Fibre and fermented foods for IBD: why "good for your gut" isn't always that simple.
You've probably heard it. Eat more fibre, add some sauerkraut or kefir, feed your microbiome. It's sensible-sounding advice, it's everywhere, and most of the time it's offered with real warmth by people who want to help.
There's good reason behind it too. Fibre and fermented foods can both influence the gut microbiome, which is why they feature so often in advice for people to support their gut health.
But here's the bit that doesn't get said often enough. You can follow all of it, do everything you've been told, tick every box, and still feel rubbish. And when that happens, it's very easy to quietly assume you're the one getting it wrong.
I'm here to tell you though - you're not! More often than not, the advice simply isn't lined up to where your gut is right now. Looking a little deeper and understanding how both fibre and fermented products work, as well as implications for situations like IBD, can really help you to move forward with confidence.
Why isn't "eat more fibre" one instruction?
We tend to talk about fibre as though it's a single thing we need more or less of, and it isn't. The soluble-versus-insoluble distinction is a useful place to start, although it isn't the whole story either.
Some soluble fibres dissolve in water and form a soft gel. Psyllium is a good example, and foods such as oats, cooked carrots and the flesh of fruit can also provide soluble fibre. Certain soluble fibres can help with stool consistency, while many are fermented by gut bacteria into short-chain fatty acids such as butyrate, which help support the cells lining the gut and play a role in immune regulation.
For many people with IBD, softer and more soluble forms of fibre can be easier to tolerate than rougher, more structural fibre, particularly when guts are feeling irritated or sore.
Insoluble fibre is found in foods such as wheat bran, fruit and vegetable skins, nuts, seeds and raw leaves. It tends to add bulk and can speed transit, which may be helpful in a settled gut but can be more challenging if the bowel is inflamed, narrowed or particularly sensitive.
There's another wrinkle too. Some soluble fibres are also highly fermentable. Inulin and chicory, for example, can feed gut bacteria very effectively but may also create significant gas and bloating in a sensitive gut. So "soluble" doesn't automatically mean "comfortable". Fermentability matters, texture matters, and the individual gut matters too.
Timing makes a difference as well. Fibre in comfortable remission is a different conversation from fibre during active disease. Although you may hear about fibre restriction banded about quite widely, current IBD guidance doesn't ask everyone to restrict fibre routinely, and actually advocates for ensure good levels when the gut can tolerate it. It does, however, recognise that some people, particularly those with symptomatic strictures, may need to modify the texture of fibrous foods rather than simply cutting fibre out.
The same vegetable cooked down soft may be much better tolerated than it is raw.
If you're wondering where to begin, two principles can help:
Lead with variety rather than chasing a fibre target. Different plant foods bring different fibres and plant compounds to the microbiome
Building up gradually gives you a much clearer sense of what your gut actually tolerates. If unsure, start low and slow!
What are fermented foods really doing?
Fermented foods have picked up an almost magical reputation: kefir, sauerkraut, kimchi, live yoghurt, kombucha. They're often described as a way of putting "good bacteria" into the gut, where they move in and improve the microbiome.
The real story is way more interesting than that.
Some fermented foods do contain live microorganisms, although not all do by the time we eat them (especially where pasturised), others may be destroyed by stomach acid. The important part is though - those microbes don't necessarily need to settle permanently in the gut to have an effect.
The process of fermentation changes the food itself. Microbes produce a wide range of compounds during the process, and these, alongside any live organisms that make it through to our gut, interact with our own population of microbes. It's this blend that helps contribute to the effects we see. And it's also one reason why reactions can differ so much. Our microbiomes aren't identical, even within the same families.
One influential study behind a lot of the recent interest came from Stanford researchers in 2021. Over 17 weeks, healthy adults followed either a fermented-food-rich diet or a fibre-rich one. The fermented-food group developed greater microbiome diversity and lower levels of several inflammatory proteins, while simply increasing fibre didn't increase microbiome diversity in the same way over the study period, although it did affect other aspects of gut and immune function.
This surprised some, who believed that it was fibre that was the main contributor to building a stronger microbiome. So just worth stressing - both fibre and fermented foods were helpful - just in slightly different ways, with fermented foods showing just how valuable they are as an addition to 'normal' diets (back on this point later!).
It's worth saying that the study was based on healthy adults, not people with IBD, and even within the study people didn't all respond alike. Both of those points are really quite useful to bear in mind.
Why might fermented foods make you feel worse?
This is the part that can often catch people out. And it can have a relevance specifically to IBD.
Fermentation doesn't only produce compounds we'd think of as helpful. Microbes can also make substances called biogenic amines, including histamine. Amounts vary enormously depending on the food, the microbes involved and how it is made and stored, so "fermented food" isn't really one uniform category either.
For most people that won't be a problem. But some people are much more sensitive to histamine than others, and how histamine is produced and broken down in the gut is becoming an interesting area of IBD research.
There are several connections including the intestinal lining, immune cells and gut microbes. I'll come back to this another time, because histamine is a whole big and very important subject all on its own. If you're keen not to miss, but not yet signed up to the newsletter, you can do so here.
For now, the practical steer is a simple one. If you want to try fermented foods, start small and introduce one thing at a time. That way, if something doesn't suit you, you have a much better chance of seeing what changed rather than being left guessing.
It's also worth noticing any wider pattern. Histamine-related symptoms can extend beyond the gut, although symptoms such as itching, headaches, flushing or disturbed sleep can of course have many other causes.
None of this means fermented food is unhealthy, or that anyone reacting to it has failed at "eating for their microbiome".
It may simply not be the right food for that person, in that amount, at that point.
Fibre and fermented foods in IBD: why "more" isn't always better.
Fibre and fermented foods in IBD need a little nuance. Both can be very useful, and both may need to be approached with a little care.
How well you tolerate them can depend on disease activity, where inflammation sits, whether narrowing or surgery has changed the gut, bowel habits, the existing microbiome and individual sensitivity to fermentation or particular foods.
And all of that can change over time.
So needing softer vegetables today doesn't mean avoiding fibre forever, and finding fermented foods difficult now doesn't mean your microbiome can never be supported.
Often the useful way to proceed isn't more restriction at all, but finding what works now whilst keeping an eye on how you can broaden carefully and sensitively later.
And if something does leave you feeling worse, that's information, not a verdict on your willpower, nor a message that you've done anything wrong. The most important thing we can do is listen to our bodies!
So the better question isn't simply whether fibre or fermented foods are "good", but what your gut can handle now, and what it's ready for next.
The advice wasn't necessarily wrong. It just wasn't tailored.
There's a lot of confident microbiome advice around, and much of it starts from perfectly reasonable science. But no general rule can fully account for an inflamed, sensitive or structurally changed gut, and that's as true here, or anywhere else.
If you've done all the "right" things and still feel worse, the answer isn't necessarily to try harder. The instructions just weren't written for your gut, where it is right now.
So perhaps the most useful way to think about supporting the microbiome in IBD is not in terms of perfect foods or perfect rules, but in terms of what is right for you just now and what you may be able to work towards later.
References
Hashash JG, Elkins J, Lewis JD, Binion DG (2024). AGA Clinical Practice Update on Diet and Nutritional Therapies in Patients With Inflammatory Bowel Disease: Expert Review. Gastroenterology, 166(3), 521–532. doi:10.1053/j.gastro.2023.11.303
Wastyk HC et al. (2021). Gut-microbiota-targeted diets modulate human immune status. Cell, 184(16), 4137–4153. doi:10.1016/j.cell.2021.06.019
Turna NS, Chung R, McIntyre L (2024). A review of biogenic amines in fermented foods: occurrence and health effects. Heliyon, 10(2), e24501. doi:10.1016/j.heliyon.2024.e24501
Kanta D, Katsamakas E, Berg Gudiksen AM, Jalili M (2025). Histamine Metabolism in IBD: Towards Precision Nutrition. Nutrients, 17(15), 2473. doi:10.3390/nu17152473
Comments