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What is small intestinal bacterial overgrowth (SIBO)?

The small bowel, also known as the small intestine, is part of the digestive system that connects the stomach with the large bowel or colon. The main purpose of the small intestine is to digest and absorb food into the body. The small bowel is divided into three parts:  the duodenum (which food from the stomach empties into), the jejunum and the ileum (which empties undigested food into the large intestine or colon).

The entire gastrointestinal tract, including the small intestine, normally contains bacteria. The number of bacteria is greatest in the colon but much lower in the small intestine. Also, the types of bacteria within the small intestine are different to the types of bacteria within the colon. Small intestinal bacterial overgrowth (SIBO) refers to a condition in which abnormally large numbers of bacteria are present in the small intestine, while the types of bacteria found in the small intestine are more like the bacteria found in the colon.

Small intestinal bacterial overgrowth (SIBO) is also known as small bowel bacterial overgrowth syndrome (SBBOS).

What causes small intestinal bacterial overgrowth?

The gastrointestinal tract is a continuous muscular tube which digesting food travels along on its way to the colon. Normally, the coordinated action of the muscles of the stomach and small intestine propels the food from the stomach, through the small intestine and into the colon.

This muscular action also sweeps bacteria out of the small intestine and limits the numbers of bacteria in the small intestine.  However, when a condition interferes with the normal activity in the small intestine, this can result in SIBO, by allowing bacteria to stay longer and multiply in the small intestine. The lack of normal muscular activity also may allow bacteria to spread backwards from the colon and into the small intestine.

Many conditions are associated with SIBO. A few are common:

  • Neurological and muscular diseases can alter the normal activity of the intestinal muscles. Diabetes mellitus damages the nerves that control the intestinal muscles. Scleroderma damages the intestinal muscles directly. In both cases, abnormal muscular activity in the small intestine allows SIBO to develop.
  • Partial or intermittent obstruction of the small intestine can interfere with the transport of food and bacteria through the small intestine and can result in SIBO. Causes of obstruction leading to SIBO include adhesions - or scarring - from previous surgery and Crohn's disease.
  • Diverticuli (small pouches) of the small intestine that allow bacteria to multiply inside diverticuli.

What are small intestinal bacteria overgrowth symptoms?

The symptoms of SIBO include:

  • Excess wind
  • Abdominal bloating and distension
  • Diarrhoea
  • Constipation
  • Abdominal pain

When the overgrowth is severe and prolonged, the bacteria may interfere with the digestion and the absorption of food, so that deficiencies of vitamins and minerals may develop. Patients may also lose weight. Patients with SIBO sometimes also report symptoms that are unrelated to the gastrointestinal tract, 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 a diagnosis is made.

WebMD Medical Reference

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