SIBO (Small Intestinal Bacterial Overgrowth): Causes, Diagnosis, and Treatment

SIBO (Small Intestinal Bacterial Overgrowth) is a condition in which bacteria proliferate in excessive numbers in the small intestine. Under normal conditions the small intestine contains fewer than 100,000 organisms per milliliter — far less than the colon. In SIBO, bacterial populations exceed this threshold, impairing digestion and nutrient absorption. ## Symptoms Common presentations include bloating, abdominal pain, excessive gas, diarrhea, and constipation. Chronic cases can progress to malabsorption of fats, proteins, and carbohydrates, leading to nutritional deficiencies — particularly vitamin B12 deficiency (bacteria consume B12 before it can be absorbed) and iron deficiency anemia. Fatigue and weight loss may follow in prolonged cases. ## Causes and Risk Factors SIBO develops when the normal mechanisms that keep bacteria out of the small intestine are compromised. Key risk factors include impaired gut motility (slow intestinal movement allows bacteria to accumulate), anatomical abnormalities such as diverticula or surgical blind loops, reduced stomach acid production (proton pump inhibitors increase risk), immune deficiency, and prior resection of the ileocecal valve (which normally blocks colonic bacteria from migrating upward). ## Diagnosis The most common clinical test is the hydrogen/methane breath test: the patient ingests glucose or lactulose, and breath samples are analyzed over 2–3 hours. Bacteria fermenting the sugars produce hydrogen or methane gas, which is absorbed and exhaled. Elevated hydrogen correlates with IBS-D; elevated methane correlates with IBS-C. The gold standard is jejunal aspirate culture (>100,000 bacteria/mL), but this is invasive and rarely performed. ## Treatment The preferred antibiotic is Rifaximin (Xifaxan): Gut-Targeted Antibiotic for IBS-D, a minimally absorbed, gut-targeted antibiotic. Off-label SIBO dosing is typically 1,200 mg/day for two weeks. An elemental diet — pre-digested nutrients that starve bacteria by being absorbed before reaching the bacterial population — normalizes breath tests in approximately 73% of cases over two weeks. Prokinetic drugs (to restore gut motility) are used post-treatment to reduce relapse. ## Relationship to IBS The overlap between SIBO and IBS is clinically significant. Some studies report that up to 80% of IBS patients test positive for SIBO via breath test. IBS-D patients tend to show elevated hydrogen; IBS-C patients show elevated methane. The mechanism is contested — it is unclear whether breath test abnormalities reflect true bacterial overgrowth or simply altered gut fermentation patterns. Nonetheless, rifaximin is FDA-approved for IBS-D, partly because treating the presumed bacterial component reliably reduces symptoms. Irritable Bowel Syndrome (IBS): Overview, Diagnosis, and Statistics Rifaximin (Xifaxan): Gut-Targeted Antibiotic for IBS-D