SIBO: The Overlooked Gut Condition
Small Intestinal Bacterial Overgrowth, or SIBO, occurs when bacteria that normally reside in the large intestine colonise the small intestine in excessive numbers. The small intestine is designed to have a relatively low bacterial population — typically fewer than 10,000 organisms per millilitre of fluid. In SIBO, this count can exceed 100,000 per millilitre.
This matters because the small intestine is where most nutrient absorption occurs. When large populations of bacteria set up residence there, they ferment carbohydrates before your body can absorb them, compete for nutrients, produce toxic metabolites, and damage the intestinal lining. The result is a constellation of symptoms that can be debilitating and difficult to diagnose.
How the Small Intestine Stays Clean
Under normal conditions, several mechanisms prevent bacterial overgrowth in the small intestine:
- The migrating motor complex (MMC): a cyclical wave of muscular contractions that sweeps through the small intestine every 90 to 120 minutes during fasting. Think of it as a housekeeper that pushes bacteria and debris toward the colon. The MMC only activates when you are not eating, which is one reason constant snacking can be problematic
- Stomach acid: hydrochloric acid acts as a sterilisation barrier, killing many bacteria before they reach the small intestine
- The ileocecal valve: a one-way valve between the small and large intestine that prevents backflow of colonic bacteria
- Bile acids: bile has natural antimicrobial properties that help control bacterial populations
- Immunoglobulin A (IgA): secreted into the gut lumen, IgA neutralises and clears bacteria
SIBO develops when one or more of these protective mechanisms fails.
Common Causes of SIBO
Impaired Motility
The most common cause of SIBO is impaired motility — reduced or absent migrating motor complex activity. This can result from food poisoning (which can damage the nerves controlling the MMC through an autoimmune mechanism), diabetes (which causes autonomic neuropathy), hypothyroidism (which slows gut motility), opioid medications, or chronic stress.
Low Stomach Acid
Long-term proton pump inhibitor (PPI) use raises stomach pH above the threshold needed to kill ingested bacteria. A meta-analysis found that PPI users had a 53% increased risk of developing SIBO compared to non-users. Advanced age, chronic gastritis, and H. pylori infection also reduce stomach acid output.
Structural Abnormalities
Surgical alterations to the gastrointestinal tract (including gastric bypass, ileocecal valve removal, and intestinal resections), diverticulosis, and adhesions from prior surgeries can create pockets where bacteria accumulate or impair the normal flow of intestinal contents.
Immune Deficiency
Conditions that impair IgA production — including IgA deficiency (the most common primary immunodeficiency, affecting roughly 1 in 500 people), chronic stress, and HIV — reduce the gut's ability to control bacterial populations.
The Three Types of SIBO
SIBO is classified by the type of gas that overgrown bacteria produce:
- Hydrogen-dominant SIBO: associated primarily with diarrhoea. Hydrogen-producing bacteria ferment carbohydrates rapidly, drawing water into the intestines through osmotic effects
- Methane-dominant (IMO): now technically classified as intestinal methanogen overgrowth. Methane-producing archaea slow gut transit, leading to constipation. Methane also directly impairs serotonin function
- Hydrogen sulphide SIBO: the newest recognised subtype, associated with diarrhoea, visceral hypersensitivity, and a distinctive rotten-egg odour to gas and stool
Why SIBO Is Underdiagnosed
SIBO shares symptoms with IBS, and many gastroenterologists do not routinely test for it. Studies suggest that up to 78% of people diagnosed with IBS actually have SIBO as the underlying cause. Standard IBS management (dietary restriction, antispasmodics) may partially manage symptoms but does not address the bacterial overgrowth itself.
Diagnosis
The primary diagnostic tool is a lactulose or glucose breath test. After drinking a sugar solution, you breathe into collection bags at timed intervals over 2 to 3 hours. Elevated hydrogen or methane in early collections indicates bacterial fermentation occurring in the small intestine. Small bowel aspirate culture (obtained during endoscopy) is considered the gold standard but is invasive and less commonly performed.
SIBO and GutIQ
GutIQ includes specific assessment parameters designed to identify symptom patterns consistent with SIBO, including the timing and character of bloating, bowel habit patterns, response to fibre, and history of food poisoning or PPI use. While GutIQ does not replace breath testing, it can help you understand whether SIBO is a likely contributor to your symptoms and guide your next steps.