SIBO and methane overgrowth, explained honestly
What it actually is
Your colon contains an enormous bacterial population. Your small intestine is supposed to contain far fewer, kept down by stomach acid, bile, immune defences and, importantly, by movement. Between meals a wave called the migrating motor complex sweeps the small intestine clear, and it is often described as the gut's housekeeper.
SIBO is what happens when that population gets too large. The bacteria ferment carbohydrate before you have absorbed it, and they do it in a narrow tube that is not built to accommodate gas. That anatomical detail explains the symptom pattern: bloating that builds through the day, visible distension after meals, and a stomach that is flat in the morning and looks pregnant by evening.
Hydrogen, methane, and why the distinction changes the plan
Breath tests measure two gases, and they point in opposite clinical directions.
| Hydrogen-predominant (SIBO) | Methane-predominant (IMO) | |
|---|---|---|
| Organism | Bacteria | Methanogenic archaea, chiefly Methanobrevibacter smithii. Not bacteria, which is why the name changed. |
| Typical bowel pattern | Looser stools, diarrhoea-predominant | Constipation. Methane appears to slow transit directly. |
| Fibre response | Variable | Often worse. Adding fibre to a slow, methane-producing gut frequently increases distension. |
| Usual treatment | Rifaximin | Rifaximin plus neomycin, since rifaximin alone is less effective here. |
This is the single most practically useful thing on the page. Someone with methane-positive overgrowth who is told to eat more fibre for their constipation is being given advice that will probably make them feel worse, and they will conclude that their gut is broken rather than that the advice was wrong for their subtype.
The tests, and their real limits
The reference standard is quantitative culture of small bowel aspirate, and the ACG's 2020 guideline sets the threshold at 103 colony-forming units per millilitre, lowered from the older 105. It needs an endoscopy, so almost nobody gets it.
In practice you get a breath test. You drink glucose or lactulose and breath samples are taken at intervals. Under the 2017 North American Consensus, a rise in hydrogen of at least 20 parts per million above baseline within 90 minutes is taken as positive for SIBO, and methane at or above 10 parts per million at any point is taken as methane-positive.
Then the honest caveats, which the clinics selling the tests tend to underplay:
- Sensitivity and specificity against culture are limited. Reviewers routinely note there is no true diagnostic gold standard in ordinary practice.
- Lactulose accelerates transit. It is not absorbed, so it can reach the colon faster than the 90-minute window assumes, and the resulting hydrogen rise reflects the colon rather than the small intestine. Glucose is more specific but only samples the proximal small intestine.
- Preparation drives the result. A low-fermentable diet the day before, an overnight fast, no antibiotics for four weeks, no laxatives or prokinetics, and no smoking or exercise during the test. Skipping these produces false positives, and home test kits vary a great deal in how clearly they say so.
- The field genuinely disagrees. How much IBS is explained by SIBO is an open question with credible researchers on both sides. That is a reason for humility, not for dismissal.
The part most treatment plans skip
SIBO is usually a consequence of something, not a primary disease. Recurrence rates after successful treatment are high, and the reason is almost always that the underlying cause is still there.
Causes worth asking about:
- Anything that impairs clearance: previous abdominal surgery and adhesions, blind loops, strictures, resection of the ileocaecal valve.
- Motility disorders: gastroparesis, scleroderma, autonomic neuropathy in long-standing diabetes.
- Medications: opioids, which markedly slow the gut, and some anticholinergics. Long-term proton pump inhibitors are frequently blamed and the evidence is genuinely mixed rather than settled.
- Structural and systemic conditions: Crohn's disease, coeliac disease, pancreatic insufficiency.
If nobody has asked you about these, the antibiotic course is treating the smoke.
Treatment, rated
- Rifaximin. The best-studied option, minimally absorbed so it acts mostly within the gut, with around 70% eradication in meta-analysis. Expensive and not always easy to obtain depending on where you are.
- Rifaximin plus neomycin for methane-positive results.
- Elemental diet. Pre-digested formula for two to three weeks, starving the bacteria of substrate. There is some evidence and it is genuinely gruelling, expensive, and hard to sustain. It is not a casual option.
- Herbal antimicrobials. Often presented as equivalent to rifaximin on the strength of a single small retrospective study. That study is real and its quality is low, and one small retrospective comparison is not a basis for equivalence claims. Some of these preparations also interact with medications.
- Prokinetics between courses are used to support the migrating motor complex and reduce recurrence. Reasonable in principle, and the evidence is modest.
Where tracking fits
Two places, and neither is diagnostic.
Before testing, a symptom log tells a clinician whether your pattern actually fits: whether distension genuinely builds through the day, whether it tracks fermentable carbohydrate, and what your stool form is doing, which is what separates the hydrogen and methane pictures. That is a better use of an appointment than describing a bad week from memory. See what a food diary should show your doctor.
After treatment, it tells you whether the course actually worked, which is harder to judge than it sounds. Symptoms fluctuate, courses are taken during bad stretches, and improvement afterwards is partly expected regardless. Comparing your symptom rate after treatment against your own established baseline is the only way to separate a real response from regression to the mean. That comparison is what Minthe's method is built to make.
Minthe is a wellness and self-tracking tool, not a medical device. It does not diagnose or treat any condition and isn't a substitute for professional medical advice. If you have red-flag symptoms such as blood in your stool, unintentional weight loss, difficulty swallowing, persistent vomiting or a fever alongside gut symptoms, see a clinician rather than tracking them.
Sources
- 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, et al. Hydrogen and methane-based breath testing in gastrointestinal disorders: the North American Consensus. American Journal of Gastroenterology, 2017.
- Quigley EMM, Murray JA, Pimentel M. AGA Clinical Practice Update on Small Intestinal Bacterial Overgrowth: Expert Review. Gastroenterology, 2020.
Frequently asked
What is SIBO?
Small intestinal bacterial overgrowth is an excess of bacteria in the small intestine, which normally carries far fewer organisms than the colon. Those bacteria ferment carbohydrate before you have absorbed it, producing gas high up in the gut where there is less room for it. That is why the classic complaint is bloating that builds through the day and distension that is visibly worse after eating.
How is it diagnosed?
The reference standard is quantitative culture of fluid aspirated from the small intestine, and the ACG 2020 guideline uses a threshold of at least 10 to the power 3 colony-forming units per millilitre. That requires an endoscopy, so in practice most diagnosis is by breath test, using either glucose or lactulose and measuring hydrogen and methane. The ACG recommends breath testing for suspected SIBO in patients with IBS.
What is the difference between SIBO and IMO?
Intestinal methanogen overgrowth involves methanogens, which are archaea rather than bacteria, so calling it "methane SIBO" was always inaccurate and the name was changed. It matters clinically, not just taxonomically: methane is associated with slow transit and constipation rather than diarrhoea, it responds to a different antibiotic combination, and adding fibre often makes it worse rather than better.
How is SIBO treated?
Usually with antibiotics. Rifaximin is the best studied and is poorly absorbed, so it acts largely within the gut, with a meta-analysis reporting an overall eradication rate of around 70%. When methane is positive, rifaximin is commonly combined with neomycin, because rifaximin alone is less effective against methanogens. An elemental diet is an alternative with some evidence and is extremely difficult to complete. Recurrence is common, which is why identifying the underlying cause matters more than the antibiotic course.
Why do people say SIBO is over-diagnosed?
Because the tests are imperfect and the diagnosis is fashionable. Breath testing has limited sensitivity and specificity against culture. Lactulose in particular speeds up transit, so an early hydrogen rise may reflect the substrate reaching the colon rather than overgrowth in the small intestine. Preparation errors cause false positives, and there is genuine, unresolved disagreement among gastroenterologists about how much irritable bowel syndrome SIBO really explains. A positive breath test is a piece of evidence, not a verdict.