Dietary fibre !

Fibre is the part of plant food that human enzymes cannot digest. It adds bulk to stools, slows sugar absorption and helps lower cholesterol.

Updated 2026-11-01

Key facts

Fibre = plant polysaccharides (and lignin) that resist human digestive enzymes, because we cannot break bonds such as the β(1→4) links of cellulose.
Insoluble fibre (cellulose, hemicellulose, lignin; whole grains, bran, vegetables) holds water, adds bulk and speeds transit.
Soluble fibre (pectins, gums, β-glucans of oats, psyllium; fruit, legumes) forms gels, slows gastric emptying and glucose absorption, and binds bile acids, lowering LDL cholesterol.
Colonic bacteria ferment fibre to short-chain fatty acids (acetate, propionate, butyrate); butyrate is the main fuel of colonocytes.
WHO advises at least 25 g/day of naturally occurring fibre for people over 10 years of age.

1. Fibre passes the small intestine undigested and reaches the colon almost intact.
2. In the colon it is partly fermented: gases (H₂, CO₂, methane) and short-chain fatty acids are formed; the acids lower colonic pH and feed the mucosa.
3. By holding water, insoluble fibre makes larger, softer stools and lowers pressure inside the colon.
4. By binding bile acids, soluble fibre makes the liver use more cholesterol to make new bile acids, so LDL falls.
5. Gels slow the arrival of glucose in the blood: the meal's glycaemic response is flatter.
6. Fibre gives few calories (about 2 kcal/g after fermentation) and increases satiety.
7. Excess fibre can reduce absorption of some minerals (iron, zinc, calcium) and causes bloating if increased suddenly; fluids should be increased with it.

Also called: Non-starch polysaccharides, Soluble and insoluble fibre, Roughage

Low-fibre diets are linked to constipation, diverticular disease and haemorrhoids; high-fibre diets to lower risk of colorectal cancer, type 2 diabetes and cardiovascular disease.
Soluble fibre (oats, psyllium) is part of the dietary treatment of high LDL cholesterol.
Fermentable fibres can worsen gas and bloating in irritable bowel syndrome, so the type of fibre is chosen case by case.

Think of the answer, then tap the question to see it.

Why can humans not digest cellulose?

Human enzymes cannot break its β(1→4) glycosidic bonds; amylase only breaks α(1→4) bonds of starch.

How does soluble fibre lower LDL cholesterol?

It binds bile acids in the gut; the liver then uses more cholesterol to replace them and takes up more LDL from blood.

What does colonic fermentation of fibre produce, and why does it matter?

Short-chain fatty acids such as butyrate, the main fuel of colon cells, plus gases.

How much fibre does WHO recommend for people over 10 years of age?

At least 25 grams a day of naturally occurring dietary fibre.

Which type of fibre mainly adds stool bulk and speeds transit?

Insoluble fibre (cellulose, hemicellulose, lignin), from bran, whole grains and vegetables.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: Fiber: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Soluble vs. insoluble fiber: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Dietary Fiber: MedlinePlus (MedlinePlus, US National Library of Medicine, 2026); Healthy diet (World Health Organization, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.