The Ultimate Guide to Fibre

What is the importance of fibre?

Dietary fibre is the part of plant foods that resists digestion in the small intestine. Unlike carbohydrates such as sugars and starches, fibre is not broken down and absorbed as glucose. Instead, it passes into the large intestine, where it plays structural, mechanical and metabolic roles that are fundamental to human health.

For decades, fibre was thought of simply as “roughage” - useful mainly for preventing constipation. We now know that fibre does far more than keep things moving.

Large-scale meta-analyses involving hundreds of thousands of participants show that higher dietary fibre intake is associated with lower risk of:

• Cardiovascular disease.
• Type 2 diabetes.
• Colorectal cancer.
• All-cause mortality.

But these benefits do not come from one single type of fibre. They come from different fibres doing different jobs - some supporting bowel motility, others regulating blood sugar and cholesterol, and others feeding the trillions of microbes living in your gut.

To understand why fibre is so powerful for gut and overall health, we need to break it down into four key categories:

• Soluble fibre.
• Insoluble fibre.
• Prebiotic fibre.
• Resistant starch.

Each overlaps in some ways - but each has a distinct role.

First: the two classic categories

Soluble vs insoluble fibre

Soluble fibre

What it is: Soluble fibre dissolves in water and often forms a gel-like substance in the gut.

What it does:

• Slows digestion and blunts blood sugar spikes.
• Improves post-meal glucose control.
• Promotes longer satiety.
• Lowers LDL cholesterol by binding bile acids.
• Feeds gut bacteria when fermentable.

Common sources: Oats, barley, psyllium, chia, flax, legumes, apples.

Insoluble fibre

What it is: Insoluble fibre does not dissolve in water.

What it does:

• Adds bulk to stool.
• Stimulates intestinal movement.
• Speeds transit time in many individuals.

Common sources: Whole grains, wheat bran, nuts, seeds, vegetable skins, leafy greens.

Functional categories that matter for gut health

Beyond soluble and insoluble classifications, fibre can also be described based on how it behaves in the digestive system.

Prebiotic fibre

Prebiotic fibres resist digestion in the small intestine and are selectively fermented by beneficial gut bacteria.

What they do:

• Increase beneficial bacteria such as Bifidobacterium and Lactobacillus.
• Support microbial diversity.
• Promote production of short-chain fatty acids.
• Strengthen gut barrier integrity.

Examples: Inulin, fructo-oligosaccharides, galacto-oligosaccharides, some resistant starches.

Resistant starch

Resistant starch is technically a starch, but it behaves like fibre because it resists digestion and reaches the colon intact.

Types include:

• RS1 – Physically inaccessible starch in whole grains and seeds.
• RS2 – Naturally resistant starch in green bananas and raw potatoes.
• RS3 – Retrograded starch in cooked and cooled rice or potatoes.
• RS4 – Chemically modified starch.

What it does:

• Feeds butyrate-producing bacteria.
• Supports gut barrier integrity.
• May improve insulin sensitivity in some individuals.

How the fibre types compare

Why you need all four

• Insoluble fibre keeps intestinal contents moving.
• Soluble fibre stabilises blood sugar and supports heart health.
• Prebiotic fibre shapes the microbiome ecosystem.
• Resistant starch increases butyrate production for gut integrity.

Large analyses consistently associate higher fibre intake with reduced cardiovascular disease risk, reduced colorectal cancer risk, and lower all-cause mortality.

The gut-health takeaway

• Insoluble fibre supports motility.
• Prebiotic fibres and resistant starch support microbiome diversity and short-chain fatty acid production.
• Viscous soluble fibres support metabolic and cardiovascular health.

Too much of one type - particularly rapidly fermented prebiotic fibre - can cause bloating or constipation if introduced too quickly or without adequate hydration.

If you have experienced this, read Can too much fibre cause constipation? 

If you are unsure whether your gut is functioning optimally, see 5 signs of an unhealthy gut.

The bottom line

Fibre is not just about bowel movements. It plays a role in feeding beneficial microbes, producing anti-inflammatory metabolites, protecting the gut lining, supporting metabolic and heart health, and reducing long-term disease risk.

The goal is not simply more fibre. It is the right mix of fibre types. Your gut ecosystem thrives on diversity and fibre diversity is what feeds it.