SIBO: Symptoms, Testing & Treatment

What Is SIBO?

Small intestinal bacterial overgrowth (SIBO) occurs when bacteria that normally reside in the large intestine proliferate excessively in the small intestine. The small intestine is designed to have a relatively low bacterial count; its primary job is nutrient absorption, and it relies on gastric acid, bile, motility (the migrating motor complex), and the ileocaecal valve to keep bacterial populations in check.

When these protective mechanisms are impaired — by conditions such as low stomach acid, motility disorders, structural abnormalities, or immunodeficiency — bacteria migrate and multiply in the small intestine. These bacteria then ferment carbohydrates that would normally be absorbed, producing hydrogen and/or methane gas. This fermentation causes bloating, abdominal distension, pain, diarrhoea (hydrogen-dominant) or constipation (methane-dominant, now sometimes called intestinal methanogen overgrowth or IMO).

SIBO and IBS: The Overlap

The relationship between SIBO and IBS is a topic of active debate. Studies using lactulose breath testing have reported SIBO prevalence in IBS patients ranging from 4 % to 78 %, depending on the testing method and diagnostic criteria used. Some researchers argue that SIBO may be an underlying cause of symptoms in a significant subset of IBS patients, while others contend that positive breath tests may reflect rapid transit or colonic fermentation rather than true small bowel overgrowth.

What is generally accepted is that the symptoms of SIBO and IBS are often indistinguishable — bloating, pain, altered bowel habits, and flatulence occur in both. For this reason, SIBO should be considered in the differential diagnosis of patients with IBS-like symptoms, particularly those who also have risk factors such as prior abdominal surgery, proton pump inhibitor use, or diabetes with autonomic neuropathy.

How Is SIBO Diagnosed?

The gold standard for diagnosing SIBO is jejunal aspirate culture, which involves endoscopically collecting a sample of fluid from the small intestine and counting bacterial colonies. However, this method is invasive, expensive, and not widely available.

In clinical practice, hydrogen and methane breath testing is the most commonly used method. The patient ingests a substrate — usually lactulose or glucose — and exhaled breath is measured at intervals for hydrogen and methane. A rise in hydrogen of 20 parts per million (ppm) or more above baseline within 90 minutes, or a methane level of 10 ppm or higher at any point, is considered positive under the 2017 North American Consensus guidelines. Breath testing has limitations, including moderate sensitivity and specificity, but it remains the most practical and accessible diagnostic tool.

Treatment Approaches

The primary treatment for SIBO is antibiotic therapy. Rifaximin, a non-absorbable antibiotic that acts locally in the gut, is the most studied agent and is effective in approximately 50–70 % of cases for hydrogen-dominant SIBO. For methane-dominant overgrowth, rifaximin is often combined with neomycin or metronidazole, as methanogens (archaea) are less responsive to rifaximin alone.

Herbal antimicrobials — including combinations of oregano oil, berberine, neem, and allicin — have shown comparable efficacy to rifaximin in at least one comparative study published in Global Advances in Health and Medicine. Some practitioners use these as a first-line or adjunctive treatment, though more research is needed.

Addressing the underlying cause is essential for preventing recurrence. This may involve prokinetic agents to restore the migrating motor complex, reducing or discontinuing proton pump inhibitors if appropriate, managing blood sugar in diabetic patients, or correcting structural issues surgically. Without addressing root causes, SIBO recurrence rates can be as high as 40–50 % within a year.

Dietary Strategies for SIBO

Several dietary approaches are used alongside antibiotic treatment for SIBO. The low-FODMAP diet reduces the fermentable substrates available to overgrown bacteria and can significantly improve symptoms. The Specific Carbohydrate Diet (SCD) and the Bi-Phasic Diet are alternatives that aim to starve small intestinal bacteria while maintaining adequate nutrition.

Meal spacing is another important strategy. The migrating motor complex (MMC) — the cleansing wave that sweeps bacteria and debris from the small intestine — only activates during fasting. Eating every 2–3 hours inhibits the MMC, so spacing meals 4–5 hours apart and avoiding constant snacking can support bacterial clearance.