Do you often feel painfully bloated by the end of the day? Are you frequently gassy? If the answer is yes then there’s a good chance you have been told it’s irritable bowel syndrome (IBS) – but there could be another underlying reason for your symptoms that’s often missed.
Research, such as a study published in the Journal of Gastroenterology in 2020, has found that as many as half of those diagnosed with IBS also have small intestinal bacterial overgrowth (SIBO).
Just like IBS, SIBO can cause a bloated belly, stomach cramps, diarrhoea and constipation. Part of the problem is that the difference in the symptoms between the two conditions can be so subtle it’s hard to tell them apart.
But if your symptoms aren’t improving with typical IBS management, it could be that SIBO is to blame – and it’s worth checking with your doctor if you may have it, as the two conditions are managed very differently.
As its name suggests, SIBO occurs due to an overgrowth of bacteria in the small intestine – a place where there usually aren’t many. Most of the microbes in your gut (collectively known as your gut microbiome) live further down in the large intestine.
In a healthy gut, any food you haven’t broken down and absorbed in the small intestine, such as fibre, moves on to your large intestine. There, it feeds your gut bacteria, which in turn make helpful molecules for your health, along with gas as a natural side-effect.
Problems can start when these bacteria start to colonise in the small intestine, where they don’t belong. As a result, they start fermenting food too early – especially carbohydrates – and this produces gas in the small intestine.
SIBO occurs due to an overgrowth of bacteria in the small intestine. This in turn can lead to symptoms such as bloating, pain and diarrhoea
This in turn can lead to symptoms such as bloating, pain and diarrhoea.
People with IBS are more likely to have SIBO, but it’s still unclear whether SIBO is a cause, or a result, or simply a co-existing condition. You don’t need to have IBS to have SIBO, though it’s less common if you don’t have an irritable bowel.
The overgrowth of bacteria can occur for all sorts of reasons – but the cause may not always be clear. For example, it can happen after any gut surgery, as this changes how quickly or slowly food moves through the system, giving bacteria more time to grow.
Some medications, such as proton pump inhibitors (PPIs), which reduce stomach acid, have also been linked to a higher risk of SIBO. With less acid, it’s easier for bacteria to survive and reach the small intestine.
And conditions that slow digestion, such as a complication of diabetes called diabetic enteropathy (where high blood sugar leads to nerve damage in the gut) and hypothyroidism (where an underactive thyroid gland slows down mutiple body processes) can also increase the risk.
There is no single known cause of IBS – theories include changes in how the gut and brain communicate, increased gut sensitivity, or an imbalance in the gut microbiome.
But unlike SIBO, there isn’t a clear, measurable overgrowth of bacteria in the small intestine.
And this is how you can check if you have SIBO.
Most commonly doctors use a breath test, which involves drinking a sugary solution, then breathing into a tube every 15-20 minutes.
The point is to measure hydrogen and methane levels – gases made by gut microbes.
If gas levels spike too quickly after drinking the sugary solution, it may suggest bacteria are overgrowing in the wrong part of your gut.
It’s a simple test, but not always 100 per cent reliable, as other factors can affect the results, such as what you ate beforehand.
So the day before the test, steer clear of foods that cause gas (such as beans, onions, garlic and wholegrains) – then fast for eight to 12 hours before the test (only water is allowed).
If SIBO is diagnosed then a short course of antibiotics to kill off the bacterial overgrowth is normally the first-line treatment.
And this can make a difference – a review in the Journal of Neurogastroenterology and Motility in 2024 found that people with SIBO were almost three times more likely to feel better and to have a significant improvement in gut-related symptoms when given antibiotics, compared to those who got a placebo or no treatment at all.
It can, however, return and further courses of antibiotics may be needed.
Probiotics – supplements of ‘good’ bacteria – which are often recommended to help replenish your gut microbiome after a course of antibiotics, aren’t necessarily helpful for SIBO.
While the theory is that the ‘good’ bacteria might prevent an overgrowth of the problematic bacteria, the research is mixed.
Some small studies show certain strains may help, but others suggest probiotics could worsen symptoms by adding more bacteria into an already imbalanced system.
A better option – once your treatment is complete and your symptoms have settled – is to focus on adding back in the high-fibre foods such as beans, wholegrains, nuts and seeds, fruits and vegetables, to help your gut microbiome recover.
There are loads of diets out there promising to fix SIBO, but while some people do feel better cutting certain foods, diet alone usually isn’t enough to clear SIBO. Most diets focus on reducing foods that gut bacteria can easily break down, such as fibre and sugar alcohols (found in products such as gum and protein bars), and prebiotic supplements such as inulin – because these can worsen symptoms in people with sensitive guts.
Some studies do show that eating fewer FODMAPs (a group of fermentable carbs such as onions and apples) leads to less gas produced in the gut, but this really requires the help of a professional and I wouldn’t advise trying it alone. It’s a restrictive approach and isn’t meant to be followed long term.
My final warning is beware of the overhype. A 2024 review in Neurogastroenterology & Motility warned that SIBO has become a catch-all diagnosis in private clinics – it’s very easy to mis-diagnose and overtreat.
If you’re taking unnecessary treatment thanks to a mis-diagnosis, this could disrupt your microbiome – and potentially make any gut problem worse.
ASK EMILY…
‘My daughter, 24, has drunk only oat milk for the past five years. Should I worry about her bones?’
Dietitian and microbiome scientist Dr Emily Leeming replies: You’re right to check – many people don’t realise that switching to plant milks can mean losing nutrients that dairy would usually provide.
We build up most of our bone strength by our mid-20s, which is why it’s especially important your daughter considers her bone health now.
Unlike cow’s milk, oat milk isn’t naturally rich in calcium or vitamin D – the two key nutrients for strong bones. In the UK, dairy is the source of almost half our calcium intake, so cutting it out without a replacement can leave a gap.
However, these days many oat milk brands are fortified (organic and barista style ones tend not to be). Look for one with at least 120mg of calcium per 100ml (about the same amount as in cow’s milk) and added vitamin D.
You haven’t mentioned whether your daughter eats other dairy or calcium-rich foods such as yoghurt and cheese. If she does, I would urge her to aim for at least three portions a day to reach the recommended 700mg of calcium.
There are also plant-based options that are naturally high in calcium, such as beans, leafy greens, nuts and seeds – especially sesame seeds (one tablespoon contains 148mg calcium).
It is possible to get enough calcium through plants, but without dairy it isn’t easy.
If your daughter doesn’t eat these foods, then I would suggest she takes a supplement to be on the safe side. But make sure to consult her GP before making major changes, though.
Vitamin D is difficult to get from food alone. Since we rely mainly on sunlight to make our own vitamin D, a daily supplement of 10mcg is recommended in winter for most people in this country, as the sun’s rays aren’t strong enough for us to make enough ourselves. I hope that helps.
