into the colon. Even when there is no food in the small intestine, muscular activity sweeps through the small intestine from the stomach to the colon.

The muscular activity that sweeps through the small intestine is important for the digestion of food, but it also is important because it sweeps bacteria out of the small intestine and thereby limits the numbers of bacteria in the small intestine. Anything that interferes with the progression of normal muscular activity through the small intestine can result in SIBO. Simply stated, any condition that interferes with muscular activity in the small intestine allows the bacteria to stay longer and multiply in the small intestine. The lack of muscular activity also may allow bacteria to spread backwards from the colon and into the small intestine.
What are small intestinal bacteria overgrowth symptoms?

The symptoms of SIBO include:

excess gas,
abdominal bloating and distension,
diarrhea, and
abdominal pain.
A small number of patients with SIBO have chronic constipation rather than diarrhea. When the overgrowth is severe and prolonged, the bacteria may interfere with the digestion and/or absorption of food and deficiencies of vitamins and minerals may develop. Weight loss also may occur. Patients with SIBO sometimes also report symptoms that are unrelated to the gastrointestinal tract, symptoms such as body aches or fatigue. The symptoms of SIBO tend to be chronic. A typical patient with SIBO can experience symptoms that fluctuate in intensity over months, years, or even decades before the diagnosis is made.
Fra http://www.medicinenet.com/small_int...th/article.htm