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This lesson covers dietary fibre, also known as non-starch polysaccharide (NSP), as required by AQA GCSE Food Preparation and Nutrition specification 8585 (section 3.2.1). Although dietary fibre is classified as a carbohydrate, the body cannot digest it — and that is precisely why it is so important. You will learn what fibre is, its food sources, its functions in the body, and why the UK population needs to eat more of it.
Dietary fibre is the term for the parts of plant foods that the human body cannot digest or absorb. It passes through the stomach, small intestine, and colon largely intact.
The scientific term used in the AQA specification is non-starch polysaccharide (NSP) — this distinguishes fibre from starch, which is a polysaccharide the body can digest.
Exam Tip: The AQA specification uses the term NSP (non-starch polysaccharide). In the exam, you may see either "dietary fibre" or "NSP" — they mean the same thing. Use whichever term appears in the question, or use both to show your understanding.
Dietary fibre is classified into two types based on how it behaves in water:
| Type | Description | Food Sources | Function |
|---|---|---|---|
| Insoluble fibre | Does not dissolve in water. Absorbs water and swells, adding bulk to faeces. | Wholemeal bread, brown rice, bran, whole wheat pasta, vegetable skins, nuts, seeds | Adds bulk to stools, speeds up transit time through the gut, prevents constipation |
| Soluble fibre | Dissolves in water to form a gel-like substance. | Oats, barley, fruit (especially apples, citrus), vegetables, pulses (beans, lentils, chickpeas), flaxseeds | Slows digestion, helps lower blood cholesterol, helps regulate blood sugar levels |
Both types of fibre are important, and a healthy diet should include a variety of fibre-rich foods to obtain both.
graph TD
A["Dietary Fibre — NSP"] --> B["Insoluble Fibre"]
A --> C["Soluble Fibre"]
B --> B1["Does NOT dissolve in water"]
B --> B2["Adds bulk to faeces"]
B --> B3["Prevents constipation"]
B --> B4["Sources: wholemeal bread, bran, brown rice, vegetable skins"]
C --> C1["Dissolves in water — forms gel"]
C --> C2["Lowers blood cholesterol"]
C --> C3["Regulates blood sugar"]
C --> C4["Sources: oats, fruit, pulses, barley"]
style A fill:#4a90d9,color:#fff
style B fill:#e67e22,color:#fff
style C fill:#27ae60,color:#fff
| Food Source | Type of Fibre | Key Details |
|---|---|---|
| Wholemeal bread | Mainly insoluble | Made from flour containing the whole grain, including the bran |
| Brown rice | Mainly insoluble | Retains the bran layer; higher in fibre than white rice |
| Wholegrain breakfast cereals | Both | Weetabix, Shredded Wheat, bran flakes, muesli, porridge oats |
| Oats and porridge | Mainly soluble | Contain beta-glucan, a soluble fibre shown to lower cholesterol |
| Fruit | Both | Apples, pears, oranges, bananas, berries — eat with skin where possible |
| Vegetables | Both | Carrots, broccoli, peas, sweetcorn, cabbage, Brussels sprouts |
| Pulses | Both | Lentils, chickpeas, kidney beans, baked beans — excellent sources |
| Nuts | Mainly insoluble | Almonds, walnuts, Brazil nuts |
| Seeds | Both | Chia seeds, flaxseeds, sunflower seeds, pumpkin seeds |
| Wholemeal pasta | Mainly insoluble | Higher in fibre than white pasta |
| Potato skins | Mainly insoluble | Eating potatoes with their skins increases fibre intake |
Understanding the difference between wholegrain and refined is essential:
| Term | Meaning | Example |
|---|---|---|
| Wholegrain | The entire grain is used, including the bran (outer layer), germ (seed), and endosperm (starchy centre) | Wholemeal flour, brown rice, porridge oats |
| Refined | The bran and germ have been removed during processing, leaving only the endosperm | White flour, white rice, white bread |
Refining removes much of the fibre, B vitamins, and iron. This is why the government recommends choosing wholegrain carbohydrates where possible.
Exam Tip: A common exam question asks you to suggest how someone could increase their fibre intake. Always give specific swaps: replace white bread with wholemeal bread, white rice with brown rice, white pasta with wholemeal pasta, sugary cereal with porridge or bran flakes. Add fruit, vegetables, and pulses to meals.
| Function | Explanation |
|---|---|
| Aids digestion | Fibre stimulates peristalsis (the muscular contractions that move food through the digestive tract), keeping the digestive system working efficiently. |
| Prevents constipation | Insoluble fibre absorbs water, making stools softer, bulkier, and easier to pass. This prevents constipation and reduces the need to strain. |
| Reduces risk of bowel cancer | A high-fibre diet reduces the time that potential carcinogens (cancer-causing substances) spend in contact with the bowel wall. Fibre also dilutes these substances. |
| Reduces risk of diverticular disease | Diverticular disease occurs when small pouches (diverticula) form in the bowel wall due to straining from constipation. A high-fibre diet prevents this. |
| Lowers blood cholesterol | Soluble fibre binds to bile acids (which contain cholesterol) in the gut and helps remove them from the body. The liver then uses more cholesterol to make new bile acids, reducing blood cholesterol levels. |
| Helps regulate blood sugar | Soluble fibre slows the absorption of glucose into the blood, preventing rapid spikes in blood sugar. This is beneficial for people with or at risk of type 2 diabetes. |
| Promotes satiety | High-fibre foods take longer to chew and digest, helping you feel full for longer. This can help with weight management by reducing overeating. |
| Feeds gut bacteria | Some fibre is fermented by beneficial bacteria in the large intestine, supporting a healthy gut microbiome. This produces short-chain fatty acids that nourish the cells lining the colon. |
| Age Group | Recommended Daily Fibre Intake |
|---|---|
| Children aged 2–5 years | 15 g per day |
| Children aged 5–11 years | 20 g per day |
| Children aged 11–16 years | 25 g per day |
| Adults (17+ years) | 30 g per day |
Most people in the UK consume only about 18 g of fibre per day — well below the recommended 30 g for adults. Increasing fibre intake is a public health priority.
| Food | Approximate Fibre Content |
|---|---|
| 2 slices of wholemeal bread | 4.0 g |
| 2 slices of white bread | 1.4 g |
| 1 portion of porridge (40 g oats) | 3.6 g |
| 1 medium apple (with skin) | 2.4 g |
| 1 medium banana | 1.4 g |
| 80 g broccoli | 2.2 g |
| 80 g baked beans | 3.7 g |
| 80 g cooked lentils | 3.0 g |
| 1 portion of brown rice (180 g cooked) | 2.8 g |
| 1 portion of white rice (180 g cooked) | 0.4 g |
Exam Tip: The table above illustrates why wholegrain swaps are so effective — two slices of wholemeal bread (4.0 g fibre) compared to white bread (1.4 g) is a significant difference. Use specific figures like these in the exam to demonstrate your knowledge.
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