IMO (intestinal methanogen overgrowth) is the reclassification of what used to be called "methane SIBO". It differs from classic SIBO in three ways: the microorganism involved (methanogenic archaea rather than bacteria), the gas that shows up on the breath test (methane, CH4, rather than hydrogen, H2) and the predominant symptom (constipation in IMO versus diarrhoea in classic SIBO).
01 · DefinitionWhat is IMO?
IMO is an overgrowth of methanogens: microorganisms that produce methane as a by-product of their metabolism. The most relevant one in the human gut is Methanobrevibacter smithii. These microorganisms consume the hydrogen generated by other bacteria during fermentation and convert it into methane.
An important nuance: methanogens can colonise not only the small intestine but also the colon. That is why the term "methane SIBO" was imprecise: the word SIBO refers specifically to the small intestine, and methanogen overgrowth is not confined to that stretch. For this reason the literature has adopted the name IMO, which describes the phenomenon more accurately without tying it to a single location.
02 · Key differenceSIBO vs IMO: the key difference.
The confusion between the two conditions comes from the fact that they share symptoms and are detected with the same test. But there are two underlying differences worth being clear about:
- The microorganism. Classic SIBO is an overgrowth of bacteria in the small intestine. IMO is an overgrowth of methanogenic archaea, a type of microorganism distinct from bacteria, with a metabolism of its own.
- The gas. In SIBO, bacteria ferment carbohydrates and release hydrogen (H2), which is what the breath test measures. In IMO, methanogens consume that hydrogen and convert it into methane (CH4); that is why the test must measure methane as well as hydrogen to detect it.
Put another way: in hydrogen SIBO the gas the bacteria produce accumulates; in IMO that hydrogen "disappears" from the count because the methanogens turn it into methane. A test that measures only hydrogen can miss an IMO.
03 · SymptomsSymptoms.
Both conditions share a common backdrop of bloating, abdominal distension, gas and digestive discomfort. Where they tend to diverge is in bowel habit:
- Hydrogen SIBO: more often associated with diarrhoea, urgency and looser stools. Excess hydrogen and rapid fermentation speed up transit.
- IMO / methane: more often associated with constipation and persistent bloating. Methane slows intestinal transit, so stools become slower and harder to pass.
This split is a tendency observed in the literature, not an exact rule. Some people have mixed presentations and others have symptoms that don't fit the pattern neatly. If you suspect one or the other, bowel habit is a clue, but it never replaces a test and a medical assessment. These conditions also overlap with IBS, which makes diagnosis by symptoms alone more difficult.
04 · DiagnosisDiagnosis.
SIBO and IMO are diagnosed with the same type of test: a breath test. The process involves taking a fermentable substrate on an empty stomach —usually lactulose or glucose— and breathing into a device at regular intervals over several hours. The device measures the exhaled gases.
The key to distinguishing IMO is that the test measures methane (CH4) as well as hydrogen (H2). As an indicative reference often cited in the literature, a methane value of 10 ppm or more at any point in the test points towards IMO; for hydrogen SIBO, other thresholds based on H2 are used.
These figures are indicative only. The test is always interpreted and the diagnosis made by a doctor, who weighs the results alongside your symptoms, your medical history and other tests. A single value in a table is not a diagnosis.
05 · ManagementDiet and management.
The approach to SIBO and IMO is always decided with a healthcare professional. The underlying treatment (which may include medication) is beyond the scope of this article and is a matter for your doctor. On the dietary side, the Low FODMAP diet is one of the tools commonly used to reduce the fermentable substrate that feeds these microorganisms and so help manage symptoms, always under the supervision of a dietitian specialising in digestive disorders.
The Low FODMAP diet is structured in phases and is not meant to be followed strictly and indefinitely. If you want to organise your meals during the initial phase, a ready-made Low FODMAP weekly menu for seven days may help. Remember that no diet and no product treats or cures SIBO, IMO or any other digestive disorder on its own: they are management tools within a plan defined by your medical team.
06 · FAQFrequently asked questions about SIBO and IMO.
Is IMO the same as SIBO?
Not exactly. IMO (intestinal methanogen overgrowth) is the reclassification of what used to be called "methane SIBO". The name was changed because the microorganism responsible is not a bacterium but a methanogenic archaeon (chiefly Methanobrevibacter smithii), and because its overgrowth is not limited to the small intestine as happens in classic SIBO. They share symptoms and diagnostic method, but the microorganism and the gas they produce are different.
How is IMO diagnosed?
It is diagnosed with the same breath test as SIBO: you take a substrate (lactulose or glucose) and the exhaled gases are measured over several hours. The difference is that the device must measure methane (CH4) as well as hydrogen (H2). A methane value of 10 ppm or more at any point in the test is considered indicative of IMO. In any case, the test is interpreted and the diagnosis made by a doctor.
Does methane cause constipation?
Excess methane is associated with constipation and bloating. Methane slows intestinal transit, so people with IMO tend to have constipation, whereas hydrogen SIBO is more often associated with diarrhoea. This is an association observed in the literature, not an absolute rule: symptoms vary from person to person and should always be assessed by a professional.
This content is for information only and does not constitute a diagnosis or medical advice. If you have persistent digestive symptoms, consult a healthcare professional.