What is Small Intestinal Bacterial Overgrowth (SIBO)?
SIBO is a condition where excess microorganisms proliferate in the small intestine, leading to a malabsorption syndrome characterized by bloating, abdominal distention, diarrhea, and in severe cases, steatorrhea and nutritional deficiencies. 1
Definition and Core Pathophysiology
SIBO occurs when bacteria—normally present in low numbers in the small intestine—proliferate excessively, typically with anaerobic colonic-type bacteria migrating proximally. 1 The small intestine normally maintains relatively low bacterial counts compared to the colon, and this overgrowth represents a distinct pathological state. 2
The condition develops when protective mechanisms fail:
- Gastric acid secretion normally acts as the first barrier preventing bacterial survival during passage into the small bowel 1, 3
- Intestinal motility and the migrating motor complex (MMC) clear debris and bacteria during fasting; when impaired, gut stasis allows bacterial accumulation 1, 3
- Intact ileocecal valve prevents reflux of colonic bacteria backward into the small intestine 1, 2
- Intestinal immunoglobulin secretion and bacteriostatic properties of pancreatic and biliary secretions provide additional defense 1, 2
Clinical Presentation
Primary Symptoms
Bloating and abdominal distention are the hallmark symptoms, characteristically worsening after eating due to bacterial fermentation of carbohydrates. 4 Patients experience:
- Flatulence and excessive gas production from hydrogen and methane generation 4
- Abdominal pain and discomfort, particularly shortly after eating, caused by severe painful non-propulsive contractions when gut coordination is disrupted 1, 4
- Diarrhea from multiple mechanisms 5
Advanced Disease Manifestations
When bacterial overgrowth becomes severe, malabsorption develops through specific mechanisms:
- Steatorrhea (fatty stools) occurs when bacteria deconjugate bile salts and degrade pancreatic enzymes, impairing fat digestion 1, 4
- Fat-soluble vitamin deficiencies (A, E, D, K) cause night blindness, poor color vision, dry flaky skin, and ataxia 1, 4
- Vitamin B12 malabsorption develops, while folic acid and vitamin K may paradoxically be elevated as bacteria manufacture these 1
- Weight loss and malnutrition result from chronic malabsorption, though these are uncommon in mild presentations 4
- Subtotal villous atrophy may be found on histology, with occasional protein-losing enteropathy 1
Metabolic Complications
Bacteria can produce abnormal metabolites:
- D-lactic acidosis (high anion gap acidosis) from D-lactic acid production 1
- Elevated blood ammonia from bacterial ammonia production 1
Diagnostic Challenges
There is currently no gold standard for diagnosing SIBO, and commonly available methodologies have significant limitations. 1 The British Society of Gastroenterology recommends testing rather than empirical treatment to establish diagnosis and support antibiotic stewardship. 1
Available diagnostic approaches include:
- Glucose or lactulose breath tests measuring hydrogen and methane production; using both hydrogen and methane analysis increases accuracy 1, 3
- Quantitative small bowel aspiration and culture is time-consuming but can be definitive 1
- Qualitative small bowel aspiration is easier to perform; positive aspirates grow colonic bacteria 1
A critical pitfall: SIBO symptoms overlap significantly with irritable bowel syndrome (IBS), making clinical distinction challenging without diagnostic testing. 4 Normal inflammatory markers like fecal calprotectin are expected in SIBO; elevated levels should prompt investigation for alternative diagnoses. 4
Risk Factors and Underlying Conditions
SIBO develops secondary to various predisposing conditions:
- Prior gastrointestinal surgery altering anatomy 4
- Diabetes mellitus with autonomic neuropathy disrupting MMC function 3, 6
- Pancreatic exocrine insufficiency 4
- Motility disorders and chronic intestinal pseudo-obstruction 1, 6
- Proton pump inhibitor therapy compromising the gastric acid barrier 6
- Pelvic radiation therapy damaging intestinal motility mechanisms 3
- Systemic sclerosis, cirrhosis, and other systemic diseases 6
The aetiology is usually complex, with more than one factor often involved. 1
Prevalence and Epidemiology
SIBO is detected in 33.8% of patients with gastroenterological complaints who underwent breath testing, and is significantly associated with smoking, bloating, abdominal pain, and anemia. 6 The risk increases with age and does not depend on gender or race. 6