What Is SIBO?
Small Intestinal Bacterial Overgrowth (SIBO) occurs when bacteria colonise the small intestine in abnormally high numbers. The small intestine is supposed to be relatively sterile compared to the colon — it is the site of nutrient absorption, and bacterial fermentation here interferes with that process in ways that produce distinctive and often debilitating symptoms.
SIBO is found in up to 78% of IBS patients in some studies, making it one of the most clinically significant underlying mechanisms of IBS — and one of the most underdiagnosed. If you have IBS symptoms that have never fully responded to dietary changes or standard treatments, SIBO is worth investigating.
SIBO Symptoms: What Makes Them Distinctive
SIBO and IBS share many symptoms, but there are patterns that suggest SIBO specifically:
- Upper abdominal bloating: SIBO causes bloating in the upper abdomen (above the navel), beginning within 30–90 minutes of eating. Standard IBS bloating is more often lower abdominal and develops over hours. Upper bloating that appears within an hour of any meal is a hallmark SIBO pattern.
- Excessive gas and belching: Bacterial fermentation in the small intestine produces hydrogen and/or methane gases that travel upward as well as downward. Frequent belching alongside bloating is more characteristic of SIBO than of standard IBS.
- Rapid onset after eating: SIBO symptoms often develop faster than typical IBS symptoms because fermentation occurs high in the GI tract where food reaches quickly.
- Nutritional deficiencies: Bacterial overgrowth in the small intestine can interfere with absorption of fat-soluble vitamins (A, D, E, K), B12, and iron. Unexplained deficiencies alongside gut symptoms are a red flag for SIBO.
- Diarrhea or constipation: Hydrogen-dominant SIBO typically causes diarrhea. Methane-dominant SIBO (sometimes called Intestinal Methanogen Overgrowth or IMO) more often causes constipation. Mixed or alternating patterns occur in hydrogen-sulphide SIBO.
- Brain fog after eating: Bacterial metabolites produced in SIBO (including D-lactic acid) can enter the bloodstream and affect cognition. Post-meal brain fog is reported disproportionately by SIBO patients.
How SIBO Differs from Standard IBS
The distinction matters because the treatment approaches differ significantly:
- Location of fermentation: In standard IBS, fermentation occurs primarily in the colon (large intestine). In SIBO, it occurs in the small intestine. This explains why SIBO bloating is upper abdominal and rapid-onset, while IBS bloating is lower and develops over hours.
- Probiotics: Standard probiotics can worsen SIBO by adding more bacteria to an already overpopulated small intestine. If you have SIBO, probiotics that help most IBS patients may make you feel worse — a diagnostic clue in itself.
- Fibre: High-fibre foods that help IBS-C can worsen SIBO bloating because bacteria ferment the fibre in the small intestine before it reaches the colon.
- Diagnosis: SIBO is diagnosed via hydrogen and methane breath testing (lactulose or glucose breath test), not by symptoms alone. A gastroenterologist can arrange this.
The SIBO Diet: Low-Fermentation Eating
The dietary approach for SIBO focuses on removing foods that feed bacterial overgrowth in the small intestine. The most evidence-based approach is a combination of low-FODMAP and low-starch eating, sometimes called the "Low Fermentation Diet."
Foods to Reduce During SIBO Treatment
- All FODMAPs: Fermentable carbohydrates are the primary fuel for small intestinal bacteria. Following a full low-FODMAP protocol (not just partial restriction) is usually necessary.
- Starches: Starchy vegetables (potatoes, sweet potatoes, corn) and refined grains ferment readily. Rice is generally better tolerated than wheat or oats during active SIBO.
- Sugar and sweeteners: Simple sugars and artificial sweeteners both feed bacteria and should be minimised.
- Fibre supplements: Psyllium, inulin, and FOS (fructooligosaccharides) are highly fermentable and should be avoided during SIBO treatment.
Foods Generally Well-Tolerated with SIBO
- Proteins: eggs, chicken, fish, meat (no breading or sauces)
- Low-FODMAP vegetables cooked (zucchini, carrots, green beans, spinach)
- White rice (lower fermentation than other grains)
- Olive oil, coconut oil
- Lactose-free dairy or hard aged cheeses (low lactose)
Why Food Tracking Is Especially Important with SIBO
SIBO symptoms are highly dose-dependent and food-combination-dependent. A small portion of a high-fermentation food may be tolerated; a large portion of the same food may cause severe bloating within an hour. Two foods that are individually tolerated may, when combined, produce symptoms. This complexity makes SIBO one of the hardest conditions to manage through memory and intuition alone.
Systematic tracking of meals alongside symptoms — with timestamps that capture the rapid-onset pattern of SIBO (30–90 minutes) as well as delayed effects (6–12 hours for motility changes) — is the most reliable way to map your individual tolerance thresholds.