SIBO is a pathologically excessive growth of bacteria in the small intestine.
Core Mechanism: Excessive numbers of bacteria in the small intestine (<10^5 organisms/mL is normal), leading to fermentation of carbohydrates, malabsorption, and mucosal inflammation.

Etiology

↓ Motility: Conditions that cause stasis, allowing bacteria to proliferate. Includes diabetes (autonomic neuropathy), scleroderma, intestinal pseudo-obstruction, and post-operative adhesions.
Anatomic Abnormalities: Surgical blind loops (e.g., Billroth II, Roux-en-Y), small bowel diverticula, strictures (Crohn’s disease), or ileocecal valve resection.
↓ Acid/Enzyme Secretion: Achlorhydria (chronic PPI use), atrophic gastritis, or pancreatic insufficiency.

Pathophysiology

The following result from bacterial overgrowth.
- ↑ Deconjugation of the bile acids
- ↑ Production of folate
- ↓ absorption of vitamin B12, fat-soluble vitamins, zinc, and iron