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What Is Small Intestinal Bacterial Overgrowth (SIBO)?

Learn what SIBO is, some of the potential causes of SIBO, and how you may be able to treat SIBO.


A healthy digestive system is equipped with a system of checks and balances, keeping bacteria where they ought to be and keeping their numbers at appropriate levels. However, for several reasons, things can get out of balance, and the number of bacteria in the digestive system, especially in the upper part of the digestive system, can increase to abnormally high levels. When this happens a condition called SIBO, or Small Intestinal Bacterial Overgrowth, may develop. This may lead to negative effects on health and well-being.

What Is SIBO?

SIBO, or Small Intestinal Bacterial Overgrowth, describes a condition either where the total number of bacteria in the small intestine increases, or there is a change in the diversity of bacterial species in the small intestine. To qualify as SIBO, these physiological changes must be accompanied by gastrointestinal discomfort or symptoms of digestive distress (Bures et al., 2010).

In a healthy digestive system, there are usually fewer bacteria in the upper part of the digestive tract compared to the lower part of the digestive tract. Gut bacteria are usually at their lowest concentrations in the small intestines and their highest concentrations in the colon (Stearns et al., 2011). In SIBO, this balance is altered and there are excessive bacteria in the small intestines, leading to gastrointestinal symptoms (Pimental et al., 2020). Moreover, the bacteria found in the upper digestive tract of people with SIBO are from species more typically found in the lower parts of the digestive system.

Symptoms of SIBO

Excessive bacteria in the small intestine can interfere with how nutrients like carbohydrates, proteins, lipids, and vitamins are metabolized and absorbed. Bacteria in the small intestines may metabolize the foods you eat before your body’s cells have the time to extract and absorb nutrients. The metabolites that these bacteria produce may then cause gastrointestinal discomfort. In addition, elevated levels of gut bacteria or their metabolites may “leak” out of the intestine, leading to an inflammatory immune response (Ghoshal & Ghoshal, 2017). The effects of excessive bacteria in the small intestine may lead to the gastrointestinal or nonspecific symptoms of SIBO.

The most common symptoms of SIBO are digestive discomfort and can include (Rao & Bhagatwala, 2019)

  • Diarrhea

  • Constipation

  • Abdominal pain or cramping

  • Bloating or abdominal distension

  • Flatulence

  • Belching

  • Indigestion

  • Nausea

In more severe or extreme cases, symptoms of SIBO can include:

  • Steatorrhea (fatty stool)

  • Weight loss

  • Anemia

  • Vitamin deficiencies (especially of the fat-soluble vitamins A, D, E, and K)

  • Mucosal inflammation

Tests for SIBO

The gold standard for SIBO testing involves taking a sample from the small intestine and testing it for bacterial content (Choung et al., 2011). This is done via a medical procedure called an endoscopy in which a long tube is interested through the mouth and is guided into the intestines. This procedure is invasive, time-consuming, expensive, and requires sedation (Pimental et al., 2020).

A simple, non-invasive, inexpensive alternative to endoscopic testing is a breath test. Breath tests rely on the fact that human cells do not produce the gases hydrogen and methane (Pimental et al., 2020). If these gases are detected in the breath, bacteria capable of producing these gases are implicated. Bacteria in the intestines metabolize the foods that you eat and produce these gases, which are then absorbed into your bloodstream and then eventually expelled through your lungs when you breathe (Gasbarrini et al., 2007). In a SIBO breath test, you ingest a specific carbohydrate (usually glucose or lactulose). You then provide a breath sample every fifteen minutes for anywhere between 90 to 240 minutes (Resaie et al., 2017).

How to Treat SIBO

The goal of treatment for SIBO is the relief of uncomfortable symptoms and restoring the small intestine’s ability to clear food and absorb nutrients. This can be achieved by treating any underlying diseases, prescribing a course of SIBO-specific antibiotics, or using a SIBO diet.

Dr. Mark Pimentel, a prominent SIBO researcher and gastroenterologist at Cedars-Sinai hospital, suggests that anyone who suspects they may have SIBO first undergo breath testing to confirm this and to determine which type of bacteria are present in excessive numbers. A course of antibiotics can then be prescribed to decrease bacterial numbers. Once the SIBO has been brought under control through the use of antibiotics, dietary changes may keep the GI tract functioning. This may involve changing the content of what you eat, taking in fewer complex carbohydrates that support intestinal bacteria. Dietary changes to reign in SIBO may also involve increasing the spacing between meals, allowing your digestive system ample time to process and digest food. This may mean a period of 4 to 5 hours between meals, although longer periods, such as in intermittent fasting, may also be beneficial.

In Sum

The bacteria that live in the human gut are essential to good health and well-being. They play a crucial part in extracting nutrients from the food that you eat and in creating the chemical building blocks of your cells. However, when these bacteria become too plentiful, or when they migrate to places where they don’t belong, you may experience symptoms of gastrointestinal discomfort.

Luckily, relief may be possible. Straightforward, noninvasive laboratory tests exist to determine whether you have excessive bacteria in your digestive tract. These tests may even be able to determine the specific bacterial species that are present in excess. Treatment may also be relatively straightforward with many people responding well to a short course of antibiotics followed by dietary changes.

References

  • Bures, J., Cyrany, J., Kohoutova, D., Förstl, M., Rejchrt, S., Kvetina, J., ... & Kopacova, M. (2010). Small intestinal bacterial overgrowth syndrome. World journal of gastroenterology: WJG, 16(24), 2978.

  • Choung, R. S., Ruff, K. C., Malhotra, A., Herrick, L., Locke III, G. R., Harmsen, W. S., ... & Saito, Y. A. (2011). Clinical predictors of small intestinal bacterial overgrowth by duodenal aspirate culture. Alimentary pharmacology & therapeutics, 33(9), 1059-1067.

  • Gasbarrini, A., Lauritano, E. C., Gabrielli, M., Scarpellini, E., Lupascu, A., Ojetti, V., & Gasbarrini, G. (2007). Small intestinal bacterial overgrowth: diagnosis and treatment. Digestive diseases, 25(3), 237-240.

  • Ghoshal, U. C., & Ghoshal, U. (2017). Small intestinal bacterial overgrowth and other intestinal disorders. Gastroenterology Clinics, 46(1), 103-120.

  • Pimentel, M., Saad, R. J., Long, M. D., & Rao, S. S. (2020). ACG clinical guideline: small intestinal bacterial overgrowth. Official journal of the American College of Gastroenterology| ACG, 115(2), 165-178.

  • Rao, S. S., & Bhagatwala, J. (2019). Small intestinal bacterial overgrowth: clinical features and therapeutic management. Clinical and translational gastroenterology, 10(10).

  • Stearns, J. C., Lynch, M. D., Senadheera, D. B., Tenenbaum, H. C., Goldberg, M. B., Cvitkovitch, D. G., ... & Neufeld, J. D. (2011). Bacterial biogeography of the human digestive tract. Scientific reports, 1(1), 1-9.

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