SIBO vs IBS: Are Your “IBS” Symptoms Really Bacterial Overgrowth?

Irritable bowel syndrome (IBS) is often described as a functional gut disorder with no clear cause.

In reality, this means many GPs and doctors can give you the IBS label, but still can’t fully explain why your gut symptoms are so debilitating.

The good news is that there may be more going on beneath the surface.

One often overlooked contributor to IBS-type symptoms is small intestinal bacterial overgrowth – an abnormal build-up of bacteria in the small intestine that may be linked to your symptoms.

Key statistics between IBS and SIBO

Did you know that, on average, about one in three people diagnosed with IBS also has SIBO on testing?

The numbers in the research vary widely – from as low as 4% to almost 80% – and this largely depends on how SIBO is tested (different breath tests versus small-bowel cultures, and how the results are interpreted). But when you stand back and look at all the data together, a consistent picture emerges:

In other words, for a sizeable proportion of patients, “IBS” and “SIBO” are linked and addressing small intestinal bacterial overgrowth may indeed eliminate your symptoms of IBS.

In this article I will explore SIBO vs IBS in more detail: what each condition is, why their symptoms overlap so much, how we can test for SIBO in clinic, and how your IBS symptoms may be alleviated if SIBO is the cause of your underlying IBS symptoms.

What is IBS?

IBS is a symptom-based diagnosis. It is defined by Rome criteria, which look at recurrent abdominal pain associated with a change in bowel habit (diarrhoea, constipation or a mixture of both), plus other features such as bloating and urgency, in the absence of structural disease on standard tests.

Key points about IBS

• It affects roughly 4–10% of adults worldwide.
• It is more common in women and often starts before the age of 50.
• It is a functional gut–brain disorder, involving changes in motility, sensitivity of the gut nerves, immune activation and the microbiome, rather than visible damage to the bowel lining.

Typical IBS symptoms include

• Bloating and abdominal discomfort or pain
• Diarrhoea, constipation, or alternating between the two
• Urgency, incomplete evacuation, mucus in the stool
• Symptoms that often flare with stress, hormones or certain foods

IBS is usually diagnosed after “red flag” conditions (such as inflammatory bowel disease, coeliac disease and bowel cancer) have been excluded.

What is SIBO?

SIBO (small intestinal bacterial overgrowth) occurs when excessive numbers of bacteria colonise the small intestine, the section of gut designed mainly for digestion and nutrient absorption.

Normally, the small intestine contains relatively low bacterial counts compared with the colon. Protective mechanisms – stomach acid, bile, digestive enzymes, the migrating motor complex and a competent ileocaecal valve – all help to keep bacteria in check. When those defences are disrupted, bacteria from the large bowel can move up into the small bowel and overgrow.

Common SIBO symptoms include

• Marked bloating and visible distension, especially after meals
• Excessive gas, belching and flatulence
• Diarrhoea, constipation or mixed bowel habit
• Abdominal discomfort or pain
• Fatigue, brain fog
• In more established SIBO you can see low iron, vitamin B12 and fat-soluble vitamins such as vitamin D, which may show up as fatigue, hair shedding or poor immunity.

Estimates suggest that SIBO may affect up to 10–15% of the general population, though high-quality epidemiology is limited and figures vary by test method and population.

What symptoms are different in IBS compared to SIBO?

Clinically, IBS tends to be more pain-predominant, whereas SIBO often shows up as very prominent bloating and distension with or without much pain – although in real life there is a lot of overlap.

A 2024 study directly comparing IBS and SIBO backed this up: IBS patients had the most severe symptoms and were especially marked by intense abdominal pain and frequent diarrhoea, whereas bacteria enriched in the SIBO group correlated more strongly with constipation.PubMed

When they looked at stool samples, the IBS group also had a more “disrupted” mix of bacteria in the large intestine. Certain potentially harmful bugs (including types of E. coli and related species) were more common in IBS, and these were linked with more pain, bloating and changes in bile acids – substances that help you digest fat.

In the SIBO group, the pattern was a bit different. The bacteria that were more common in SIBO were more strongly linked with constipation, and the way the bacteria interacted with each other in the gut was more complex. A family of bacteria called Ruminococcaceae seemed to sit at the “centre” of this network, and they were associated with a different set of gut chemicals related to certain hormones and the breakdown of amino acids.

Together, these findings suggest that, even when symptoms overlap, IBS and SIBO often involve different microbiome changes and metabolic pathways – which is why treatment needs to be tailored, rather than “one size fits all”.

Gut Microbiome testing or SIBO tests?

For some people, a careful history and SIBO breath test are enough to guide treatment. In more complex cases – for example long-standing symptoms, lots of food reactions or a history of antibiotics or infections –we may also consider a stool-based gut microbiome test such as the GI-MAP test.

This doesn’t diagnose SIBO (which is a problem in the small intestine), but it does look at the bacteria, yeasts and inflammation markers in the large intestine.

It can pick up patterns such as overgrowth of opportunistic bugs (like certain E. coli-type species), low levels of helpful bacteria, signs that bile acids aren’t being handled properly, or problems breaking down and absorbing foods. All of this helps us tailor your plan – for example, which foods to focus on, whether certain probiotics or prebiotics are likely to help or hinder, and what kind of antimicrobial or gut-healing support is most appropriate for you.

Can SIBO cause fat malabsorption?

In more pronounced cases, SIBO can also interfere with bile acids – the “detergents” your liver makes to help you digest and absorb fat. Normally, bile acids are released into the small intestine, mix with the fats in your food, and then get reabsorbed further down the gut so they can be reused. When there is too much bacteria in the small intestine, those bacteria can deconjugate (chemically change) the bile acids so they no longer work properly.

When this happens, two things can go wrong:

• Some of the altered bile acids leak into the large bowel and pull water with them, which can trigger loose, watery stools or urgent diarrhoea.
• At the same time, fat is not broken down and absorbed as well, so it can pass through into the stool. This can cause oily, pale stools that float, leave a film in the toilet, or are difficult to flush away – doctors call this fat malabsorption or steatorrhoea.

If this carries on for a while, you may not absorb fat-soluble vitamins A, D, E and K as well as you should. Over time this can contribute to problems such as dry skin, poor night vision, low vitamin D, weaker bones, easy bruising or heavier periods, alongside the gut symptoms themselves.

Are there different types of SIBO?

There isn’t just one type of SIBO. Different gas patterns on breath testing can point to slightly different clinical pictures:

• Hydrogen-dominant SIBO – more often linked with diarrhoea and IBS-D.
  
• Methane-dominant overgrowth (now called intestinal methanogen overgrowth, IMO) – more often linked with constipation and IBS-
  
• Hydrogen sulphide-dominant SIBO – a less common pattern that may be associated with looser stools and “sulphur”-smelling gas; research here is still early.

SIBO vs IBS: how often do they overlap?

Over the last few years, several large reviews have tried to understand how often IBS and SIBO occur together.

• One big analysis from 2018 pulled together 50 studies (8,398 people with IBS and 1,432 without). It found that about 38% of IBS patients had SIBO, and that patients with IBS were nearly five times more likely to have SIBO than those without IBS.
  
• A follow-up review in 2020, found a 31% rate of SIBO in IBS, with a 3.7-fold higher risk compared with controls – and almost five-fold higher when only healthy controls were used.
  
• The data indicates that roughly one in three people with IBS has SIBO, with especially high rates in:
  
  • Diarrhoea-predominant IBS (IBS-D)
  • Methane-positive, constipation-predominant IBS (IBS-C)
    

In simple terms: if you’ve been told you have IBS, there is a reasonable chance that SIBO is part of the picture – particularly if your main issues are diarrhoea or very gassy, constipated bowels.

Some dramatic different ranges have been quoted – for example, between 4% and 78% of IBS patients have SIBO, depending on the study. This huge variation comes down to differences in:

• Who was studied (primary care vs tertiary centres)
• Which test was used (glucose vs lactulose breath test vs jejunal aspirate culture)
• Cut-off values and interpretation criteria

But when you step back and look across the literature, a consistent picture appears:

Roughly one in three people with IBS-type symptoms has objective evidence of SIBO on testing.

For those patients, addressing SIBO can be a key part of improving bloating, bowel habit and their daily quality of life and finally getting to the root cause of their symptoms.

Why do IBS and SIBO look so similar?

IBS and SIBO can produce near-identical symptoms, which is one reason they are so often confused.

In SIBO, bacteria in the small intestine ferment carbohydrates before they have been fully absorbed. This fermentation produces large amounts of hydrogen, methane and sometimes hydrogen sulphide gas. These gases:

• Distend the small bowel, causing bloating, discomfort and pain
• Alter motility – methane in particular has been shown to slow intestinal transit and promote constipation
• May increase intestinal permeability- ‘leaky gut’ and low-grade inflammation, feeding into the gut–brain axis and IBS-type symptoms

In IBS, similar symptoms can arise from:

• Hypersensitive gut nerves- the bowel isn’t “damaged”, but it feels pain more intensely.
• Altered motility patterns (too fast or too slow)- the gut can move too fast (diarrhoea), too slowly (constipation), or swing between the two.
• Low-grade immune activation in the bowel wall-a low level of inflammation in the gut wall that doesn’t show up like classic IBD, but still irritates the bowel.
• Changes in the gut microbiome– an imbalance in the mix of bacteria in the large intestine, even without true SIBO in the small intestine. Sometimes strains of candida as well as bacteria can overgrow

Risk factors that make SIBO more likely

Not everyone with IBS-type symptoms will have SIBO, so it helps to know who is at higher risk. Reviews of SIBO highlight several risk factors:

Key risk factors

• Motility disorders
  Conditions that slow the migrating motor complex (MMC) – these include post-infectious food poisoning, gastroenteritis, diabetes-related neuropathy, scleroderma, connective tissue disorders or chronic pseudo-obstruction – increase SIBO risk.
  
• Structural or surgical changes to the gut
  Small-bowel resections, blind loops, strictures, adhesions and a weak or absent ileocaecal valve all encourage stasis and back-flow of colonic bacteria.
  
• Long-term acid suppression
  Long-term use of acid-suppressing medicines such as proton pump inhibitors (PPIs) – for example omeprazole or lansoprazole – can increase the risk of SIBO in some people.”Proton pump inhibitors (PPIs) reduce gastric acid, one of the key defences against incoming bacteria. Meta-analyses show higher odds of SIBO in long-term PPI users, particularly when SIBO is diagnosed by aspirate culture.
  
• Chronic opioid use and other motility-slowing drugs
  Opiates and some anti-diarrhoeal medications slow transit, allowing bacteria to overgrow.
  
• Systemic illnesses
  Liver disease, pancreatitis, coeliac disease, inflammatory bowel disease and systemic sclerosis are all associated with higher SIBO rates.

Other conditions that have been associated with a higher prevalence of SIBO include:

• Functional dyspepsia (upper-gut pain, early fullness, nausea)
• Coeliac disease (especially if not fully controlled)
• Inflammatory bowel disease
• Non-alcoholic fatty liver disease
• Chronic pancreatitis
• Systemic sclerosis
• Some neurological conditions such as Parkinson’s disease

Even in people without obvious risk factors, a history of severe food poisoning, travel bugs or repeated use of acid suppressing medication can trigger a lasting change in motility and microbiota that increases the likelihood of SIBO and IBS-type symptoms.

How we investigate SIBO vs IBS

IBS: a diagnosis of pattern and exclusion

IBS is diagnosed clinically, using Rome criteria and a careful history, plus examination and basic tests (such as bloods, coeliac screen and in some cases endoscopy or colonoscopy) to exclude other pathology.

There is no single “IBS test”. Instead, we look for a characteristic pattern of symptoms, lack of red flags, and normal structural investigations.

In functional and nutritional practice, we will often also use a stool-based gut microbiome test, such as the GI-MAP, in people with IBS-type symptoms. This doesn’t diagnose IBS (or SIBO) on its own, but it can give very useful extra information about what is happening in the large intestine – for example: whether there is overgrowth of opportunistic bacteria or yeasts, whether beneficial bacteria are low, how well you are digesting and absorbing food, and whether there are signs of inflammation or immune activation. All of this helps us personalise your plan – from diet and probiotics to antimicrobial and gut-healing support – rather than relying on a generic “one size fits all” IBS protocol.

SIBO: breath tests and their limitations

For SIBO, the most widely used tools in practice are hydrogen and methane breath tests. You drink a sugar solution (usually lactulose or glucose), and breath samples are collected over time to measure gas production as the substrate travels through your gut.

• Glucose breath tests are more specific but may miss overgrowth in the distal small bowel.
• Lactulose breath tests are more sensitive but prone to false positives if colonic fermentation is misinterpreted as small-bowel overgrowth.

There is still no universally accepted gold standard for SIBO diagnosis. Even jejunal aspirate cultures – often described as the reference method – have their own technical challenges. This is why prevalence figures vary so widely between studies

In clinic, we do not rely on the test in isolation. We interpret breath test results alongside:

• Your symptoms and their pattern
• Risk factors and past medical/surgical history
• Response to previous treatments

That combination is what helps us decide whether treating SIBO is likely to make a meaningful difference for you.

SIBO vs IBS: does treatment look different?

Yes – once SIBO is identified as a driver of IBS-type symptoms, the treatment plan usually needs to include both:

1. Strategies that address the bacterial overgrowth, and
2. Approaches that support motility, gut–brain function and long-term symptom control.

Treating SIBO itself

Options include:

• Non-absorbed antibiotics
  Rifaximin is the best-studied antibiotic for SIBO and IBS-D, with multiple trials showing symptom improvement and normalisation of breath tests in a proportion of patients.In methane-positive, constipation-dominant cases, adding a second antibiotic such as neomycin has been shown to improve eradication rates.
  
• Herbal antimicrobials
  Herbs such as oregano, neem, berberine and certain botanical combinations are alternatives or adjuncts to rifaximin. Early trials suggest they may help eradicate SIBO in many patients, and we have had great success stories. Please read reviews of our success on google at our sister clinic ibs-solutions.co.uk where we hold consultations.
  
• Elemental diets
  A short course of an elemental diet – a liquid formula containing pre-digested nutrients – can “starve out” excess bacteria while still nourishing the patient. Studies in SIBO and IBS have shown high rates of symptom improvement and breath-test normalisation after 2–3 weeks, although this approach is demanding and should be supervised.

Whatever route is chosen, identifying and addressing underlying risk factors (motility, medications, structural issues) is vital to reduce relapse risk.

Supporting motility and preventing relapse

Because SIBO is often a relapsing condition, we place a lot of emphasis on what happens after the initial clearance phase:

• Prokinetics
  Prokinetics support the migrating motor complex between meals and can help keep bacteria moving in the right direction. Options include ginger or prescription agents (such as low-dose prucalopride) and, in some cases, herbal prokinetics, depending on the patient.
  
  
• Dietary strategies
  Reducing fermentable carbohydrates (for example through a structured low FODMAP approach or a tailored modification of it) can lower symptom burden and make the small-bowel environment less favourable for overgrowth, especially in the short to medium term.
  
• Digestive support
  Where appropriate, pancreatic or bile support, stomach acid support (if low) or targeted digestive enzymes may be used to help restore more normal digestion and reduce fermentable substrate reaching the small intestine.
  
• Careful use of probiotics
  The role of probiotics in SIBO remains controversial. Some patients find they help; others feel worse, especially if started too early. We usually introduce them cautiously and on a case-by-case basis once overgrowth has been addressed.
  
• Some studies suggest that specific probiotics or yeast such as Saccharomyces boulardii may help when used alongside antibiotics, particularly for diarrhoea and to support the gut barrier, although the evidence is still emerging and guidelines are cautious about recommending any one product.

IBS-focused care still matters

Even when SIBO is present, IBS-focused strategies remain important:

• Identifying and managing dietary triggers beyond FODMAPs- e.g do you have a food intolerances? Lactose intolerance? Yest intolerance?
  
• Supporting stress resilience and the gut–brain axis (sleep, nervous system regulation, psychological support)- Is stress cauisng many of your symptoms or are your symptoms causing you anxiety and stress?
  
• Are your IBS symtoms due to other conditions such as pelvic floor dysfunction, endometriosis or histamine-related issues where relevant

When should you suspect SIBO under an IBS label?

Although every case is individual, SIBO is particularly worth considering if:

• Bloating is severe, visible and reliably worse after eating
• You experience significant gas, belching or foul-smelling wind
• Symptoms began after a bout of food poisoning, a GI infection or abdominal surgery
• You have been on long-term acid suppressants, opiates or other motility-slowing drugs
• There is a known connective tissue, motility or systemic condition associated with SIBO

In these situations, SIBO testing may be able tp clarify whether bacterial overgrowth is part of the picture and, guide a more tailored treatment plan.

Final thoughts: SIBO vs IBS – it’s not always either/or

• IBS is a broad umbrella for a set of chronic gut symptoms and gut–brain changes.
• SIBO is present in about one in three IBS patients
• SIBO tends to be bloating-predominant, IBS more pain-predominant, but there is substantial overlap.

If you suspect SIBO may be involved in your case, the next step is a careful assessment of your history, risk factors and symptom pattern, alongside appropriate testing. Book a consultation today

From there we can decide together whether a SIBO-focused approach is likely to help and how it can be integrated with a broader IBS management plan tailored to you.

FAQs: SIBO vs IBS

Is SIBO the same as IBS?

No. IBS is a syndrome – a label for a cluster of symptoms such as pain, bloating and altered bowel habit, once other conditions have been ruled out. SIBO is a specific diagnosis describing an abnormal build-up of bacteria in the small intestine.

The two often overlap. Many people with SIBO meet criteria for IBS, and around one in three people labelled with IBS will have SIBO on breath testing. For those patients, SIBO is one of the underlying drivers of their “IBS” symptoms.

Can you have IBS without SIBO?

Yes – absolutely. Plenty of people with IBS-type symptoms do not have SIBO on testing.

Their symptoms may be driven by:

• Gut–brain axis changes
• Post-infectious IBS
• Food intolerances or FODMAP sensitivity
• Pelvic floor dysfunction
• Coeliac disease, bile acid diarrhoea or other conditions

This is why a good assessment looks beyond a single test and considers the whole picture.

What are the key differences between SIBO and IBS symptoms?

On the surface, symptoms can look very similar – bloating, gas, abdominal discomfort and unpredictable bowels.

Clues that SIBO may be present include:

• Very prominent, visible bloating that worsens through the day or after meals
• Lots of gas (belching and/or flatulence), sometimes with a strong odour
• A clear trigger such as food poisoning, abdominal surgery or long-term acid suppression
• A pattern where standard IBS treatments only help partially or briefly

Even then, symptoms alone can’t diagnose SIBO – they simply flag that testing might be useful.

How accurate are SIBO breath tests?

Hydrogen–methane breath tests are a practical way to investigate SIBO but they are not perfect. Recent reviews and guidelines emphasise that both lactulose and glucose tests can give false positives and false negatives, especially if protocols and cut-offs vary.

Because of this, breath test results are best interpreted:

• Alongside your symptoms and risk factors
• With a clear understanding of the test protocol used
• As part of a wider clinical picture, not as a yes/no in isolation

Can SIBO be cured, or does it always come back?

For some people, SIBO can be cleared and stay away for long periods, especially if there’s no major underlying motility or structural issue.

For others, SIBO behaves more like a chronic, relapsing condition. Initial treatment may bring good relief, but symptoms return if underlying drivers (such as slow motility or ongoing medications) are not addressed.

Do I have to follow a strict low FODMAP diet if I have SIBO?

Not necessarily.

A low FODMAP diet can temporarily reduce fermentable carbohydrates and ease bloating while the gut recovers, but it’s not the only option and is not meant to be permanent. For many people, a gentler, personalised reduction of key triggers (such as onions, garlic, wheat, certain fruits or pulses) works just as well.

The goal is always to liberalise the diet as far as your symptoms and gut function allow, rather than keeping you on a very restricted regime long term.

Are probiotics helpful or harmful if you have SIBO?

It depends.

Some people with SIBO feel better on the right probiotic; others feel more bloated, especially if probiotics are introduced too early, at high doses, or with strains that increase gas production.

In practice, we:

• Focus first on reducing overgrowth and supporting motility
• Introduce probiotics cautiously, at low doses, once symptoms are more stable
• Tailor strain choice and dosing to the individual

If you feel worse when you start a probiotic, it’s usually a sign to pause and review rather than to “push through”.

Should I test for SIBO or just try treatment?

There are pros and cons to both approaches.

Testing can:

• Clarify whether SIBO is present and which gas pattern (hydrogen, methane or both) dominates
• Help tailor treatment choices
• Provide an objective marker to re-test in more complex cases

In some simple, low-risk situations, a carefully supervised therapeutic trial may be considered without testing. But for most people with complex or long-standing IBS-type symptoms, structured testing plus an individualised plan is more reliable than guessing.

I have endometriosis and IBS – could SIBO also be involved

Possibly, yes.

Studies show that women with endometriosis are more likely to have an IBS diagnosis than women without endometriosis, and the two conditions can easily be confused because of overlapping symptoms like abdominal pain, cramping, bloating and bowel habit changes.

On top of this, some specialist clinics and emerging research suggest that SIBO may be more common in women with endometriosis, particularly when there are adhesions, repeated pelvic surgery, marked constipation or prominent “endo belly” bloating.

That doesn’t mean everyone with endometriosis automatically has SIBO. But if you have:

• Endometriosis
• An “IBS” label
• Plus severe bloating, distension and/or constipation that hasn’t responded well to standard IBS advice

…then it is worth a more detailed review of your history and, where appropriate, targeted SIBO breath testing rather than assuming all of your symptoms are gynaecological.

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    Author – Victoria Tyler BSc Hons MBANT
    Victoria Tyler owns and runs two busy clinics : Nutrition and Vitality- her testing centre and the IBS and Gut Disorder Clinic where she offers consultations virtually and in London.
    
    Nutrition and Vitality, along with the IBS and Gut Disorder Clinic, were founded with the goal of helping patients alleviate IBS symptoms by uncovering and addressing the root causes of their digestive issues.
    As a Registered Nutritional Therapist, Victoria holds a BSc (Hons) in Nutritional Therapy and has trained with the Institute of Functional Medicine. She is also accredited by BANT and CNHC.
    Before transitioning into health, Victoria earned a degree in Economics and an MBA, working with corporations including Canon and Vodafone. However, her own health challenges led her to pursue a career in Nutritional Therapy.
    With a passion for learning, Victoria is committed to staying at the forefront of Functional Medicine. She helps patients manage IBS and other digestive disorders, including SIBO, Candida, and IBD, by identifying and addressing their root causes.
    Victoria strongly believes that every symptom has an underlying cause, and there is always a solution to every health condition. To learn more, or to see patient reviews, visit Victoria’s profile on Google.