Soluble vs Insoluble Fibre: What's the Difference and Which Do You Need?

Myth: all fibre is the same.

Here's the one you actually need for your gut health.

By Bridget Foliaki-Davis, professional chef and qualified functional nutritionist | Updated August 2026

Key facts

• Dietary fibre comes in two main types: soluble fibre, which dissolves in water and forms a gel in the gut, and insoluble fibre, which adds bulk and keeps digestion moving.
• Australian guidelines set an Adequate Intake for dietary fibre of 25g a day for women and 30g for men (NHMRC Nutrient Reference Values).
• Analysis of the 2011-12 Australian National Nutrition and Physical Activity Survey found that only 28.2% of Australian adults met the fibre Adequate Intake, with a median intake of 20.7g a day.
• A third category, fermentable prebiotic fibres such as GOS (galacto-oligosaccharides) and inulin, feeds beneficial gut bacteria directly.
• Boost Protocol, sold by Bridget's Healthy Kitchen in Sydney, Australia, is a prebiotic GOS syrup containing one ingredient. It is heat-stable, so it can be stirred into hot drinks or used in cooking, and a jar holds 60 serves.
• In a 12-week double-blind, placebo-controlled trial of 44 overweight and obese prediabetic adults aged 45 to 70, a daily 15g dose of GOS significantly increased beneficial Bifidobacterium in the gut, though it did not change insulin sensitivity (Canfora et al., Gastroenterology, 2017; 153(1):87-97).
• In a randomised, double-blind, placebo-controlled study of 200 healthy volunteers, GOS produced the strongest bifidogenic effect of every prebiotic substance tested, while long-chain inulin showed no significant bifidogenic effect (Bouhnik et al., American Journal of Clinical Nutrition, 2004; 80(6):1658-1664).
• GOS acts selectively rather than broadly: across trials, it consistently increased Bifidobacterium specifically, rather than shifting the wider bacterial population, which is why it works best alongside a varied diet of plants.
  

Let's talk fibre. Because somewhere along the way, "eat more fibre" became the whole conversation, as if fibre were one single thing you could simply get more of. Like water. Or sleep.

Here's the truth: telling someone to "eat more fibre" is a bit like telling a cook to "add more seasoning". Which seasoning? For what dish? Salt and cinnamon are both seasonings, and I promise you they are not interchangeable.

Fibre works the same way. There are different types; they do genuinely different jobs in your body, and if your gut has been grumbling at you lately, there's a very good chance you're getting plenty of one type and almost none of the one your gut bacteria are quietly begging for.

That third type is the one almost nobody talks about. We'll get there. First, let's sort out the two you've heard of.

Soluble fibre: the gel-maker

Soluble fibre dissolves in water. In your gut, it forms a soft gel, and that gel does lovely, unglamorous work: it slows digestion down, helps you feel satisfied after a meal, and research suggests it supports healthy blood sugar balance by softening the spike after you eat.

Where you'll find it: oats, chia seeds, psyllium, apples, citrus, beans and lentils.

If you've ever made overnight oats and watched them turn thick and creamy, you've seen soluble fibre do its thing in a jar. That same gelling happens inside you. I find that genuinely delightful, but then again I'm a chef.

Insoluble fibre: the broom

Insoluble fibre doesn't dissolve. It passes through you largely intact, adding bulk and helping everything keep moving. Think of it as the kitchen broom of your digestive system. Not fancy. Absolutely essential.

Where you'll find it: wheat bran, brown rice, nuts, seeds, and the skins of fruit and veg. Yes, the skins. This is your permission slip to stop peeling everything.

Most of us, especially those of us who grew up with "roughage" and bran cereal, associate all fibre with this type. Which is exactly how the myth started. Bran is fibre, bran is beige and worthy; therefore all fibre is beige and worthy.

Not quite.

So which one do you need?

Both. That's the honest answer, and anyone who tells you otherwise is selling something too simple.

But here's where it gets interesting. Soluble and insoluble describe what fibre does physically, how it behaves in water. There's another way to sort fibre that matters even more for your gut health: whether your gut bacteria can eat it.

And this is the part of the story the bran cereal never told you.

The third type: prebiotic fibre, the one your gut actually feeds on

Your large intestine is home to trillions of bacteria, and the helpful ones have a favourite food. It isn't protein. It isn't the broom-style roughage, which mostly sweeps past them. It's fermentable prebiotic fibre: types like GOS (galacto-oligosaccharides) and inulin that travel undigested to the lower gut, where your good bacteria ferment them and multiply.

When those bacteria feed well, they produce short-chain fatty acids, compounds that research links to digestive wellbeing and a healthy gut lining. In plain terms: prebiotic fibre isn't just passing through. It's groceries for the good guys.

And GOS in particular has been properly studied, which I love. In one 12-week trial published in Gastroenterology, 44 adults aged 45 to 70 took either 15g of GOS a day or a placebo. The GOS group saw a significant increase in Bifidobacterium, one of the beneficial families we most want to see thriving. Worth saying plainly: that same study looked at insulin sensitivity and found no change, so this is a gut-microbiome finding, not a blood sugar one. I'd rather tell you exactly what the science says than stretch it.

There's an older study I like even more, because it put the prebiotics head to head. Two hundred healthy volunteers, double-blind and placebo-controlled, testing seven different non-digestible carbohydrates. GOS came out strongest of the lot for feeding Bifidobacterium. Long-chain inulin, in that particular trial, didn't shift it at all.

Now, here's where I have to be fair to inulin, because I sell it and I cook with it constantly. That result was for one specific form of inulin, and other fructan fibres have performed well elsewhere. Inulin earns its place in my pantry as a brilliant sugar substitute that happens to be fibre. It's a baking ingredient. GOS is a daily gut habit. Different jobs, both useful, and I'd rather tell you which is which than pretend one fibre does everything.

One more thing worth knowing, and most brands won't tell you: prebiotic fibre is selective, not broad. GOS narrows in on Bifidobacterium rather than lifting your whole microbial garden.

Which brings me to the thing I say to Campers more than anything else.

Eat the rainbow, and then eat some more of it

Your gut isn't home to one kind of bacteria. It's home to hundreds of species, and they don't all eat the same thing. Different plants carry different fibres, and different fibres feed different residents. Which means a narrow diet feeds a narrow population, however healthy that narrow diet looks on paper.

This is why I will never tell you that a spoonful of anything replaces vegetables. It doesn't. It can't. A concentrated prebiotic is very good at one specific job, and the studies show exactly that: GOS reliably feeds one beneficial family and leaves the rest of the garden much as it found it.

So the instruction from my kitchen has always been the same. Eat a wide variety of vegetables. Not more of the same three. Different colours, different families, different textures, across the week. Cabbage this week, fennel next. Something green, something orange, something you've never cooked before. Leave the skins on. Use the stalks.

That variety is the foundation. The spoonful is the top-up for the one fibre that's genuinely hard to get enough of from food alone. In that order, always.

While you're at it, add the ferments. Kimchi, sauerkraut, kefir, live yoghurt. Those bring live bacteria to the table rather than feeding the ones already there, which is a different job altogether, and they've been eaten across half the world's cuisines for centuries because they taste good. A forkful of kimchi next to dinner does more for your week than most things you can buy in a capsule. Just check they're properly fermented rather than pickled in vinegar; the real ones live in the fridge.

And about half of people in these studies respond strongly while others barely shift. Researchers have gone looking for what separates the two groups and haven't pinned it down. Bodies differ. Yours will tell you.

This is the fibre most of us get the least of. Onion, garlic, leek, slightly green bananas and legumes all contain some. But the amounts in a normal day's eating are modest, which is why prebiotic fibres are the gap worth closing first.

It's also why we sell Boost Protocol. After years of coaching Campers through gut-health cooking, I kept hitting the same wall: the fibre people most needed was the one hardest to eat enough of. So I went looking for the best GOS I could find.

Sourcing is most of a chef's job, and I am fussy about it. I wanted GOS in a format people would actually keep using, with nothing else in the jar padding it out. What I found was a syrup: one ingredient, no fillers, no added sweeteners, and sweet enough that taking it doesn't feel like a chore.

The format was the deciding factor. It's heat-stable, so it goes straight into your morning coffee or tea, or into whatever you're cooking. That matters more than it sounds, because the hardest part of any daily habit is remembering it, and the easiest fix is to hang it on something you already do. No powders, no capsules, no cardboard, and no new routine to build.

One honest note: if your gut isn't used to fermentable fibre, start small. Half a teaspoon, then build. Your bacteria adjust; give them a week or two of grace. If you've tried adding fibre before and felt like a balloon by day three, you're not alone! 

Related: Bloating | Symptoms, Causes + Prevention Tips

Your one-day fibre upgrade

Here's how simple this gets. No overhaul, just three swaps across one day:

Breakfast: my chia breakfast pudding (recipe below), which quietly covers all three types before you've had your second coffee.

Lunch: whatever you're having, plus half an avocado and a scatter of seeds (soluble and insoluble in one easy addition).

Dinner: cook your onion and garlic base properly, the way every good dish starts anyway (prebiotic), and leave the skin on your roast veg (insoluble).

That's it. Three types, one ordinary day of delicious food. This is not about restriction. It's about feeding yourself, and the trillions of tiny tenants doing good work on your behalf.

Recipe: My Healthy Chia Breakfast Pudding

This one has a story. On a trip away years ago, I ate one of the best chia puddings of my life. Utterly delicious, and absolutely doused in raw cane sugar. I posted a photo, got inundated with requests for a healthier version, licked my spoon, and got to work. The result has been a Camper favourite ever since, and it happens to be a perfect all-three-types breakfast.

Serves 1 | Gluten-free, sugar-free, dairy-free, vegan | Soak 2-12 hours, so start the night before

Ingredients

• 3 tbsp chia seeds
• 125ml light coconut milk
• 125ml coconut water
• ½ green apple, peeled and diced small
• 1 tbsp Pure As Inulin Powder (or swap in Sweet As Fibre Syrup; it works beautifully)
• Pinch each of cinnamon, mixed spice and ginger
• 50g frozen mixed berries
• 1 tbsp unsweetened coconut yoghurt, to serve
  

Method

1. Stir the chia seeds into the coconut milk and coconut water, cover, and soak in the fridge for 2 to 12 hours.
2. Microwave the apple for 60 to 90 seconds until tender, then stir through half the inulin and the spices.
3. Bring the berries and the remaining inulin to a gentle boil in a small pot, then take off the heat.
4. Stir the soaked chia (loosen with a splash more coconut milk if lumpy), then layer: chia, berries, chia, apple, yoghurt, remaining berries. Devour.
   

Full recipe with photos: Bridget's Healthy Chia Breakfast Pudding

Nutritional note: soluble fibre from the chia, insoluble fibre from the seed hulls and apple, prebiotic fibre from the inulin. All three types in one jar, and it tastes like the holiday version.

FAQ

What is the difference between soluble and insoluble fibre? Soluble fibre dissolves in water and forms a gel that slows digestion and supports satiety and healthy blood sugar balance. Insoluble fibre doesn't dissolve; it adds bulk and keeps digestion regular. Most whole plant foods contain a mix of both, in different ratios.

What is prebiotic fibre? Prebiotic fibre is fermentable fibre, such as GOS or inulin, that resists digestion and reaches the large intestine intact, where it feeds beneficial gut bacteria. Food sources include onion, garlic, leek, legumes and just-ripe bananas; concentrated sources include GOS syrups and inulin powder.

How much fibre do I need a day? Australian guidelines set an Adequate Intake of 25g a day for women and 30g for men. Most of us fall well short: an analysis of national survey data found only 28.2% of Australian adults reached that target, with a median intake around 20.7g. Building up gradually, with plenty of water, is gentler than a sudden jump.

Which fibre is best for gut health? Your gut does best with variety, but fermentable prebiotic fibres like GOS give beneficial bacteria their preferred food, so they're the smart gap to close if your diet is already decent on wholegrains and veg. Boost Protocol is a 100% GOS syrup with no fillers. Start with small amounts and build slowly.

Is a prebiotic the same as a probiotic? No. Prebiotics are fibres that feed the beneficial bacteria already in your gut. Probiotics are live bacteria you swallow with the aim of adding more. Probiotics add, prebiotics feed, and most people who are generally well get more from feeding what they already have. Full comparison here.

Is GOS the same as inulin? No. Both are prebiotic fibres, but they behave differently. In one head-to-head trial, GOS produced a stronger increase in Bifidobacterium than long-chain inulin. Inulin is excellent as a fibre-rich sugar substitute in baking; GOS suits a daily gut-health habit. Many people use both, for different jobs.

Does cooking destroy fibre? No. Cooking softens fibre and can change its texture, but the fibre itself survives your saucepan just fine. Cook your vegetables however you love them; eaten beats perfect, every time.

Ready to feed the good guys?

Boost Protocol is 100% GOS syrup, one ingredient, 60 serves per jar. Heat-stable, so it goes straight into your morning coffee. Start at half a teaspoon and build from there. Or grab the Fibremaxxing Bundle and cover all three fibre types in one delivery.

 

 

About the author. Bridget Foliaki-Davis is a professional chef with more than 30 years of experience and a qualified functional nutritionist. She refined her classical French technique at the Paul Bocuse Institute in Lyon, cooked as VIP chef for Google Australia, and is the author of 5 cookbooks and winner of 5 Gourmand World Cookbook Awards. She founded Bridget's Healthy Kitchen and the 28 Day Boost Camp, and lost over 30kg through her own cooking, maintaining her health for close to a decade. She lives in Sydney with her family, who run the business together.

This content is for educational purposes only and is not medical advice. It is not intended to diagnose, treat, cure, or prevent any disease. Always consult your doctor or a qualified healthcare professional before changing your diet, especially if you have a medical condition or take medication.