Bloated by mid-afternoon, gassy after meals that used to be fine, and wondering if it could be SIBO? SIBO, or small intestinal bacterial overgrowth, means more bacteria than expected in your small intestine, and its symptoms overlap heavily with everyday bloating and IBS.
SIBO has been linked to bloating, abdominal pain, gas and diarrhoea [1]. It cannot be confirmed from symptoms alone. A clinician may use a hydrogen and methane breath test, while direct sampling of small intestine fluid is invasive and rarely used in routine practice [1][3].
What is SIBO?
Most gut bacteria live in the colon, and the small intestine normally holds far fewer [4]. The ACG guideline considers 10³ colony-forming units per millilitre or more in fluid from the duodenum suggestive of SIBO, although how SIBO should be defined and tested is still debated [1][4]. It can be associated with factors that alter the gut's normal defences or movement, such as previous abdominal surgery, slower gut transit time and some medicines [1][4]. For context, see what is actually living in your gut and what a healthy gut microbiome looks like, or our gut health glossary.
What are the symptoms of SIBO?
The most commonly reported are bloating, abdominal discomfort, gas, diarrhoea and, in some people, constipation [1]. Methane is made by archaea called methanogens rather than bacteria, so methane-positive cases, often linked with constipation, are now usually called intestinal methanogen overgrowth, or IMO [1]. Because symptoms overlap with bloating and gas, food intolerance, general signs your gut health is off and IBS in daily life, symptoms alone cannot confirm SIBO. A pooled analysis found SIBO was more common in people with IBS, though results varied by test [5].
| SIBO | IBS | Food intolerance | |
|---|---|---|---|
| What it is | Too many bacteria in the small intestine [1] | A disorder of gut-brain interaction with recurrent tummy pain and bowel changes [6] | Difficulty digesting specific foods |
| How it is assessed | Clinical assessment, often with a breath test [1][3] | Symptom pattern, after other causes are ruled out [6] | Guided elimination and reintroduction |
When should you see a GP?
SIBO symptoms overlap with conditions that need checking quickly, including coeliac disease and bowel cancer. Clinicians look out for specific warning signs, sometimes called red flags, that mean a symptom should be assessed rather than monitored [6].
See a GP promptly, rather than waiting to track your symptoms, if you have:
- symptoms that started after the age of 50 [6]
- weight loss you cannot explain [6]
- blood in your poo or bleeding from your bottom [6]
- iron-deficiency anaemia [6]
- a family history of bowel cancer, inflammatory bowel disease or coeliac disease [6]
Ask for an urgent GP appointment or call 111 if your poo is black or dark red, or you have bloody diarrhoea [7].
How is SIBO tested?
Usually with a breath test: you drink a glucose or lactulose solution, then breathe into tubes over two to three hours. The 2017 North American Consensus proposed a hydrogen rise of 20 parts per million or more within 90 minutes as positive, and methane of 10 parts per million or more as methane-positive [2]. Interpretation is still debated. Preparation, the sugar used and gut transit speed can all affect results, and fast transit can cause an early rise that mimics SIBO, so a positive test needs reading alongside symptoms [3]. Commercial tests differ in instructions and interpretation, as our guide to every gut health test method explains. Treatment is decided by a clinician and may involve antibiotics [1], which can affect the wider microbiome, as explored in how antibiotics affect gut health.
What should you track if you think you have SIBO?
If none of the red flags above apply, a record can help. Tracking cannot diagnose SIBO, but recording symptoms, stool patterns, meals and possible triggers can give a clinician a clearer picture over time. Note when bloating starts after eating, your stool consistency on the Bristol Stool Chart, how often you go, and any stress or poor sleep. Our guides to building a gut health baseline and the gut health metrics worth tracking walk through it. Talk to a GP or dietitian before cutting out whole food groups.
Can SIBO come back after treatment?
SIBO can recur, particularly when an underlying factor that contributed to the overgrowth remains present [1].
Do probiotics help with SIBO?
The evidence is mixed. The American College of Gastroenterology guideline found too little good-quality evidence to recommend them [1], and the same probiotic may not work the same way for everyone. Our explainer on probiotics vs prebiotics covers the basics.
Is there a SIBO diet?
No single diet has strong evidence as a treatment. Some clinicians use short-term dietary changes to ease symptoms alongside treatment, ideally with a dietitian [1]. Our guide to the gut health diet covers the broader evidence.
Check the red flags first. If any apply, book a GP appointment now. If none do, a two-week record turns "I always feel bloated" into information a clinician can use. Our guide to tracking your gut health at home shows simple ways to start.
References
- Pimentel M, Saad RJ, Long MD, Rao SSC. ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth. American Journal of Gastroenterology. 2020;115(2):165-178.
- Rezaie A, Buresi M, Lembo A, et al. Hydrogen and methane-based breath testing in gastrointestinal disorders: the North American Consensus. American Journal of Gastroenterology. 2017;112(5):775-784.
- Hammer HF, Fox MR, Keller J, et al. European guideline on indications, performance, and clinical impact of hydrogen and methane breath tests in adult and pediatric patients. United European Gastroenterology Journal. 2022;10(1):15-40.
- Bushyhead D, Quigley EMM. Small intestinal bacterial overgrowth: pathophysiology and its implications for definition and management. Gastroenterology. 2022;163(3):593-607.
- Shah A, Talley NJ, Jones M, et al. Small intestinal bacterial overgrowth in irritable bowel syndrome: a systematic review and meta-analysis of case-control studies. American Journal of Gastroenterology. 2020;115(2):190-201.
- Chey WD, Kurlander J, Eswaran S. Irritable bowel syndrome: a clinical review. JAMA. 2015;313(9):949-958.
- NHS. Symptoms of bowel cancer. nhs.uk.
This content is for informational purposes only and does not constitute medical advice. If you have concerns about your digestive health, please speak to a qualified healthcare professional.
