SIBO is a pathologically excessive growth of bacteria in the small intestine.

Etiology

Structural abnormalities
- Postsurgical complications, e.g.:
  - Short bowel syndrome
  - Blind loop syndrome: bacterial overgrowth in the bypassed intestinal segment (blind loop), e.g., due to gastrectomy
  - Adhesions
Motility disorders
- Irritable bowel syndrome (IBS)
- Paralytic ileus, e.g., due to opiate use
- Systemic disorders, e.g., diabetic autonomic neuropathy, systemic sclerosis, amyloidosis
Biochemical abnormalities
- Achlorhydria, e.g., due to atrophic gastritis
- Chronic pancreatitis
Immune disorders
- Common variable immunodeficiency
- Selective IgA deficiency
- HIV

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