SIBO: What Is It, What Causes It and How Can Nutrition Help?
If you struggle with persistent bloating, excessive gas, abdominal discomfort or changes in your bowel habits, you may have come across the term SIBO.
Small intestinal bacterial overgrowth — usually shortened to SIBO — has become a popular topic in the gut-health world, particularly among people experiencing IBS-type symptoms.
But what actually is SIBO? Why does it happen? How do you know if you have it? And where does nutrition fit into the picture?
The answer is a little more nuanced than some of the information you may find online.
SIBO is a genuine clinical phenomenon, but it is also an area where the science is still evolving. Symptoms can overlap with many other digestive conditions, breath testing has limitations, and simply trying to "kill the bacteria" without considering why the overgrowth developed in the first place may miss an important part of the picture.
In this guide, I'll explain what we currently understand about SIBO, how it is investigated, and how nutritional therapy can form part of a personalised approach to supporting digestive health.
So, what exactly is SIBO?
Your digestive tract is home to trillions of microorganisms. This community — often referred to as the gut microbiome — plays an important role in digestion, metabolism and immune function.
Most of the bacteria in your digestive tract live in the large intestine, or colon.
The small intestine is different. It contains microorganisms too, but normally in much lower numbers.
SIBO occurs when there is an abnormal increase in microorganisms in the small intestine.
These microorganisms can ferment carbohydrates and produce gases, including hydrogen. Some microorganisms also produce methane.
This can contribute to symptoms such as bloating, abdominal discomfort, excessive wind and changes in bowel habits.
However, SIBO isn't simply a case of having "too many bad bacteria".
The small intestine has several mechanisms that help keep microbial populations under control, including stomach acid, bile, pancreatic secretions and normal intestinal movement.
One particularly interesting mechanism is the migrating motor complex (MMC).
You can think of the MMC as a kind of housekeeping system for the small intestine. Between meals, waves of muscular contractions help move residual food and microorganisms through the digestive tract.
If normal motility is disrupted, there may be greater opportunity for microorganisms to accumulate.
And this is where the question of why SIBO developed in the first place becomes so important.
What are the symptoms of SIBO?
There isn't one symptom that tells you that you have SIBO.
The symptoms can overlap considerably with IBS and other gastrointestinal conditions.
Commonly reported symptoms include:
Bloating
Abdominal pain or discomfort
Excessive flatulence
Belching
Diarrhoea
Constipation
Alternating constipation and diarrhoea
Feeling excessively full after eating
Nausea
Changes in bowel habits
In more significant cases, problems with digestion and absorption of nutrients can occur.
But here's the important part:
Having these symptoms does not necessarily mean you have SIBO.
Bloating, gas and altered bowel habits are incredibly common and can have many possible causes.
Coeliac disease, IBS, inflammatory bowel disease, carbohydrate malabsorption, constipation, pelvic floor dysfunction and other gastrointestinal conditions can all produce similar symptoms.
This is why I wouldn't recommend assuming that SIBO is the answer simply because you are bloated.
Hydrogen, methane and IMO: what's the difference?
If you've looked into SIBO testing, you may have come across the terms hydrogen-positive, methane-positive and IMO.
This can sound complicated, but the basic idea is fairly straightforward.
Hydrogen
Some intestinal microorganisms produce hydrogen when they ferment carbohydrates.
Higher hydrogen levels on a breath test can therefore provide evidence of a hydrogen-producing microbial overgrowth, when interpreted according to accepted testing criteria.
Hydrogen-predominant patterns are often associated with diarrhoea, although symptoms don't always follow a neat pattern.
Methane
Methane is different.
It isn't produced by bacteria. It is produced by microorganisms called archaea, also known as methanogens.
This is why you may see the term intestinal methanogen overgrowth (IMO) rather than methane-SIBO.
Methane production is particularly associated with slower intestinal transit and constipation.
So, if someone has significant constipation alongside elevated methane, the constipation itself becomes an important part of the clinical picture — not simply a symptom to be covered up.
Why might someone develop SIBO?
This is perhaps the most important question to ask.
Rather than thinking about SIBO as a completely separate condition, it can be more useful to consider the factors that may have created an environment where microbial overgrowth was more likely.
Slower intestinal motility
Normal movement through the digestive tract helps prevent microorganisms from accumulating in the small intestine.
Anything that significantly slows transit may therefore be relevant.
This can include chronic constipation and certain gastrointestinal or neurological conditions.
Previous gastrointestinal infection
Some people develop ongoing digestive symptoms following food poisoning or gastroenteritis.
Changes in gut function and motility following an infection may be relevant in some individuals.
Structural changes
Previous abdominal surgery, adhesions, strictures, diverticula or other structural abnormalities can sometimes interfere with normal movement through the digestive tract.
These situations require appropriate medical assessment rather than nutritional therapy alone.
Certain medications
Some medications can influence gut motility or the gastrointestinal environment.
This doesn't mean that medication is necessarily the "cause" of SIBO, and prescribed medication should never be stopped without discussing it with the prescribing clinician.
It is simply one part of the overall history that may be relevant.
Other digestive conditions
SIBO can occur alongside other gastrointestinal problems, including IBS and conditions affecting digestion or intestinal motility.
This is one reason why a good assessment looks beyond the breath-test result.
The American Gastroenterological Association recommends that management considers potentially correctable underlying causes and any nutritional consequences, rather than focusing solely on the overgrowth itself.
How do you test for SIBO?
One of the most commonly used non-invasive investigations is a hydrogen and methane breath test (HMBT).
You drink a measured solution containing a carbohydrate substrate, usually glucose or lactulose, and then provide a series of breath samples over a period of time.
If microorganisms ferment the substrate, gases including hydrogen and methane are produced. Some of these gases are absorbed into the bloodstream and eventually exhaled through the lungs, where they can be measured.
Glucose or lactulose?
Both glucose and lactulose are used for breath testing.
Glucose is absorbed relatively early in the small intestine, while lactulose travels further through the digestive tract.
Both tests have limitations, and results need to be interpreted in the context of symptoms, intestinal transit and the way the test was performed.
This is worth knowing because SIBO breath testing isn't a perfect test.
There are differences between testing protocols and some disagreement within gastroenterology about the sensitivity and specificity of breath testing. UK guidance has historically been particularly cautious about relying on breath tests alone.
More recent UK work has focused on standardising hydrogen/methane breath-testing protocols because differences in how tests are performed and interpreted can make results difficult to compare.
What does a positive test mean?
The North American Consensus criteria commonly used in clinical practice consider:
A rise in hydrogen of 20 ppm or more within 90 minutes to be consistent with a positive hydrogen breath test.
Methane of 10 ppm or more at any point during testing to indicate methane positivity.
Methane positivity is generally referred to as intestinal methanogen overgrowth (IMO) rather than SIBO because methanogens are archaea rather than bacteria.
The important thing is that the numbers shouldn't be interpreted in isolation. The pattern, symptoms, preparation and clinical context all matter.
What does nutritional therapy for SIBO involve?
This is where a personalised approach is particularly important.
There is a temptation to approach SIBO with a very restrictive diet and a long list of supplements.
But more restriction isn't necessarily better.
Nutritional therapy should be about understanding the individual picture and supporting digestive health alongside appropriate medical care.
There are several areas that may be considered.
1. Reducing digestive symptoms
Certain carbohydrates are fermented more readily by gut microorganisms.
For someone experiencing significant bloating and discomfort, temporarily reducing highly fermentable foods may help reduce symptoms.
A low-FODMAP diet is one approach that has good evidence for improving symptoms in people with IBS.
However, it is important to understand what this does — and what it doesn't do.
A low-FODMAP diet can reduce the amount of fermentable carbohydrate reaching the gut and may therefore reduce symptoms.
It should not automatically be presented as a treatment or cure for SIBO.
Long-term unnecessary restriction can reduce dietary variety and fibre intake, which is why a structured approach where dietary restriction, if appropriate, is used for a defined period and foods are subsequently reintroduced according to individual tolerance.
2. Looking at meal timing and motility
The migrating motor complex is active between meals.
For some people, allowing reasonable periods between meals may be worth considering as part of a broader digestive-health strategy.
This doesn't mean fasting all day or following rigid meal schedules.
Instead, the focus is on avoiding constant grazing where this is practical, while still meeting your nutritional and energy requirements.
If constipation is present, addressing it is particularly important.
This may involve looking at:
Fibre intake and fibre type
Fluid intake
Physical activity
Meal patterns
Toilet habits
Stress and sleep
Whether medication may be contributing
Whether further investigation is needed
3. Supporting nutritional adequacy
Very restrictive diets are sometimes used in an attempt to control SIBO symptoms.
But if your diet becomes smaller and smaller because you're afraid of triggering symptoms, it can become difficult to obtain enough fibre, protein, vitamins and minerals.
In more significant cases of SIBO, nutritional deficiencies can also occur.
This is why nutritional adequacy should remain a priority.
The aim isn't to create a diet where you can only eat a handful of "safe" foods.
It is to find the least restrictive approach that gives you good symptom control while maintaining nutritional variety.
4. Antimicrobial treatment
If testing and clinical assessment suggest SIBO or IMO, antimicrobial treatment may form part of management.
Prescription antibiotics may be used by medical practitioners in appropriate cases.
In nutritional therapy and functional medicine settings, botanical antimicrobial protocols are also sometimes used.
However, this is an area where it is important to be honest about the evidence.
There is research into a number of botanical compounds, but the evidence base is much less established than the evidence for some prescription approaches, and products, doses and combinations vary considerably.
"Natural" does not automatically mean safe.
Some herbs and concentrated botanical products can interact with medicines or be unsuitable for people with particular health conditions.
For this reason, antimicrobial supplements should be considered on an individual basis, rather than following a generic internet protocol.
5. Looking beyond the overgrowth
Perhaps the most important part of the process is asking:
Why did this happen?
If someone has chronic constipation, for example, simply focusing on microbial overgrowth without addressing bowel transit may leave an important part of the picture untouched.
Likewise, if there is an underlying gastrointestinal condition or structural problem, that needs appropriate medical investigation.
The AGA's clinical guidance emphasises identifying and correcting underlying causes where possible, alongside addressing nutritional consequences and the clinical symptoms themselves.
This is one reason I don't see SIBO management as a simple "kill and move on" process.
What about probiotics?
This is another area where the answer isn't simply "yes" or "no".
Probiotics are often recommended for digestive health, but there isn't a universal probiotic protocol for SIBO.
Some people tolerate particular strains well. Others find that introducing probiotics makes their bloating or digestive symptoms worse.
The evidence is still developing, so it is preferable to consider probiotics within the context of the individual rather than automatically adding them to every SIBO protocol.
Can SIBO come back?
It can.
And this is another reason why understanding the wider picture matters.
If the factors that contributed to the original problem remain — for example, significant constipation or impaired intestinal motility — symptoms may return.
This doesn't mean that treatment has "failed".
It means that the underlying factors may need further attention.
The goal should therefore be broader than simply trying to reduce microorganisms.
It is about understanding the digestive system as a whole and supporting the factors that help it function normally.
When should you speak to your GP?
Digestive symptoms are common, but persistent or unexplained symptoms shouldn't simply be assumed to be SIBO, IMO, or a combination of the two.
Speak to your GP or another appropriate healthcare professional if you have:
Unexplained or persistent weight loss
Blood in your stool
Persistent vomiting
Significant or worsening abdominal pain
Persistent diarrhoea
Difficulty swallowing
Anaemia or suspected nutritional deficiency
A significant change in bowel habits
A family history of bowel cancer, inflammatory bowel disease or coeliac disease
Symptoms that are new, severe or concerning
These symptoms may require medical investigation.
Nutritional therapy can complement appropriate medical care, but it should not replace diagnosis or treatment from your GP, gastroenterologist or other medical professional.
My approach to SIBO and digestive health
SIBO can be a frustrating piece of the digestive-health puzzle.
If you've spent months — or even years — dealing with bloating, gas, constipation or diarrhoea, it can be tempting to search for the one food, supplement or protocol that will finally solve the problem.
In reality, digestive health is rarely that simple.
My approach is to look at the whole picture: your symptoms, bowel habits, medical history, diet, lifestyle, medications, previous digestive problems and, where appropriate, functional testing.
Where SIBO is suspected, breath testing may provide useful information — but it is only one piece of the puzzle.
The aim of nutritional therapy isn't to put you on an increasingly restrictive diet or give you a long list of supplements.
It is to help you understand your digestive symptoms, identify relevant contributing factors, support nutritional adequacy and make realistic, personalised changes that fit into your life.
Because your digestive system is more than a collection of symptoms.
It is a whole system — and sometimes understanding the bigger picture is where things start to make sense.
Wondering whether SIBO could be part of the picture?
If persistent bloating, changes in bowel habits, abdominal discomfort or digestive symptoms are affecting your day-to-day life, it can be helpful to step back and look at the bigger picture rather than simply removing more and more foods.
In my nutritional therapy practice, I take a personalised approach to digestive health, considering symptoms, diet, lifestyle, gut function, motility and relevant testing alongside your medical history.
If you’d like support making sense of your digestive symptoms and understanding what might be contributing, book a free explore call to find out more about working together.
A note about this article
This article is for general educational purposes and is not intended to diagnose, treat or cure SIBO or any other medical condition. Nutritional therapy is a complementary approach and does not replace appropriate medical diagnosis or treatment.
If you have persistent or concerning digestive symptoms, please speak to your GP or another appropriately qualified healthcare professional.
References
American Gastroenterological Association. Clinical Practice Update on Small Intestinal Bacterial Overgrowth (SIBO). Gastroenterology, 2020.
Pimentel M, Saad RJ, Long MD, Rao SSC. ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth. American Journal of Gastroenterology, 2020.
Rezaie A, et al. Hydrogen and Methane-Based Breath Testing in Gastrointestinal Disorders: The North American Consensus. American Journal of Gastroenterology, 2017.
Association of Gastrointestinal Physiologists / British Society of Gastroenterology. Proposed Standardised Testing Protocol for Hydrogen/Methane Breath Testing.
British Society of Gastroenterology. Guidelines for the Investigation of Chronic Diarrhoea in Adults.