What is Small Intestinal Bacterial Overgrowth (SIBO)?

NOTE: This article was originally published on September 1, 2019, and was updated on August 18, 2026. This is the first article in a series of three about Small Intestinal Bacterial Overgrowth (SIBO) and covers what SIBO is, how common it is, and its primary symptoms. The second article in the series is about Diagnosing Small Intestinal Bacterial Overgrowth (SIBO), and the third and final article is about treating SIBO, including antimicrobials and dietary changes.

What is SIBO?

Normally, the small intestine, which is made up of the duodenum, jejunum, and ileum [1,2], contains very few bacteria; however, when bacteria that normally populate the large intestine (colon) “spill over” into the small intestine, this is called Small Intestinal Bacterial Overgrowth (SIBO). 

SIBO has traditionally been classified as either methane-dominant SIBO, which slows gut transit time resulting in constipation, or hydrogen-dominant SIBO, which increases gut transit time resulting in diarrhea [1]. 

In 2020, the American College of Gastroenterology expanded the definition into three distinct microbial subtypes, of which only two are bacterial in origin [2]. These three subtypes are;

Hydrogen-Dominant SIBO caused by bacteria that break down carbohydrates, producing hydrogen gas, which accelerates gut motility, resulting in diarrhea, gas, and bloating,

Intestinal Methanogen Overgrowth (IMO), formerly called “methane-dominant SIBO“; however, since the methane gas is produced by single-celled organisms called archaea and not bacteria, the subtype is classified as Intestinal Methanogen Overgrowth (IMO) rather than SIBO. The methane produced by these archaea delays gut transit time and drives chronic constipation,

Hydrogen Sulfide SIBO: A recognized third gas subtype driven by sulfate-reducing bacteria in the small bowel. Elevated hydrogen sulfide levels drive rapid transit time (diarrhea), stool urgency, visceral hypersensitivity, and abdominal pain [2].

The body has several mechanisms to prevent bacteria from the large intestine ending up in the small intestine, including:

High stomach acid, which kills most bacteria in the small intestine, 

The ileocaecal valve, which acts as a barrier between the small and large intestines,

Secretions from the pancreas and bile also help keep bacteria from reproducing in the wrong areas.

What Are Some Causes and Risk Factors For SIBO?

SIBO can occur for several reasons, including low stomach acid (achlorhydria), pancreatic insufficiency, anatomical abnormalities such as obstructions or diverticula, motility disorders (dysmotility) common in diabetes, and Ehlers-Danlos Syndrome (hEDS). While heavy alcohol consumption was long a known risk factor [3], it is now known that there is an association between moderate alcohol consumption (one drink per day for women) and SIBO [4], likely due to injury to mucosal cells and slowed intestinal contractions, which contributes to dysmotility.

How Common is SIBO?

The prevalence of SIBO among young and middle-aged adults is currently estimated to range between 2.5% and 22% [5], and the risk increases with age.

Why Does the Risk of SIBO Increase With Age?

The risk of developing SIBO increases steadily with age due to age-related physiological declines, such as decreased stomach acid production (hypochlorhydria), slowed “transit time” (decreased gut motility), higher incidence of comorbidities like type 2 diabetes, diverticulosis, and the frequent use of proton pump inhibitors.

What are the Symptoms of SIBO?

Many symptoms of SIBO overlap with Irritable Bowel Syndrome (IBS), including abdominal pain, bloating, gas, and alternating bouts of diarrhea or constipation. 

Clinical Reflections

Some people are diagnosed with IBS but find that their symptoms don’t improve despite implementing appropriate dietary changes, and in such cases, it is worth investigating whether SIBO may be present. The next article, Diagnosing Small Intestinal Bacterial Overgrowth (SIBO), explains how SIBO is diagnosed. The first step toward effective treatment and recovery is understanding the underlying mechanisms, from stomach acid to intestinal motility, which is what this article provides.

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Quick Clinical Summary

Q: What is Small Intestinal Bacterial Overgrowth (SIBO)?

A: SIBO is a clinical condition characterized by an abnormal increase in the number and/or type of bacteria in the small intestine. Specifically, it involves bacteria that are normally found in the large intestine migrating or ‘spilling over’ into the small intestine, where they do not belong.

Q: What are the risk factors for developing SIBO?

A: Primary risk factors include low stomach acid (achlorhydria), pancreatic insufficiency, anatomical abnormalities such as obstructions or diverticula, motility disorders (dysmotility) common in diabetes and Ehlers-Danlos Syndrome (hEDS), even moderate alcohol consumption, and increased age.

Q: Can SIBO cause symptoms outside of the digestive tract?

A: Yes. While the most common symptoms are bloating, gas, and abdominal pain, SIBO has been clinically associated with systemic symptoms like joint pain and chronic widespread pain conditions such as fibromyalgia.

References

1. Bures J, Cyrany J, Kohoutova D, et al. Small intestinal bacterial overgrowth syndrome. World Journal of Gastroenterology. 2010 Jun 28;16(24):2978-90. [https://doi.org/10.3748/wjg.v16.i24.2978]
2. Pimentel, M., Saad, R. J., Long, M. D., & Rao, S. S. C. (2020). ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth. American Journal of Gastroenterology, 115(2), 165–178. https://doi.org/10.14309/ajg.0000000000000501 
3. Hauge T, Persson J, Danielsson D. Mucosal bacterial growth in the upper gastrointestinal tract in alcoholics (heavy drinkers). Digestion. 1997;58(6):591-5. [https://doi.org/10.1159/000201507]
4. Gabbard SL, Lacy BE, Levine GM, et al. The impact of alcohol consumption and cholecystectomy on small intestinal bacterial overgrowth. Digestive Diseases and Sciences. 2014 Mar;59(3):638-44. [https://doi.org/10.1007/s10620-013-2960-y]
5. Efremova, I., Maslennikov, R., Poluektova, E., et al. (2023). Epidemiology of small intestinal bacterial overgrowth. World Journal of Gastroenterology, 29, 3400–3421. https://doi.org/10.3748/wjg.v29.i22.3400 

6. Silva, B. C. D., Ramos, G. P., Barros, L. L., et al. (2025). Diagnosis and treatment of small intestinal bacterial overgrowth: An official position paper from the Brazilian Federation of Gastroenterology. Arquivos de Gastroenterologia, 62. https://doi.org/10.1590/s0004-2803.24612024-107

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