Types of SIBO: Hydrogen, Methane, Mixed and Hydrogen Sulfide
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- 6 min read
Maybe its SIBO.
What is Small Intestinal Bacterial Overgrowth (SIBO)?
Small intestinal bacterial overgrowth (SIBO) occurs when too many bacteria, particularly those more commonly found in the large intestine, are present in the small intestine, where bacterial numbers are usually very low. Normally, easily digested carbohydrates are broken down and absorbed in the upper small intestine. This means they are no longer available for microbes further along the digestive tract to ferment. In SIBO excess bacteria or archaea are present higher up in the small intestine, so fermentation and gas production begin earlier than they should. As a result, both easily digested and harder-to-digest carbohydrates may be fermented by these overgrown microorganisms which is what causes symptoms.
Symptoms of SIBO
Common digestive symptoms include abdominal pain, bloating, visible distension, excessive gas, flatulence and diarrhoea, constipation. More than two-thirds of people with SIBO report a number of symptoms. The type and severity of symptoms may depend on the extent of the bacterial overgrowth and the level of inflammation affecting the intestinal lining. Many people live with seemingly random digestive symptoms for years or have them dismissed as “just IBS”, particularly when standard Medicare-funded PCR stool testing does not identify a pathogen from the lower bowel. However, these tests do not assess bacterial or methanogen overgrowth in the small intestine, so a negative stool result does not make SIBO any less real.
How does SIBO develop?
SIBO commonly develops when food and waste move too slowly through the digestive tract. This may occur following an illness or gastrointestinal infection, such as traveller’s diarrhoea or food poisoning, which can disrupt the nerves that control intestinal movement. Motility may also slow after surgery or anaesthesia, or because of endometriosis, adhesions, scar tissue or other structural changes that restrict normal movement through the bowel. Slower transit interferes with the digestive tract’s natural ability to clear bacteria and gives bacteria and archaea more time to ferment food residue. Other contributing factors may include reduced stomach acid and the backflow of contents from the large intestine into the small intestine due to poor function of the ileocecal valve.
SIBO can also cause symptoms beyond the digestive system. These may include fatigue, skin changes, headaches, joint pain, low mood, anxiety and brain fog. Due to the lack of specific symptoms, the diagnosis of SIBO requires comprehensive diagnostic assessment.
Can SIBO Affect Your Overall Health?
When digestion and absorption are significantly affected, SIBO may contribute to weight loss or difficulty gaining weight, fatty or oily stools, vitamin and mineral deficiencies, low blood protein levels, anaemia and, in more severe cases, malnutrition. Research has found associations between SIBO and a wide range of health conditions. A literature review published in 2024 grouped these associated conditions into 12 categories:
Gastrointestinal conditions: irritable bowel syndrome (IBS), non-alcoholic fatty liver disease (NAFLD), Crohn’s disease, coeliac disease, cirrhosis and pancreatitis
Autoimmune conditions: including systemic sclerosis
Cardiovascular conditions: heart failure, deep vein thrombosis and coronary artery disease
Metabolic conditions: diabetes, obesity and high blood lipids
Endocrine conditions: hypothyroidism, Graves’ disease and Hashimoto’s disease
Kidney conditions: chronic and acute kidney injury
Skin conditions: rosacea and psoriasis
Neurological conditions: Alzheimer’s disease, Parkinson’s disease and multiple sclerosis
Developmental conditions: autism spectrum disorder
Mental health conditions
Genetic conditions: including cystic fibrosis
Gastrointestinal cancers
**It is important to understand that an association does not necessarily mean SIBO caused the condition, or that the condition caused SIBO. Some of these findings may relate more broadly to changes in the gut microbiome rather than to SIBO specifically. Together, however, the research highlights the important relationship between the gut microbiome, digestive function and overall health.
The Four Microbial Overgrowth Patterns
SIBO is often discussed as though it were one condition, but testing can reveal four primary microbial gas patterns: hydrogen-dominant SIBO, intestinal methanogen overgrowth, combined hydrogen and methane overgrowth, and intestinal sulfide overproduction. Identifying the dominant pattern matters because each can produce different symptoms and may require a different treatment approach. There are four primary microbial gas patterns commonly discussed in relation to SIBO.
Hydrogen dominant SIBO: linked to IBS-diarrhoea type, and diarrhoea.
Intestinal methanogen overgrowth (IMO): originally called methane-dominant SIBO. However, as Methanobrevibacter smithii is now classified as archaea rather than bacteria, the term IMO has been adopted. Intestinal methanogen overgrowth is associated with IBS-constipation type and manifests with symptoms like bloating, abdominal discomfort, and constipation. There is a positive association between an elevated level of methane and constipation as methane slows down intestinal transit. Moreover, methane is correlated to a severity of constipation. Hydrogen dominant SIBO and IMO frequently present concurrently;
Methane-hydrogen dominant SIBO: mixed-type.
Hydrogen-sulfide-SIBO: linked to IBS-D with symptoms that usually extend past the gut such as fatigue, joint pain, histamine intolerance.
Hydrogen-Dominant SIBO
Hydrogen-dominant SIBO occurs when excessive bacteria in the small intestine ferment carbohydrates and produce high levels of hydrogen gas. This can speed up movement through the bowel and is commonly associated with diarrhoea, bloating, gas and abdominal discomfort. Over time, disrupted digestion and nutrient absorption may contribute to deficiencies in vitamin B12, fat-soluble vitamins and minerals. In SIBO, carbohydrate fermentation primarily produces hydrogen gas which is why is it called hydrogen-dominant SIBO.
Methane-Dominant SIBO: Intestinal Methanogen Overgrowth IMO
Intestinal methanogen overgrowth (IMO), previously called methane-dominant SIBO, involves an overgrowth of methane-producing microorganisms called archaea, particularly Methanobrevibacter smithii in the small intestine. Archaea are biologically different from bacteria, which is why IMO is now recognised as a separate condition to classic SIBO.
IMO can also cause different symptoms because methane can slow the movement of food and waste through the digestive tract, making IMO usually associated with constipation. Elevated methane levels reduce intestinal motility, leading to constipation, abdominal distension, bloating, and reflux.
In IMO, archaea convert hydrogen to produce methane which can be seen on breath testing. Complete and lasting recovery can be challenging, and some people experience periods in which symptoms improve and later return. Which is why we test, and treat accordingly using a phased approach rather than using random anti-microbials and hoping for the best.
Combined Hydrogen and Methane Overgrowth: Mixed-Type SIBO
Combined hydrogen and methane overgrowth occurs when both gases are produced at elevated levels. Symptoms may fluctuate between diarrhoea and constipation and can include abdominal pain, bloating, reflux, nausea and persistent fatigue. Because the balance of hydrogen and methane can influence symptoms and treatment response, testing is important for developing an individualised treatment approach.
Hydrogen-sulphide Dominant SIBO: intestinal sulfide overproduction (ISO)
Hydrogen sulfide–dominant overgrowth, sometimes referred to as intestinal sulfide overproduction (ISO), occurs when gut microbes produce excessive hydrogen sulfide gas. Although ISO appears to be less common than hydrogen SIBO or IMO, it may occur alone or alongside either or both conditions. ISO has been associated with diarrhoea, urgency, bloating, abdominal pain and greater overall symptom severity. In one study, people with ISO alone reported the highest pain and symptom severity scores compared to SIBO and IMO. However, these findings show an association and do not yet prove that hydrogen sulfide directly causes every symptom.
Hydrogen sulfide is primarily produced by gut bacteria, including sulfate-reducing bacteria. In normal amounts, it has important roles within the body, but excessive production may irritate the intestinal environment and contribute to symptoms. Even low-to-moderate levels may be relevant, with emerging research suggesting that untreated ISO could encourage the development of additional microbial overgrowth patterns and further gas production.
Why Identifying the Type Matters
SIBO, IMO and ISO can produce overlapping symptoms, but the microorganisms involved and gases produced are not the same. This means that a treatment that is appropriate for one pattern may be ineffective or poorly tolerated in another. Breath testing helps identify whether hydrogen, methane or hydrogen sulfide is elevated so that treatment can be tailored rather than relying on random antimicrobials and hoping for the best.
Experiencing persistent bloating, altered bowel habits, abdominal discomfort or unexplained digestive symptoms? Book an appointment to discuss whether testing may be appropriate for you.
Quick SIBO FAQs
What are the different types of SIBO? The four main microbial gas patterns are hydrogen-dominant SIBO, methane overgrowth (IMO), combined hydrogen and methane overgrowth, and hydrogen sulfide overproduction (ISO).
What is the difference between SIBO and IMO? SIBO is an overgrowth of bacteria in the small intestine, while IMO involves methane-producing archaea that may overgrow in the small or large intestine.
Which type of SIBO causes constipation? Intestinal methanogen overgrowth is most commonly associated with constipation because methane can slow intestinal transit.
Which type of SIBO causes diarrhoea? Hydrogen-dominant SIBO and hydrogen sulfide overproduction (IMO) are more commonly associated with diarrhoea, although symptoms vary between individuals.
Can you have hydrogen and methane overgrowth together? Yes, hydrogen SIBO and methane overgrowth can occur together, producing a mixed pattern with symptoms that may alternate between diarrhoea and constipation.
Does a standard stool test detect SIBO? No, a standard stool test primarily assesses the large intestine and cannot reliably identify microbial overgrowth within the small intestine.
How are SIBO, IMO and ISO tested? They are usually investigated with a non-invasive breath test that measures hydrogen, methane and, where available, hydrogen sulfide after drinking a carbohydrate solution.
