Diet and deficiency
The components of a balanced diet and what happens when one is missing.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 7.1.1Describe the components of a balanced diet and the effects of deficiencies.
4 minute read
Diet and deficiency
A balanced diet contains all the required nutrients in the correct proportions and amounts for the individual — and what is correct varies with age, sex, activity and pregnancy.
What a diet must contain, and why
- Carbohydrates — the main source of energy.
- Fats (lipids) — energy storage, insulation, and making cell membranes.
- Proteins — growth and repair of tissues.
- Vitamins — small amounts needed for specific processes.
- Minerals — ions such as calcium and iron.
- Fibre (roughage) — provides bulk so muscles can push food along the gut by peristalsis; without it, constipation.
- Water — the solvent in which all reactions occur and substances are transported.
Deficiency diseases worth knowing
- Vitamin C deficiency → scurvy (bleeding gums, poor wound healing), because vitamin C is needed to make collagen.
- Vitamin D or calcium deficiency → rickets in children, as bones are not properly hardened.
- Iron deficiency → anaemia, because iron is needed to make haemoglobin, so less oxygen can be carried.
Malnutrition works both ways
Malnutrition means an unbalanced diet, not only too little. Persistent energy intake above what is used leads to obesity, raising the risk of heart disease and type 2 diabetes.
Think of it like this
A balanced diet is like a full toolbox. Having ten hammers and no screwdriver is not "mostly fine" — a missing tool stops a whole job, which is why a single vitamin deficiency causes such specific, dramatic symptoms.
Worked examples
Method, step by step
A person's diet lacks iron. Explain why they feel tired.
- 1Iron is needed to make haemoglobin in red blood cells.
- 2Less haemoglobin means less oxygen can be transported around the body.
- 3Cells receive less oxygen, so aerobic respiration is reduced and less energy is released.
Without iron, less haemoglobin is made, so less oxygen is carried to cells, reducing aerobic respiration and energy release — causing tiredness (anaemia).
Common misconceptions
- Thinking malnutrition only means starvation. Obesity is a form of malnutrition too, because the diet is unbalanced.
- Believing fibre is digested. It is not — its value is precisely that it stays in the gut and provides bulk for peristalsis.
- Assuming any anaemia is iron deficiency alone; the exam link required is iron → haemoglobin → oxygen transport.
In the exam
- Deficiency questions want the mechanism, not just the disease name: "iron is needed to make haemoglobin, so less oxygen is carried" scores far better than "anaemia".
- If asked why a pregnant woman needs more calcium and iron, link to the fetus building bones and making its own blood.