SIBO Mechanisms & Diagnostic Limits
SIBO is over-diagnosed by breath testing and under-explained mechanistically. True small-bowel bacterial overgrowth is a downstream consequence of motility failure, bile acid dysregulation, and host bioenergetic collapse — not a primary infection. Recurrence is the rule because antimicrobials never address the upstream cause.
Why breath tests are fundamentally unreliable
Lactulose and glucose breath tests have poor sensitivity and specificity. Rapid orocecal transit produces false positives. Hydrogen-only protocols miss methane and hydrogen sulfide producers. The 90-minute cutoff has no validation against gold-standard small bowel aspirates.
Migrating motor complex failure
The MMC sweeps the small bowel between meals, mechanically clearing residual bacteria. Vagal dysfunction, hypothyroidism, opioid use, and post-infectious autoimmunity to vinculin all impair MMC function. Without functional MMC, even normal bacterial loads accumulate in the small bowel.
The host energy connection
Small bowel enterocytes also depend on mitochondrial energy. When systemic inflammation, NAD⁺ depletion, and oxidative stress impair enterocyte function, bile acid handling, antimicrobial peptide production, and barrier integrity all decline — creating the conditions for chronic overgrowth regardless of microbial composition.
Key terms
- SIBO
- Small Intestinal Bacterial Overgrowth — abnormally elevated bacterial counts in the small bowel, typically downstream of motility, immune, or bioenergetic failure.
- MMC
- Migrating Motor Complex — cyclic peristaltic sweeps of the small bowel between meals that mechanically clear residual bacteria.
- Vinculin antibodies
- Autoantibodies to a cytoskeletal protein in the interstitial cells of Cajal, produced after acute gastroenteritis; impair MMC function and predispose to post-infectious IBS-SIBO.
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Frequently asked
Should I get a breath test?
Breath tests have well-documented sensitivity and specificity problems and rarely change clinical management. Mechanistic case reasoning — motility, bile flow, immune competence, bioenergetic capacity — is more useful than a binary breath result.
Why does my SIBO keep coming back after rifaximin?
Because rifaximin treats the bacterial population without addressing motility failure, bile dysregulation, or host energy collapse. The terrain that selected for overgrowth is still present, so the population reassembles.