SIBO: Small Intestinal Bacterial Overgrowth and Treatment Strategies

Small intestinal bacterial overgrowth (SIBO) occurs when bacteria that normally colonize the colon proliferate in the small intestine, where bacteria are normally sparse.

Normal vs Abnormal

The colon contains trillions of bacteria fermenting fiber. The small intestine normally contains fewer than 100,000 bacteria per mL—tight junction controls prevent bacterial overgrowth. When small intestinal bacterial counts exceed 100,000/mL, symptoms typically develop.

Symptoms

Bloating and abdominal distension occur shortly after eating. Gas accumulation causes discomfort and visible abdominal enlargement. Diarrhea or constipation results from altered colonic transit and bacterial fermentation patterns.

Malabsorption symptoms—fatigue, muscle weakness, nutritional deficiency—occur when bacteria prevent nutrient absorption or when bacterial damage increases intestinal permeability.

Predisposing Factors

Reduced stomach acid predisposes to SIBO—hydrochloric acid kills ingested bacteria. Medications reducing acid production increase SIBO risk. Structural abnormalities slowing small intestinal transit (adhesions, strictures, diverticulae) allow bacterial overgrowth.

Immune defects, particularly IgA deficiency, predispose to SIBO. Prior gastric surgery alters anatomy, increasing SIBO risk. Motility disorders—pseudo-obstruction, gastroparesis—allow bacterial overgrowth.

Diagnosis

Breath testing identifies SIBO. Bacteria ferment carbohydrates, producing hydrogen and methane detected in breath. Glucose or lactulose hydrogen breath tests differentiate SIBO from other causes of symptoms.

Endoscopy with small intestinal sampling is more definitive but invasive. Most clinicians use breath testing for initial diagnosis.

Treatment Approaches

Antibiotics targeting small intestinal bacteria are standard therapy. Rifaxomicin is often first-line—non-absorbed and targets small intestine specifically. Success rates are 60-70%. However, recurrence is common (up to 50% within one year).

Herbal antimicrobials (berberine, oregano oil, Japanese knotweed) show promise in some studies. They may cause fewer adverse effects than antibiotics, though evidence is less extensive.

Dietary Management

Low-carbohydrate diets starve SIBO bacteria. Elemental diets (easily absorbed liquids requiring minimal digestion) provide nutrition while starving bacteria. These approaches work alongside antimicrobial therapy.

Addressing Predisposing Factors

If possible, increase stomach acid through dietary means (bitter herbs, proper meal timing) or address underlying causes. Treating motility disorders prevents recurrence.

Prevention of Recurrence

Many people experience SIBO recurrence. Addressing underlying predisposing factors is essential for durable treatment. Gut motility enhancement through exercise and dietary fiber helps prevent recurrence.