IBS, SIBO, or IMO? Why the Difference Matters
by Dr. Cheri Wood
If you’ve been told you have IBS (irritable bowel syndrome) but still struggle with bloating, gas, constipation, or diarrhea, you’re not alone. IBS affects approximately 4–14% of the global population, depending on diagnostic criteria used.
While IBS is a very real diagnosis, it often describes a pattern of symptoms rather than identifying the underlying cause.
IBS (irritable bowel syndrome) symptoms include:
Recurrent abdominal pain
Changes in bowel habits (diarrhea, constipation, or both)
Bloating, gas, or abdominal distention
There are four types of IBS:
IBS-C (constipation predominant)
IBS-D (diarrhea predominant)
IBS-M (mixed constipation and diarrhea)
IBS-U (unclassified)
What IBS does not specify is why those symptoms are occurring. One commonly overlooked contributor is Small Intestinal Bacterial Overgrowth (SIBO) or Intestinal Methanogen Overgrowth (IMO).
What Is SIBO?
SIBO stands for Small Intestinal Bacterial Overgrowth.
Under normal circumstances, the majority of bacteria reside in the large intestine. The small intestine, where most digestion and nutrient absorption occur, contains relatively low bacterial counts.
In SIBO, excessive bacteria colonize the small intestine. When carbohydrates are consumed, these bacteria ferment them prematurely, producing gases such as hydrogen (and sometimes hydrogen sulfide).
This excess gas production can lead to:
Significant bloating (often worse as the day progresses)
Abdominal discomfort or cramping
Diarrhea or loose stools
Increased belching or flatulence
Food intolerances, especially to high-FODMAP foods
One classic presentation is minimal bloating upon waking, with progressive distention after meals and throughout the day.
What Is IMO?
IMO stands for Intestinal Methanogen Overgrowth.
While historically grouped under SIBO, IMO is technically distinct. Methane is not produced by bacteria but by archaea, specifically Methanobrevibacter smithii, which consume hydrogen and produce methane gas. The term “IMO” was introduced in updated breath testing guidelines to reflect this distinction.
Methane has been shown to slow intestinal transit time. As a result, IMO is more commonly associated with:
Constipation
Hard or infrequent stools
Sensation of incomplete evacuation
Persistent bloating
Early satiety (feeling of fullness)
Research demonstrates an association between methane production on breath testing and constipation-predominant IBS.
While hydrogen-dominant SIBO often presents with diarrhea, methane-predominant IMO more frequently presents with constipation, though overlap patterns do occur. Beyond bowel patterns, prolonged SIBO and IMO may also impair absorption of nutrients such as vitamin B12 and fat-soluble vitamins.
Why This Distinction Matters
IBS is a symptom-based diagnosis, whereas SIBO and IMO are physiologic conditions that may represent an underlying, and often treatable, cause of those symptoms.
Treatment strategies differ depending on gas pattern:
Hydrogen-predominant overgrowth may respond to targeted antimicrobial strategies and temporary carbohydrate modification.
Methane-predominant overgrowth often requires combination therapy and additional motility support.
Preventing recurrence often involves addressing underlying drivers such as slowed motility, impaired digestive function, and gut–brain nervous system regulation.
Diagnosis is typically made via breath testing, which measures hydrogen and methane gas production after ingestion of a carbohydrate substrate.
If someone is treated only under the IBS umbrella without identifying whether an overgrowth is present, symptoms may persist or recur.
Is SIBO or IMO testing right for you?
If your digestive symptoms feel stuck, cyclical, or only partially responsive to dietary changes, it may be time to look deeper.
With appropriate testing and an individualized plan, meaningful improvement is possible.
If you’re curious whether SIBO or IMO could be contributing to your symptoms, our team at Neighborhood Naturopathic is here to help guide you through next steps. I invite you to schedule a visit with myself or another provider in the clinic to learn more.
See you in clinic,
Dr. Wood
References
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Bin Waqar, S. H., & Rehan, A. (2019). Methane and Constipation-predominant Irritable Bowel Syndrome: Entwining Pillars of Emerging Neurogastroenterology. Cureus, 11(5), e4764. https://doi.org/10.7759/cureus.4764
Rezaie, A., Buresi, M., Lembo, A., Lin, H., McCallum, R., Rao, S., Schmulson, M., Valdovinos, M., Zakko, S., & Pimentel, M. (2017). Hydrogen and Methane-Based Breath Testing in Gastrointestinal Disorders: The North American Consensus. The American journal of gastroenterology, 112(5), 775–784. https://doi.org/10.1038/ajg.2017.46