SIBO (Small Intestinal Bacterial Overgrowth)

Small intestinal bacterial overgrowth occurs when colonic-type bacteria colonise the small intestine, where counts should be low. Fermenting carbohydrate proximally produces gas, distension and often diarrhoea or, in methane-predominant cases, constipation. It can also impair absorption: bacterial deconjugation of bile salts causes fat malabsorption, and bacterial consumption of B12 produces deficiency alongside paradoxically elevated folate. The critical clinical point is that SIBO is a consequence, not a primary disease. The small bowel is kept relatively sterile by gastric acid, pancreatic enzymes, bile, an intact ileocaecal valve and the migrating motor complex — the sweeping wave of motility that clears the small bowel between meals. Overgrowth follows when one of those fails: post-infectious IBS damaging motility, scleroderma, diabetic autonomic neuropathy, long-term proton pump inhibitor use, adhesions, diverticula or previous gastrointestinal surgery. Recurrence rates after treatment are high precisely because the underlying defect usually persists. Diagnosis is imperfect. Breath testing after glucose or lactulose is standard but has meaningful false positive and negative rates, especially with lactulose, and jejunal aspirate culture is invasive and rarely performed. Methane-predominant overgrowth, now more accurately termed intestinal methanogen overgrowth, involves archaea rather than bacteria and responds to different antibiotics.

• •Bloating and abdominal distension, often worsening through the day
• •Excessive gas
• •Diarrhoea, or constipation in methane-predominant cases
• •Abdominal pain or cramping
• •Fatigue and brain fog
• •Unintentional weight loss in severe cases
• •B12 deficiency with normal or high folate

• •Impaired migrating motor complex, often post-infectious
• •Long-term proton pump inhibitor use reducing gastric acid
• •Structural issues: adhesions, strictures, diverticula, blind loops
• •Previous gastrointestinal surgery or ileocaecal valve resection
• •Scleroderma and other connective tissue disease
• •Diabetic autonomic neuropathy
• •Chronic pancreatitis reducing enzymatic defence

{"Failure of the migrating motor complex to clear the small bowel between meals","Loss of the acid, bile and enzymatic barriers that normally limit colonisation","Anatomical stasis from adhesions, strictures or surgical blind loops"}