Quick answer: “Methane SIBO” is now called intestinal methanogen overgrowth (IMO). It describes too many methane-producing microbes, mainly an archaeon called Methanobrevibacter smithii, which can live in the small or large intestine. Methane slows the movement of the gut, so IMO is commonly associated with constipation, hard stools and bloating. A breath test with methane of 10 ppm or more at any point is the usual cut-off. Medical options are decided by your doctor; food, fibre choices, meal spacing and motility support may help alongside.
If your breath test came back “methane positive” and you have struggled with constipation for years, the two are very likely connected. Methane overgrowth behaves differently from the hydrogen-type SIBO most articles describe, and that changes what tends to help. If you are new to SIBO, start with what SIBO is and why it happens, or read how it differs from IBS in SIBO or IBS?
What is methane SIBO, and why is it now called IMO?
SIBO means an excess of bacteria in the small intestine. Methane, however, is not made by bacteria. It is made by archaea, a separate group of single-celled organisms, and the main one in humans is Methanobrevibacter smithii. Because these organisms are not bacteria, and because they can overgrow in the large intestine as well as the small intestine, the American College of Gastroenterology guideline proposed the term intestinal methanogen overgrowth, or IMO.1
Methanogens feed on hydrogen produced by other gut bacteria and turn it into methane. That is why some people have high methane but little hydrogen on their test: the archaea are “using up” the hydrogen.
Why does methane cause constipation?
Methane is not an inert gas in the gut. In experimental work, infusing methane into the small intestine slowed transit by around 59% and increased non-propulsive contractions, the kind that churn rather than move contents forward.3 A recent meta-analysis found that people with IMO are significantly more likely to report constipation than people without it.4
This creates a loop: slower transit gives microbes more time to ferment, which can mean more gas and more bloating, which can in turn feed more methane production.
What are the symptoms of IMO?
- Constipation, often long-standing, with hard or pellet-like stools (Bristol type 1–2)
- Bloating and visible abdominal distension, often worse as the day goes on
- A feeling of incomplete emptying
- Less urgency or diarrhoea than in hydrogen-type SIBO
- Symptoms that did not change much with fibre supplements, or got worse with them
None of these are specific to IMO. The same picture can come from pelvic floor problems, thyroid issues, medication side effects or a low-fibre, low-fluid diet. A breath test helps make sense of the pattern but does not explain everything on its own.
How is methane SIBO tested?
IMO is assessed with a lactulose or glucose breath test that measures both hydrogen and methane. The North American Consensus uses a methane level of 10 ppm or more at any point during the test as positive.2 Because methane can come from the large intestine, an early rise is less important than for hydrogen.
Preparation matters: antibiotics, some supplements and laxatives can change results, and the day-before diet must be followed carefully. Your doctor or the testing lab will give you the protocol. Read chronic constipation: the root causes for other factors worth looking at at the same time.
When to see your doctor first
- Blood in the stool or black, tarry stools
- Unintended weight loss, anaemia or night sweats
- A change in bowel habit after the age of 50, or a family history of bowel cancer or inflammatory bowel disease
- Constipation with severe pain, vomiting or inability to pass wind (seek urgent care)
- New constipation after starting a medication
What may help with methane-type constipation?
Medical options for IMO, such as specific antibiotic combinations, are decided by your doctor.1 Nutritional therapy works alongside that, looking at why the methanogens were able to thrive and at what keeps the gut moving. Approaches commonly explored include:
- Meal spacing. Leaving 3–4 hours between meals, without grazing, gives the migrating motor complex (the gut’s “cleaning wave”) time to work.
- Fibre type, not just quantity. Some people with IMO feel worse with large amounts of fermentable fibre. Gentler options are often introduced slowly. In one trial, two green kiwifruit a day improved constipation as well as psyllium and prunes, and were well tolerated.5
- Fluids and movement. Enough water, plus a daily walk, both support transit.
- Toilet routine and posture. A regular time after breakfast, and a footstool to raise the knees, can make emptying easier.
- Motility support. Ginger and certain other options are sometimes used to support gut motility. These are optional, individual and should be checked with your doctor if you take medication.
- Looking upstream. Thyroid function, iron, stress, previous food poisoning and past antibiotic courses can all play a part.6
Constipated, bloated and methane positive?
In a free 15-minute call we can talk through your test results and history and see whether working together makes sense for you.
Book your free 15-minute call →Why does IMO keep coming back?
Recurrence is common when the reason for the overgrowth is still there. Slow motility, a past bout of food poisoning, structural issues or ongoing constipation can all allow methanogens to rebuild. That is why a root-cause approach looks beyond the breath test result to what is keeping transit slow.
How can nutritional therapy help?
Nutritional therapy does not replace your doctor’s assessment or medical care. What it can offer is time to map your symptoms, diet, routines and history, and to build a personalised, step-by-step plan around food, meal rhythm and lifestyle that works alongside your medical care. Any supplements are optional and chosen for you individually.
Frequently asked questions
Is methane SIBO the same as SIBO?
Not exactly. Classic SIBO is an overgrowth of bacteria in the small intestine and is linked to hydrogen. Methane is made by archaea, which can overgrow in the small or large intestine, so the current term is intestinal methanogen overgrowth (IMO).
What methane level is positive on a breath test?
Under the North American Consensus, a methane level of 10 ppm or more at any point during the breath test is considered positive.
Can IMO cause diarrhoea?
It is much less typical. Methane tends to slow the gut, so constipation is the usual pattern. Some people have both hydrogen and methane raised and may alternate between constipation and loose stools.
Should I follow a low-FODMAP diet for methane SIBO?
A low-FODMAP diet can ease bloating in the short term for some people, but it is restrictive and not meant to be followed long term. It is best done with guidance and followed by a structured reintroduction.
Do probiotics help with IMO?
The evidence is mixed and depends on the strain. Some people feel better and some feel more bloated. If you want to try one, it is worth doing so with guidance and one change at a time.
References
- Pimentel M, et al. ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth. Am J Gastroenterol. 2020. PubMed: PMID 32023228
- Rezaie A, et al. Hydrogen and Methane-Based Breath Testing in Gastrointestinal Disorders: The North American Consensus. Am J Gastroenterol. 2017. PubMed: PMID 28323273
- Pimentel M, et al. Methane, a gas produced by enteric bacteria, slows intestinal transit and augments small intestinal contractile activity. Am J Physiol Gastrointest Liver Physiol. 2006. PubMed: PMID 16293652
- Mehravar S, et al. Symptom Profile of Patients With Intestinal Methanogen Overgrowth: A Systematic Review and Meta-analysis. Clin Gastroenterol Hepatol. 2025. PubMed: PMID 39147218
- Chey SW, et al. Exploratory Comparative Effectiveness Trial of Green Kiwifruit, Psyllium, or Prunes in US Patients With Chronic Constipation. Am J Gastroenterol. 2021. PubMed: PMID 34074830
- Barlow GM, et al. Modern concepts of small intestinal bacterial overgrowth. Curr Opin Gastroenterol. 2025. PubMed: PMID 40960427
This article is for educational purposes and is not medical advice. Nutritional therapy does not replace medical care. Always work with your doctor for any persistent or new symptoms and before making changes to your care, medication or supplements.