Diet

1. Carbohydrates. Main source of ENERGY. Found in bread, rice, pasta, potatoes, sugar.

2. Proteins. GROWTH and REPAIR of tissues; making enzymes, antibodies, hormones. Found in meat, fish, eggs, beans, dairy.

3. Fats (lipids). Long-term energy STORE; insulation; making cell membranes; carries fat-soluble vitamins. Found in butter, oil, nuts, fatty meat.

4. Vitamins. Small but vital — many roles. Cambridge focus: vitamins C and D. See dedicated section.

5. Mineral ions. Many roles. Cambridge focus: calcium and iron. See dedicated section.

6. Water. ~60% of body. Solvent for chemical reactions; transport (blood plasma); temperature regulation (sweating); medium for digestion.

7. Fibre (roughage). Indigestible plant material (mostly CELLULOSE). Provides BULK so the gut can grip and push food along (peristalsis). PREVENTS CONSTIPATION. Found in fruit, vegetables, wholegrain bread, beans.

Cambridge tip. Cambridge mark schemes credit BOTH water AND fibre. Listing only the "main" five (carbs, protein, fat, vitamins, minerals) loses 2 marks.

Vitamin C (ascorbic acid).

• Source: citrus fruits (oranges, lemons), tomatoes, peppers, blackcurrants.
• Function: needed to make COLLAGEN — the protein in connective tissue, blood vessel walls, gums.
• Deficiency: SCURVY → bleeding gums, weakness, slow wound healing.
• Vitamin C is destroyed by heat — overcooking vegetables removes much of it.

Vitamin D.

• Source: oily fish, dairy products, eggs. Also MADE in the skin from sunlight.
• Function: helps the gut ABSORB CALCIUM ions from food.
• Deficiency: RICKETS in children (soft, bowed bones — legs curve outward). In adults: weak bones (osteomalacia).
• Children with little sun exposure (long winters, indoor lifestyles, head-covering clothing) at risk → governments fortify foods.

Worked qualitative. A child has bowed legs and thin wrist bones. Pictures show the family lives in northern Scandinavia. Likely cause?

• Vitamin D deficiency from lack of sunlight.
• Without vitamin D, calcium isn't absorbed → bones softer than normal → bend under weight → rickets.
• Treatment: vitamin D supplements + dairy + sunlight when possible.

Cambridge tip. Vitamin D's job is "helps the gut absorb calcium" — not "is needed for bones directly". Linking the chain (D → Ca²⁺ absorption → bones) earns full marks.

Calcium (Ca²⁺).

• Source: dairy (milk, cheese, yoghurt), green leafy veg, fish with edible bones.
• Function: BONES and TEETH (calcium phosphate); also needed for BLOOD CLOTTING and muscle function.
• Deficiency: weak bones (rickets, osteoporosis), poor blood clotting.
• Vitamin D required for absorption — see above.

Iron (Fe).

• Source: red meat, liver, beans, dark green leafy veg (spinach), wholegrain breads.
• Function: makes HAEMOGLOBIN — the oxygen-carrying protein in red blood cells.
• Deficiency: ANAEMIA — fewer / weaker red blood cells, less O₂ carried, fatigue, pale skin, breathlessness.
• Cooking in cast-iron pans adds traces of iron to food.

Worked qualitative. A vegan teenager complains of constant tiredness and looks pale. Why?

• Iron-deficiency anaemia is a risk for vegans because the most iron-rich foods are animal-based (meat, liver).
• Plant iron is harder for the body to absorb than animal iron.
• Fix: eat iron-rich plants (spinach, lentils, tofu) WITH vitamin C (boosts absorption); supplements if needed.

Cambridge tip. When asked "why iron?", say "to make haemoglobin which carries oxygen". Don't just say "for blood".

Daily energy requirement depends on:

1. Age.

• Babies: high energy per kg (rapid growth + small body).
• Teenagers: very high total intake (growth spurt + activity).
• Adults (sedentary): moderate intake.
• Elderly: lower (slowing metabolism, less activity).

2. Sex.

• Males generally need more — more muscle mass = higher resting metabolic rate.

3. Activity level.

• An office worker needs ~2,500 kcal/day; an Olympic swimmer in training needs 6,000+.

4. Pregnancy / breastfeeding.

• Extra ~300-500 kcal/day to support fetus or milk production.
• Higher protein, calcium, iron, folate needs.

5. Body size.

• Bigger person = more cells = more energy for maintenance.

6. Climate.

• Cold climates: more energy used to keep warm.

Worked qualitative. A woman who is breastfeeding eats 2,800 kcal/day instead of her usual 2,200. Why?

• Producing breast milk requires energy (milk is mostly water, fat, sugar, protein).
• The body uses ~500 extra kcal/day to make milk.
• Without extra food intake, the mother's own body stores deplete → fatigue, weight loss, lower milk supply.

Cambridge tip. When asked which group has highest energy demand per kg of body, the answer is BABIES (not athletes). Per kg body weight, infants are growing the fastest.

Malnutrition = a diet that's UNBALANCED. Includes:

1. Starvation. Total food intake too low. Body breaks down its own protein and fat. Severe weight loss, weakness, organ failure, death.

2. Kwashiorkor. Severe PROTEIN deficiency in children, often when carbs are still available. Symptoms: swollen belly (oedema, due to low blood protein → fluid leaks out), thin limbs, poor growth, brittle hair.

3. Anaemia. Low haemoglobin. Cause: iron deficiency (most common), or vitamin B12. Symptoms: tiredness, pale skin.

4. Scurvy. Vitamin C deficiency. Bleeding gums, slow healing.

5. Rickets. Vitamin D / calcium deficiency in children. Bowed legs, soft bones.

6. Obesity. EXCESS food intake (especially fats and sugars). BMI > 30. Linked to type 2 diabetes, heart disease, joint problems, some cancers. Strictly a form of malnutrition.

Worked qualitative. A toddler has a bloated belly, thin arms and legs, and pale orange-tinged hair. The family eats mainly cassava (a starchy staple). What's wrong?

• Protein deficiency (cassava is almost pure carbohydrate).
• Classic KWASHIORKOR.
• Treatment: gradual reintroduction of protein-rich foods (milk powder, beans, egg).

Cambridge tip. OBESITY is a form of malnutrition — students often forget this. Malnutrition isn't just "too little".