Small intestinal bacterial overgrowth (SIBO) occurs when there is an abnormal increase in the overall bacterial population in the small intestine — particularly types of bacteria not commonly found in that part of the digestive tract. This condition is sometimes called blind loop syndrome.

small intestinal bacterial overgrowth (SIBO) commonly results when a circumstance — such as surgery or disease — slows the passage of food and waste products in the digestive tract, creating a breeding ground for bacteria. The excess bacteria often cause diarrhea and may cause weight loss and malnutrition.

While SIBO is often a complication of stomach (abdominal) surgery, this condition can also result from structural problems and some diseases. Sometimes surgery is needed to correct the problem, but antibiotics are the most common treatment.


Signs and symptoms of SIBO often include:

  • Loss of appetite
  • Abdominal pain
  • Nausea
  • Bloating
  • An uncomfortable feeling of fullness after eating
  • Diarrhea
  • Unintentional weight loss
  • Malnutrition

When to see a doctor

Bloating, nausea and diarrhea are signs and symptoms of many intestinal problems. See your doctor for a full evaluation — especially if you've had abdominal surgery — if you have:

  • Persistent diarrhea
  • Rapid, unintentional weight loss
  • Abdominal pain lasting more than a few days

If you have severe abdominal pain, seek immediate medical care.


Small intestinal bacterial overgrowth (SIBO) can be caused by:

  • Complications of abdominal surgery, including gastric bypass for obesity and gastrectomy to treat peptic ulcers and stomach cancer
  • Structural problems in and around your small intestine, including scar tissue (intestinal adhesions) that can wrap around the outside of the small bowel, and bulging pouches of tissue that protrude through the wall of the small intestine (intestinal diverticulosis)
  • Certain medical conditions, including Crohn's disease, radiation enteritis, scleroderma, celiac disease, diabetes or other conditions that can slow movement (motility) of food and waste products through the small intestine
Gastrointestinal tract

Gastrointestinal tract

Your digestive tract stretches from your mouth to your anus. It includes the organs necessary to digest food, absorb nutrients and process waste.

Why small intestinal bacterial overgrowth (SIBO) develops

The small intestine is the longest section of your digestive tract, measuring about 20 feet (6.1 meters). The small intestine is where food mixes with digestive juices and nutrients are absorbed into your bloodstream.

Unlike your large intestine (colon), your small intestine normally has relatively few bacteria due to rapid flow of contents and the presence of bile. But in SIBO, stagnant food in the bypassed small intestine becomes an ideal breeding ground for bacteria. The bacteria may produce toxins as well as interfere with the absorption of nutrients. The breakdown products following bacterial digestion of food can also trigger diarrhea.

Risk factors

Factors that increase your risk of SIBO include:

  • Gastric surgery for obesity or ulcers
  • A structural defect in the small intestine
  • An injury to the small intestine
  • An abnormal passageway (fistula) between two segments of bowel
  • Crohn's disease, intestinal lymphoma or scleroderma involving the small intestine
  • History of radiation therapy to the abdomen
  • Diabetes
  • Diverticulosis of the small intestine
  • Adhesions caused by previous abdominal surgery


Small intestinal bacterial overgrowth (SIBO) can cause escalating problems, including:

  • Poor absorption of fats, carbohydrates and proteins. Bile salts, which are normally needed to digest fats, are broken down by the excess bacteria in your small intestine, resulting in incomplete digestion of fats and diarrhea. Bacterial products may also harm the mucous lining (mucosa) of the small intestine, resulting in decreased absorption of carbohydrates and proteins.

    Bacteria can compete for available food. And compounds produced through the bacterial break-down of stagnant food can also trigger diarrhea. Together, these effects of bacterial overgrowth result in diarrhea, malnutrition and weight loss.

  • Vitamin deficiency. As a result of incomplete absorption of fats, your body can't fully absorb the fat-soluble vitamins A, D, E and K. Bacteria in the small intestine synthesize as well as use vitamin B-12, which is essential for the normal functioning of your nervous system and the production of blood cells and DNA.

    The overgrowth of bacteria can result in B-12 deficiency that can lead to weakness, fatigue, tingling, and numbness in your hands and feet and, in advanced cases, to mental confusion. Damage to your central nervous system resulting from B-12 deficiency may be irreversible.

  • Weakened bones (osteoporosis). Over time, damage to your intestine from abnormal bacterial growth causes poor calcium absorption, and eventually may lead to bone diseases, such as osteoporosis.
  • Kidney stones. Poor calcium absorption may also eventually result in kidney stones.

Jan 06, 2022

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