Small Intestinal Bacterial Overgrowth and Irritable Bowel Syndrome: A Bridge between Functional Organic Dichotomy

FODMAP DietGastrointestinal MotilityGut DysbiosisGut MicrobiotaHydrogen Breath Test

Small Intestinal Bacterial Overgrowth and Irritable Bowel Syndrome: A Bridge between Functional Organic Dichotomy

Why this matters for your gut health

IBS was long assumed to be mostly psychological, but this review consolidates evidence that gut dysbiosis — specifically small intestinal bacterial overgrowth (SIBO) — plays a genuine organic role in a subset of IBS patients. This reframing has direct clinical implications: it justifies testing for and treating SIBO (with antibiotics, probiotics, dietary changes) rather than treating IBS purely as a functional/psychosomatic disorder.

Summary

The review covers gut flora physiology, the protective mechanisms that normally prevent SIBO, methods of diagnosing SIBO (jejunal aspirate culture, glucose and lactulose hydrogen breath tests), and why prevalence estimates vary so widely (4–78% in IBS patients) depending on test method and population. It examines risk factors for SIBO among IBS patients, the microbiology of overgrowth, proposed mechanisms linking SIBO to IBS symptoms (gas production, bile acid deconjugation, immune activation, visceral hypersensitivity), and treatment approaches — with rifaximin emerging as the preferred antibiotic.

Key findings

SIBO prevalence ranges from 4–78% in IBS patients vs. 1–40% in controls, largely due to differing diagnostic criteria.
Glucose hydrogen breath test (GHBT) is highly specific (78–97%) but poorly sensitive (15.7–62%); lactulose breath test (LHBT) early-peak criteria are the reverse — nonspecific but often over-diagnose SIBO.
Two meta-analyses found significantly higher odds of positive SIBO tests in IBS patients vs. controls (OR 3.45–4.7 and OR 4.46).
Risk factors for SIBO in IBS include female gender, older age, diarrhea-predominant IBS, marked bloating/flatulence, PPI and narcotic use, and low hemoglobin.
Rifaximin is currently the best-supported antibiotic treatment; TARGET 1–3 trials showed meaningful symptom relief over placebo.
Probiotics and dietary interventions (e.g., low-FODMAP diets) show promise but need more controlled trials specific to SIBO-positive IBS patients.

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Citation

Ghoshal UC, Shukla R, Ghoshal U. Small Intestinal Bacterial Overgrowth and Irritable Bowel Syndrome: A Bridge between Functional Organic Dichotomy. Gut Liver. 2017;11(2):196–208.