"Eat More Fibre!" ...But Which Fibre?
You need to eat more fibre."
It's probably one of the most common pieces of digestive health advice.
Constipated? Eat more fibre.
Want a healthier gut microbiome? Eat more fibre.
High cholesterol? Eat more fibre.
Want to reduce your risk of bowel disease? You guessed it: eat more fibre.
And broadly speaking, it's good advice.
But fibre isn't one thing. If you already have digestive symptoms, simply piling more fibre onto your plate is likely to make you feel considerably worse.
The type of fibre, how fermentable it is, how much you're currently eating, and how quickly you increase it matters.
So rather than telling you to just "eat more fibre", let's look at which fibres might actually suit your gut, and how to increase them without causing havoc.
First: what actually is fibre?
Dietary fibre is made up of carbohydrates found in plant foods that aren't completely digested in the small intestine.
Instead, they continue into the large intestine, where they can do all sorts of useful things.
Depending on the type of fibre, it may:
add bulk to stools
absorb water and soften stools
help regulate bowel movements
feed beneficial gut bacteria
support production of short-chain fatty acids such as butyrate
support blood sugar regulation
help maintain healthy cholesterol levels
increase satiety
But different fibres behave very differently once they're inside your digestive tract.
That's where things get interesting.
Soluble fibre: the sponge
Soluble fibre dissolves in water and, in many cases, forms a gel-like substance in the digestive tract.
This can help soften hard stools, while also helping to firm up loose stools by absorbing excess water.
Sources include:
oats
chia seeds
ground flaxseed
psyllium husk
apples
pears
kiwi
carrots
sweet potato
For people with sensitive digestion, soluble fibre is often a useful place to start.
Psyllium in particular is interesting because it can help regulate stool consistency in both constipation and diarrhoea.
But even here, more isn't automatically better. Introduce concentrated sources such as psyllium gradually and make sure you're drinking enough fluid alongside them.
Insoluble fibre: the broom
Insoluble fibre doesn't dissolve in water, instead, it adds bulk to stools and can help move material through the digestive tract hence the simplistic but useful analogy of a broom.
You'll find it in foods including:
wheat bran
wholegrains
nuts and seeds
vegetable skins
leafy vegetables
For someone with a healthy digestive system, these foods can be a valuable part of a varied diet.
However, if you have a very sensitive or irritated gut, active diarrhoea, significant urgency or certain inflammatory or structural bowel conditions, suddenly adding lots of coarse insoluble fibre may be the last thing your digestive system wants.
This is one reason why telling everyone with constipation to eat bran isn't particularly helpful.
Fermentability: the piece people often miss
Some fibres are highly fermentable. This means our gut bacteria rapidly break them down and use them as food. Fermentation produces compounds called short-chain fatty acids, including butyrate, which play important roles in maintaining the health of the colon and supporting the gut environment.
But fermentation also produces gas - that's where a supposedly gut-healthy food can become uncomfortable.
If you have a sensitive digestive system, suddenly eating large quantities of highly fermentable fibre will likely cause:
bloating
excessive gas
abdominal discomfort
cramping
changes in bowel movements
Highly fermentable foods include many:
beans
lentils
chickpeas
onions
garlic
leeks
asparagus
certain fruits
inulin and chicory-root fibres added to some "high fibre" foods
This doesn't make these foods bad - many of them are incredibly nutritious foods that can support a diverse gut microbiome, but your specific tolerance matters.
Someone who currently eats very little fibre and experiences significant bloating should not start their gut-health journey with a giant bowl of chickpeas, onions and garlic.
Start lower and slower.
A particular note on legumes
Beans, lentils and chickpeas deserve their own mention.
They're fantastic sources of fibre, plant protein, minerals and polyphenols.
But they can also be very fermentable.
For someone with IBS, significant bloating, excessive gas or generally sensitive digestion, suddenly adding large portions of legumes can cause havoc.
That doesn't mean you should or need to avoid them forever.
If you want to introduce them, try:
starting with a very small portion
choosing canned legumes and rinsing them thoroughly
increasing the amount gradually
observing your individual tolerance
And importantly: you do not have to eat legumes to achieve a high-fibre diet.
Resistant starch: food for your microbes
Resistant starch behaves a little differently from ordinary starch.
Rather than being completely digested in the small intestine, some reaches the large intestine where it can be fermented by gut bacteria.
Sources include:
cooked and cooled potatoes
cooked and cooled rice
cooked and cooled pasta
oats
beans and lentils
slightly green bananas
Resistant starch can support production of short-chain fatty acids*, including butyrate**.
Again, though, tolerance varies.
If you currently have significant digestive symptoms, introduce these foods gradually rather than suddenly trying to maximise resistant starch intake.
*A quick note on short-chain fatty acids (SCFAs): When certain fibres are fermented by our gut bacteria, they produce compounds called short-chain fatty acids. Primarily acetate, propionate and butyrate. These aren't just waste products of fermentation; they have important roles in gut and wider health.
**Butyrate, in particular, is a major energy source for the cells lining the colon and helps support the integrity of the gut barrier. This is one of the reasons feeding our gut microbes with a varied range of fibres can be so beneficial.
So what fibre should YOU eat?
This is where I'd start thinking about symptoms rather than fibre targets alone.
If you're constipated
Consider gradually increasing:
kiwi fruit
ground flaxseed
chia seeds
oats
cooked vegetables
psyllium husk
fruit
fluids alongside your fibre intake
And remember that constipation isn't always caused by inadequate fibre.
If you're already eating plenty, simply adding more and more bulk isn't necessarily going to solve the underlying problem.
If you have loose stools or diarrhoea
Gentler soluble fibres may be better tolerated, such as:
oats
psyllium
banana
cooked carrots
peeled/cooked fruits and vegetables
Large quantities of bran, raw vegetables and coarse insoluble fibre may aggravate symptoms for some people.
Persistent diarrhoea, particularly with blood, unexplained weight loss, fever or night-time symptoms, deserves medical investigation rather than simply experimenting with fibre.
If you're very bloated or gassy
Don't assume you need more fibre immediately.
Look at what you're currently eating.
Have you suddenly increased:
legumes?
onions and garlic?
high-fibre bars or cereals?
foods containing added inulin/chicory fibre?
huge portions of vegetables?
You may benefit from initially choosing less rapidly fermentable sources of fibre and slowly building your tolerance.
The aim isn't to permanently remove every food that causes fermentation.
It's to find the amount and type your digestive system can comfortably handle and build from there where appropriate.
If your digestion is generally sensitive
Start with food texture as well as fibre type.
A bowl of raw kale, cabbage and chickpeas places a very different demand on digestion than a bowl of well-cooked carrots, courgette and sweet potato.
Try:
cooking vegetables thoroughly
peeling tougher skins initially
choosing softer fruits
introducing one new fibre source at a time
starting with small portions
You can gradually expand from there.
How to increase your fibre without regretting it
The UK recommendation for adults is around 30g of fibre per day.
But if you're currently eating 12–15g, tomorrow doesn't need to be your first 30g day.
Your digestive system, and the microbes living within it, need time to adapt.
Instead of obsessing over hitting the target immediately, increase gradually.
Week 1
Look at what you're already eating and add one small fibre-rich addition each day.
Perhaps an extra cooked vegetable with dinner or a kiwi with breakfast.
Week 2
If you're tolerating that well, add another gentle source.
Perhaps oats, ground flaxseed or chia.
Week 3
Increase the variety of vegetables and fruits you're eating.
If your digestion is comfortable, this might also be the point to experiment with a small portion of a more fermentable food such as lentils or chickpeas.
Week 4 and beyond
Continue increasing both quantity and diversity according to your tolerance.
There isn't a prize for getting there fastest.
A few things that make fibre much easier to tolerate
Increase fluids alongside fibre.
Fibre absorbs water. Increasing fibre substantially without increasing fluid intake can actually make constipation worse.
Cook your vegetables.
If your gut is sensitive, cooked vegetables are often easier to tolerate than enormous raw salads.
Chew properly.
This is far more important than anyone want to believe and so effective! Resist the habit of wolfing down your food and aim for the consistency of puree before you swallow.
Don't introduce five new things at once.
If you add flaxseed, psyllium, lentils, chia seeds and three extra portions of vegetables simultaneously and then become horribly bloated, you won't know what caused it.
Pay attention to your symptoms.
Your response is useful information.
The takeaway
I absolutely want people eating plenty of fibre.
But I don't want someone who's struggling with their digestion to hear "eat 30g of fibre a day" and immediately start adding bran cereal, fibre bars, enormous salads and spoonfuls of inulin to everything they eat.
Good digestive nutrition is more nuanced than that.
The right fibres, in the right amounts, introduced at a pace your digestive system can actually tolerate.
And over time, where appropriate, we want to build towards a varied diet containing lots of different plant foods and fibres rather than continually restricting them because they once caused symptoms.
If you'd like some support decoding what your gut is trying to tell you and putting together a personalised plan to move forward, click here to book a free discovery call with me.
We can have a chat about what's been going on and whether working together might be the right next step for you.