Can We Feed the Gut Microbiome Without Making IBS Worse? An Interesting New Fibre Study

Anyone who has followed a low-FODMAP diet for IBS will know the dilemma. Reducing highly fermentable carbohydrates can be remarkably helpful for bloating, abdominal pain and unpredictable bowels. But many of the foods we restrict are also foods that feed our gut bacteria. What do we do?

So can we have the best of both worlds: less fermentation-related discomfort, while still giving the gut microbiome enough fibre to thrive?

A small new study published in the European Journal of Nutrition suggests this may be possible — although we are not yet at the point where I would recommend everyone with IBS rush out and buy a new fibre supplement.

Why might a low-FODMAP diet affect the microbiome?

FODMAPs are short-chain carbohydrates that are not always well absorbed in the small intestine. They therefore reach the large bowel, where bacteria ferment them. This fermentation produces gas and draws water into the bowel. In people with IBS, who often have a more sensitive gut, the resulting stretching of the intestine can trigger bloating, pain and changes in bowel habit.  

Reducing FODMAPs can therefore be extremely effective for symptom management.

However, there is a catch. Some FODMAP-containing foods also provide fermentable fibres and prebiotics that gut bacteria use as fuel. Previous research has suggested that low-FODMAP diets can reduce certain bacteria, particularly Bifidobacteria, and may reduce the amount of fermentable carbohydrate reaching the colon. The effects are not uniformly negative — a recent meta-analysis cited by the researchers found that the most consistent microbiome change was a reduction in Bifidobacteria — but it does raise an important question about how best to look after the microbiome while controlling IBS symptoms.  

What did the researchers test?

The researchers recruited adults with diagnosed IBS who had already been following a low-FODMAP diet for at least a month.

After a one-week baseline period, participants were randomly allocated to one of two groups for three weeks. Both groups received psyllium husk, a soluble gel-forming fibre already familiar to many people with IBS. However, the intervention group also received high-amylose resistant starch — a form of starch that escapes digestion higher up the gut and becomes food for bacteria in the colon.

The amount was introduced gradually, increasing from 5 g of the powder per day to 40 g per day over 11 days rather than being given at the full dose immediately. This gradual introduction is an important detail when we are talking about fibre and sensitive guts.  

Only 26 people completed the trial, 13 in each group. Most were women, and the average age was around 37. So we need to remember from the outset that this was a small pilot study, rather than a definitive trial.

What happened to their IBS symptoms?

Perhaps the most clinically interesting finding was what didn't happen.

Despite adding considerably more fermentable fibre, the participants receiving the resistant-starch-and-psyllium blend did not experience worsening IBS symptoms.

Pain, bloating, constipation, diarrhoea and overall IBS symptom severity remained broadly stable. There was also no significant change in bowel movement frequency or stool type.   

This does not mean that the fibre treated their IBS. Symptoms did not significantly improve compared with placebo either.

Instead, the useful finding is that people already controlling their symptoms with a low-FODMAP diet appeared able to introduce this particular fermentable fibre combination without sacrificing that symptom control.

That is quite a different — and much more accurate — conclusion.

Their fibre intake increased substantially

The intervention successfully increased total fibre intake to around 33 g per day, while resistant starch intake increased from approximately 2 g to 9.5 g per day.   

This matters because people with IBS can easily end up eating less fibre while trying to avoid symptoms. Fibre provides more than stool bulk: different fibres provide substrates for our intestinal microbes, which can then produce metabolites such as short-chain fatty acids.

The challenge has always been finding fibres that are beneficial and tolerable.

The idea behind combining resistant starch with psyllium is particularly interesting. Previous animal research suggests the gel-forming properties of psyllium may alter where resistant starch is fermented, potentially shifting some fermentation further along the colon. However, that proposed mechanism has not yet been demonstrated directly in humans, so it remains a theory rather than an established explanation.    

What happened to the microbiome?

There were measurable changes.

The overall bacterial community at genus level differed between the groups after the intervention. One group of bacteria labelled Ruminococcus E increased substantially — around 5.5-fold — in the intervention group. Prevotella also increased.

These are interesting findings because some members of the Ruminococcus family are particularly good at breaking down resistant starch and can interact with other bacteria involved in producing beneficial fermentation products.

But there is an important nuance: overall microbial diversity did not significantly increase.

Nor should we automatically assume that increasing one bacterium means somebody now has a “healthier microbiome”. Microbiome science simply isn't precise enough yet for us to make those kinds of claims from individual bacterial changes.    

What about butyrate and inflammation?

This is another area where it is important not to get carried away.

Despite the changes in bacterial composition, there was no significant overall increase in faecal short-chain fatty acids, including butyrate.

There were interesting increases in butyrate in a very small subgroup of participants who initially had particularly low stool output, but this was a post-hoc analysis involving only a handful of people. It therefore needs confirming in a much larger trial.

Similarly, the inflammatory markers that could reliably be measured did not change significantly.  

So this study does not demonstrate that the supplement reduced inflammation or increased butyrate production across the group.

An interesting gut-brain finding — but one to treat cautiously

The researchers also reported an improvement in gastrointestinal-specific anxiety — essentially, anxiety and worry focused on gut sensations and symptoms.

That is potentially fascinating because IBS is a disorder of gut-brain interaction, and fear of symptoms can itself become part of the symptom cycle.

However, this finding needs particularly careful interpretation. Several of the psychological and quality-of-life results looked statistically significant before adjustment, but did not remain significant after correction for the large number of statistical comparisons being made.

In other words: interesting signal, absolutely worth investigating further, but not evidence that resistant starch is an anxiety treatment. The authors themselves emphasise the need for caution.    

So should people with IBS start taking resistant starch?

Not on the basis of this study alone.

This was a three-week pilot study involving just 26 people, predominantly women and with a mixture of IBS subtypes. Several findings were exploratory, and the researchers did not measure factors such as gut transit time that could influence both symptoms and microbiome results. Larger and longer studies are needed.     

It is also worth remembering that fibre tolerance is enormously individual in IBS. Resistant starch on its own can be highly fermentable and can make some people extremely bloated. The study tested a specific combination of resistant starch and psyllium that was introduced very gradually. We cannot assume that taking a large dose of potato starch, resistant-starch powder or another fermentable fibre would have the same effect.

What I think is exciting about this research

For me, the most interesting message isn't about a particular supplement.

It is the idea that treating IBS doesn't have to mean choosing between controlling symptoms and feeding the microbiome.

Low-FODMAP diets can be enormously useful, but the long-term goal should not simply be to remove more and more foods. We need to understand how to maintain symptom control while helping people achieve a varied, fibre-rich diet that their individual gut can tolerate.

This early study suggests that carefully selected and slowly introduced fibres may eventually become part of that solution.

The researchers demonstrated that a substantial increase in fibre and resistant starch could be achieved without worsening symptoms, while producing measurable changes in the gut microbiome. That's encouraging.

But it is preliminary — and at this stage I would see it as another reason to focus on personalised fibre tolerance and dietary diversity, rather than as evidence for one new miracle IBS supplement. The researchers themselves conclude that the findings are preliminary and need confirmation in larger, longer studies.    

Come see me for personalised IBS intervention :-)

Reference: Yan R, Devine A, Lo J, et al. Dietary fibre blend improves gut health in patients with irritable bowel syndrome following a low FODMAP diet: a randomised, double-blind control trial. European Journal of Nutrition. 2026;65:263. doi:10.1007/s00394-026-04113-5.