Diet Notes

Cambridge IGCSE Biology: Revision notes

Key facts

  • A balanced diet contains all nutrient types in the right proportions: carbohydrates, fats, proteins, vitamins, mineral ions, fibre and water.
  • Carbohydrates give quick energy; fats give a long-term energy store, insulation and protection.
  • Protein is for growth and repair; vitamin D helps absorb calcium; iron is needed for haemoglobin.
  • Lack of vitamin C causes scurvy; lack of vitamin D or calcium causes rickets.
  • Fibre adds bulk and prevents constipation; water is the solvent for reactions and transport.

A balanced diet

A balanced diet has all seven nutrient types, and the proportions matter as much as the nutrients themselves.

Name all the nutrient types, not just some. Each has a main job, and each comes from particular foods.

  1. 1

    Carbohydrates

    energy

  2. 2

    Fats and oils

    energy store and insulation

  3. 3

    Proteins

    growth and repair

  4. 4

    Vitamins and mineral ions

    specific metabolic and body functions

  5. 5

    Fibre

    bulk for the gut

  6. 6

    Water

    solvent for all body processes

What each nutrient does for the body.

Main job

Carbohydrates:
Energy
Fats and oils:
Energy store and insulation
Proteins:
Growth and repair
Vitamins and mineral ions:
Specific metabolic and body functions
Fibre:
Bulk for the gut
Water:
Solvent for all body processes

Sources

Carbohydrates:
Bread, rice, pasta, potatoes
Fats and oils:
Butter, oils, nuts
Proteins:
Meat, fish, eggs, beans
Vitamins and mineral ions:
Fruit, vegetables, dairy, leafy greens
Fibre:
Wholemeal foods, vegetables
Water:
Drinks, fruit and vegetables

A balanced diet must contain:

Carbohydrates and fats

Carbohydrates are the main quick energy source, and fats store more energy per gram and also insulate and protect.

Glucose is the main energy source for respiration in all cells and is essential for the brain; excess is stored as glycogen in the liver and muscles. Fats and oils are a long-term energy store under the skin and around organs. They insulate, cushion vital organs, supply essential fatty acids and help absorb fat-soluble vitamins (A, D, E, K).

010203040CarbohydrateFatNutrientEnergy (kJ per g)
Approximate energy released per gram of food

Which gives more energy per gram?

Proteins, vitamins and minerals

Protein builds and repairs tissue, vitamins and mineral ions each do a specific job.

Remember how vitamin D and calcium link: vitamin D enables the body to absorb calcium, so without enough vitamin D calcium absorption is impaired even if the diet contains plenty. Iron makes haemoglobin for oxygen transport.

  1. 1

    Sunlight on skin, or fatty fish and egg yolk

    supplies vitamin D

  2. 2

    Vitamin D enables calcium absorption

  3. 3

    Calcium builds strong bones and teeth

Vitamin D enables calcium to be absorbed.

Protein

Sources:
Meat, fish, eggs, beans, dairy
Why it is needed:
Growth and repair; enzymes and antibodies

Vitamin C

Sources:
Citrus fruits, berries, peppers
Why it is needed:
Collagen production; immune function

Vitamin D

Sources:
Fatty fish, egg yolks; made by skin in sunlight
Why it is needed:
Calcium absorption; bone strength

Calcium (mineral ion)

Sources:
Dairy, leafy greens, nuts
Why it is needed:
Bones and teeth; blood clotting; muscle contraction

Iron (mineral ion)

Sources:
Red meat, beans, fortified cereals
Why it is needed:
Making haemoglobin; oxygen transport

Which vitamin is needed to absorb calcium?

Deficiency diseases

Scurvy is caused by a lack of vitamin C and rickets by a lack of vitamin D or calcium.

State the nutrient lacking and link it to the symptoms. Vitamin C is needed for collagen, so a lack gives bleeding gums and poor wound healing. Lack of vitamin D or calcium means bones do not harden properly. Sailors with no fresh fruit used to get scurvy.

  1. 1

    Lack of vitamin C gives scurvy

    collagen is not made: bleeding gums, poor wound healing

  2. 2

    Lack of vitamin D or calcium gives rickets

    bones do not harden: soft, bowed legs in children

Deficiency diseases link a missing nutrient to symptoms.

Scurvy

Cause:
Long-term lack of vitamin C
Effects:
Bleeding gums, poor wound healing, easy bruising
Who:
Historically sailors with no fresh fruit

Rickets

Cause:
Lack of vitamin D or calcium
Effects:
Soft, weak bones; bowed legs
Who:
Mostly children, whose bones are still developing

Which deficiency causes scurvy?

Fibre and water

Fibre keeps food moving through the gut, and water is the solvent for reactions and transport.

Fibre adds bulk to the contents of the gut and helps peristalsis, which prevents constipation. Water is the solvent for metabolic reactions, transports nutrients and waste, and helps control temperature through sweating.

Labelled diagram of the human digestive system
Fibre adds bulk to the contents of the gut and helps peristalsis move food along; water is absorbed along the gut.

Fibre

Main sources:
Wholemeal bread and cereals, fruit and vegetables, beans
Importance:
Adds bulk; promotes peristalsis; prevents constipation

Water

Main sources:
Drinking water, drinks, fruit and vegetables
Importance:
Solvent for reactions; transport; temperature control

How does fibre help the gut?

Try an exam question

A child has soft, bowed legs. Name the disease and explain how the child's diet may have caused it.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.