[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Biochemistry](https://durgical.com/en/biochemistry/) › Dietary fibre

[Biochemistry](https://durgical.com/en/biochemistry/) · Topic

# 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.

## In detail

- 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

## Clinical notes

- 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.

## Related conditions

- [Diverticular disease and diverticulitis](https://durgical.com/en/conditions/diverticular-disease-and-diverticulitis/)
- [Colorectal cancer](https://durgical.com/en/conditions/colorectal-cancer/)
- [Type 2 diabetes mellitus](https://durgical.com/en/conditions/type-2-diabetes-mellitus/)
- [Irritable bowel syndrome](https://durgical.com/en/conditions/irritable-bowel-syndrome/)
- [Atherosclerosis and dyslipidaemia](https://durgical.com/en/conditions/atherosclerosis-and-dyslipidaemia/)

## Related

- [Carbohydrate chemistry](https://durgical.com/en/biochemistry/topics/carbohydrate-chemistry/)
- [Macronutrients and the energy value of food](https://durgical.com/en/biochemistry/topics/macronutrients-and-the-energy-value-of-food/)
- [Balanced diet](https://durgical.com/en/biochemistry/topics/balanced-diet/)
- [Cholesterol](https://durgical.com/en/biochemistry/molecules/cholesterol/)
- [Bile acid synthesis](https://durgical.com/en/biochemistry/topics/bile-acid-synthesis/)

## Test yourself

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.

[Add to my cards (5)](https://durgical.app/estudiar/?add=/en/biochemistry/topics/dietary-fibre/)

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

Sources: [Fiber: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002470.htm) (MedlinePlus, US National Library of Medicine, 2026); [Soluble vs. insoluble fiber: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002136.htm) (MedlinePlus, US National Library of Medicine, 2026); [Dietary Fiber: MedlinePlus](https://medlineplus.gov/dietaryfiber.html) (MedlinePlus, US National Library of Medicine, 2026); [Healthy diet](https://www.who.int/news-room/fact-sheets/detail/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).

[← All of biochemistry](https://durgical.com/en/biochemistry/)

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/biochemistry/topics/dietary-fibre/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/topics/dietary-fibre/