Introduction
Fibre has long been marketed as nature’s broom – good for keeping things moving, helpful for constipation, and something you “should” eat more of. But this oversimplifies the story. Fibre is not one thing. Different types of fibre do very different jobs in the gut, and how your body responds depends on both the type and the context.
For some people, a sudden increase in fibre leads to uncomfortable bloating and diarrhoea. For others, a carefully tailored fibre plan transforms digestion and energy. Understanding the nuances of fibre can help you work with your gut, not against it.
This article unpacks what fibre actually is, the science behind how it interacts with the gut microbiome, when it helps or hinders IBS symptoms, and how to increase it safely.
Table of Contents
- What is fibre really?
- The main types of fibre
- Soluble
- Insoluble
- Resistant starch
- Prebiotics
- Polyphenol-rich fibres
- Fibre and the gut microbiome
- Fibre and digestion: the IBS paradox
- How much fibre do you really need?
- Building your fibre tolerance step by step
- Common myths about fibre
- FAQs
- Practical checklist
1. What is fibre really?
Fibre is the portion of plant foods that resists digestion in the small intestine. Instead of being fully broken down, fibre moves into the colon where it interacts with gut bacteria. Depending on the type, fibre may:
- Absorb water and form gels
- Add bulk to stool
- Act as food for beneficial bacteria
- Produce short-chain fatty acids (SCFAs), which support gut lining and reduce inflammation
Not all fibres do all of these things – which is why variety matters.
2. The main types of fibre
Soluble fibre
- Dissolves in water, forms a gel-like substance.
- Slows digestion and helps regulate blood sugar.
- Found in oats, apples, beans, psyllium.
- Helpful for both diarrhoea and constipation when used appropriately.
Insoluble fibre
- Adds bulk, speeds up intestinal transit.
- Found in wholegrains, vegetable skins, nuts, seeds.
- Useful for sluggish bowels but can worsen loose stools.
Resistant starch
- A type of starch that resists digestion and reaches the colon.
- Fermented by bacteria into SCFAs (notably butyrate, which supports colon cells).
- Found in cooked-and-cooled potatoes, rice, green bananas.
Prebiotic fibres
- Specific fibres that selectively feed beneficial bacteria such as Bifidobacteria and Lactobacilli.
- Includes inulin, FOS, GOS.
- Found in onions, garlic, leeks, asparagus.
- Can trigger bloating if introduced too quickly.
Polyphenol-rich fibres
- Fibres bound with polyphenols (antioxidants) that also support gut bacteria.
- Found in berries, cocoa, flaxseeds.
- May play a role in gut-immune signalling.
Suggested Table:
| Fibre type | Food sources | Main actions | Who benefits most | Cautions |
|---|---|---|---|---|
| Soluble | Oats, apples, beans, psyllium | Slows digestion, softens stool | Constipation, diarrhoea regulation | Excess may cause gas |
| Insoluble | Wholegrains, veg skins, nuts | Adds bulk, speeds transit | Constipation, sluggish motility | May worsen diarrhoea |
| Resistant starch | Cooked/cooled potatoes, rice, green bananas | SCFA production, gut barrier support | Gut lining repair, microbiome diversity | Gas if overdone |
| Prebiotics | Onions, garlic, asparagus | Feed beneficial bacteria | Microbiome health | Bloating in sensitive guts |
| Polyphenol fibres | Berries, cocoa, flax | Antioxidant, microbiome signalling | General gut and immune health | Rarely problematic |
3. Fibre and the gut microbiome
When fibre reaches the colon, bacteria ferment it into SCFAs such as acetate, propionate and butyrate. These compounds:
- Nourish colon cells
- Reduce inflammation
- Improve blood sugar regulation
- Communicate with the immune system
RCTs show that diets high in diverse plant fibres increase microbiome diversity and SCFA production [1,2]. Conversely, very low-fibre diets reduce microbial richness within days.
4. Fibre and digestion: the IBS paradox
Fibre is often recommended for IBS, but results are mixed:
- Constipation-predominant IBS (IBS-C): Soluble fibre like psyllium often helps [3].
- Diarrhoea-predominant IBS (IBS-D): Insoluble fibre can worsen symptoms.
- Bloating and gas: Rapidly fermentable prebiotics (e.g., inulin) may cause discomfort if tolerance is low.
Meta-analyses show that soluble fibre improves global IBS symptoms, while insoluble fibre has little benefit [4]. This highlights the importance of type and dose.
5. How much fibre do you really need?
UK guidelines recommend 30g of fibre daily for adults. The average intake is closer to 18g. But the right dose is individual. Someone with IBS may need to build tolerance gradually from 15g towards 25–30g, while monitoring symptoms.
6. Building your fibre tolerance step by step
- Start with soluble fibre. Add oats, chia seeds, or cooked carrots.
- Add variety. Aim for 30 different plant foods each week.
- Hydrate. Fibre without water can worsen constipation.
- Introduce prebiotics slowly. Small amounts of garlic, leeks, asparagus.
- Notice your gut’s response. Keep a simple food-symptom log.
- Use supplements cautiously. Psyllium husk and PHGG are often tolerated best.
Checklist:
- Swap white bread for seeded sourdough.
- Add 1 tbsp chia to yoghurt.
- Cook and cool potatoes for resistant starch.
- Add a handful of berries daily.
- Drink 1–2 extra glasses of water.
7. Common myths about fibre
- “More is always better.” Not true – too much fibre too fast worsens IBS.
- “Low-FODMAP means low fibre.” Not necessarily – plenty of low-FODMAP fibres exist (e.g., oats, kiwifruit, chia).
8. FAQs
Q1: Does fibre always reduce bloating?
No. Certain fibres (especially prebiotics like inulin) increase gas production. It depends on your current gut microbiome and tolerance.
Q2: Should I take a fibre supplement?
Supplements can help in specific cases (e.g., psyllium for IBS-C). They should complement, not replace, whole foods.
Q3: Can fibre help diarrhoea?
Yes – soluble fibres absorb water and can slow stool transit. Insoluble fibres may worsen diarrhoea.
Q4: How long until I notice a difference?
Some benefits (like softer stools) may appear in days. Microbiome shifts usually take weeks.
Q5: Is fibre bad if I have SIBO?
Some fermentable fibres can worsen symptoms in SIBO. A phased, supervised approach is often needed.
9. Key Takeaways
- Fibre isn’t one thing – types matter.
- Soluble and prebiotic fibres are generally most supportive for gut health.
- Start low, go slow, and build variety.
- IBS responses vary – one size does not fit all.
- Whole foods > supplements for long-term microbiome health.
References
- Makki K, et al. The impact of dietary fibre on gut microbiota in health and disease. Science 2018.
- Sonnenburg ED, et al. Diet-induced extinctions in the gut microbiota compound over generations. Nature 2016.
- Ford AC, et al. Efficacy of soluble fibre in IBS: systematic review and meta-analysis. Am J Gastroenterol 2014.
- Moayyedi P, et al. The role of fibre in IBS: evidence summary. BMJ 2014.