IBS or SIBO? How to Tell the Difference (and Why It Matters)
- Hannah Sharpe da Rosa
- Mar 14
- 5 min read
Many people living with ongoing digestive symptoms are told they have irritable bowel syndrome (IBS). But for some, the real issue may be small intestinal bacterial overgrowth (SIBO).
The difficulty is that IBS and SIBO can look remarkably similar. Both can cause bloating, abdominal discomfort, wind, diarrhoea or constipation, which means it is easy for one to be mistaken for the other. In fact, some research suggests that around one in three people with IBS-type symptoms may also have SIBO.
Understanding the difference matters. When the root cause is identified more accurately, treatment can often be more targeted, more effective and more supportive of long-term digestive health.
What Is IBS?
IBS is a functional gastrointestinal disorder. In simple terms, this means it is diagnosed based on a pattern of symptoms rather than a visible structural problem in the digestive tract.
Common symptoms of IBS include:
Abdominal pain or cramping
Bloating
Excess gas
Diarrhoea, constipation, or alternating bowel habits
Changes in stool frequency or consistency
IBS is very common, affecting an estimated 1 in 7 people worldwide.
The exact cause is still not fully understood, but researchers believe IBS may involve a combination of factors, including:
Altered gut motility
Changes in the gut microbiome
Increased gut sensitivity
Stress and disruption of the gut-brain axis
Because IBS describes a cluster of symptoms rather than a single disease, the underlying drivers can vary from person to person.
What Is SIBO?
SIBO stands for small intestinal bacterial overgrowth.
It happens when bacteria that normally belong in the large intestine begin to build up in the small intestine, where bacterial levels are usually much lower. When this happens, these bacteria start fermenting carbohydrates too early in the digestive process, producing gas and leading to bloating, discomfort and other digestive symptoms.
Common symptoms of SIBO include:
Significant bloating or abdominal distension
Excess gas
Abdominal discomfort
Diarrhoea or constipation
Fatigue
Nutrient deficiencies, especially vitamin B12, iron, and fat-soluble vitamins A, D, E and K
In some cases, SIBO can interfere with digestion and absorption enough to contribute to nutritional deficiencies over time.
Why IBS and SIBO Are Often Confused
IBS and SIBO overlap considerably in the way they present. Both can cause:
Bloating
Gas
Abdominal pain
Diarrhoea
Constipation
Because IBS is often diagnosed largely from symptoms, it is possible for an underlying issue such as SIBO to be missed.
At the same time, not everyone with IBS has SIBO, and the relationship between the two is still being explored. For some people, SIBO may be a contributing factor. For others, it may not be relevant at all.
IBS vs SIBO: What Is the Difference?
Although symptoms often overlap, there are some important differences.
IBS | SIBO |
A functional disorder diagnosed by symptoms | A bacterial overgrowth in the small intestine |
Abdominal pain is often the dominant symptom | Bloating and distension are often more prominent |
Usually linked more broadly to bowel function and gut sensitivity | Linked to bacterial fermentation in the small intestine |
Rarely causes nutrient deficiencies directly | Can lead to malabsorption and nutrient deficiencies |
Multiple possible causes | More specific microbial imbalance |
In simple terms, IBS is often more pain-predominant, while SIBO is often more bloating-predominant.
Signs your IBS might actually be SIBO
Only appropriate testing can confirm SIBO, but some patterns may suggest that bacterial overgrowth could be involved.
These include:
severe bloating, especially after meals
bloating that worsens as the day goes on
excessive gas or burping
unexplained nutrient deficiencies
symptoms triggered by fermentable carbohydrates
symptoms that have not improved with typical IBS support
Some people also find that symptoms began after:
food poisoning
antibiotic use
digestive infections
medications that affect motility or stomach acid
The different types of SIBO
SIBO is not always the same. Different microbes can dominate in the small intestine and produce different gases. These gases are often measured during breath testing and can influence both symptoms and treatment strategy.
Hydrogen-dominant SIBO
This is the most commonly recognised type.
When bacteria produce excess hydrogen gas, symptoms may include:
bloating and abdominal distension
excess gas
abdominal discomfort
diarrhoea
faster gut transit
Symptoms often appear soon after eating, especially after foods high in fermentable carbohydrates such as onions, garlic, legumes and certain fruits.
Methane-dominant SIBO (IMO)
Methane-dominant SIBO is often referred to more accurately as intestinal methanogen overgrowth (IMO).
Methane is associated with slower gut motility, which is why this pattern is more often linked with:
constipation
sluggish digestion
bloating
feeling overly full after meals
Methane can also be present alongside hydrogen, so mixed patterns are common.
Hydrogen sulfide SIBO
Hydrogen sulfide SIBO is a newer and still emerging area of research.
Symptoms may include:
abdominal pain
bloating
diarrhoea
increased food sensitivities
gas with a “rotten egg” smell
This type may also be associated with irritation in the digestive tract and possible effects on gut barrier function in some individuals.
Why identifying the cause matters
If symptoms are being driven by SIBO rather than IBS alone, the most effective support may be different.
Support for SIBO often focuses on:
reducing bacterial overgrowth
supporting healthy gut motility
adjusting fermentable carbohydrate intake
helping restore the gut environment during recovery
This is why identifying the underlying cause is so important. Treating symptoms alone may only provide partial relief if the real driver has not been addressed.
SIBO treatment: what you need to know
Treatment for SIBO often involves a combination of approaches, depending on the individual.
Reducing bacterial overgrowth
This may involve prescribed antibiotics such as rifaximin, or in some cases herbal antimicrobials.
Correcting nutrient deficiencies
Because SIBO can affect absorption, short-term support may be needed for nutrients such as vitamin B12, iron, or fat-soluble vitamins.
Personalised dietary support
Some people feel better when fermentable carbohydrates are temporarily reduced. This may involve a lower carbohydrate, lactose-free, or low-FODMAP-style approach.
However, diet should always be personalised. There is no single plan that works for everyone, and highly restrictive diets are rarely the long-term answer.
Supporting gut motility
Prokinetics may be used to help support motility and reduce the risk of symptoms returning.
It is also important to explore and address any underlying factors, as SIBO can recur if issues such as poor motility, microbiome disruption, chronic stress or low stomach acid are left unresolved.
When to seek support
Persistent bloating, abdominal discomfort, excess gas and unpredictable bowel habits should not be ignored.
If you have been diagnosed with IBS but continue to struggle despite making dietary changes or trying standard treatment approaches, it may be worth exploring whether other factors, including SIBO, could be contributing.
A personalised nutrition approach can help identify triggers, support digestive health and create a plan tailored to your symptoms and your body.
Final thoughts
IBS and SIBO share many symptoms, which is why they are so often confused. But they are not the same thing.
IBS is a broad syndrome with a number of potential causes. SIBO is a more specific issue involving bacterial overgrowth in the small intestine, which may be contributing to IBS-like symptoms in some people.
When the real cause is identified, support can become much more targeted and effective. With the right assessment and a personalised plan, many people are able to make meaningful progress in their digestion, energy and overall wellbeing.
If you are struggling with ongoing bloating, bowel changes or digestive discomfort and want to better understand what may be driving your symptoms, Hannah offers personalised nutrition support designed to get to the root of the problem.
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