The big idea: Your body cannot make everything it needs from scratch. Some substances must come ready-made in your food — these are the essential nutrients.
A balanced diet supplies all the nutrient groups — carbohydrates, proteins, lipids, vitamins, minerals, fibre and water — in the right amounts.
If the diet is wrong (too little, too much, or missing a nutrient), health suffers. This is called malnutrition.
- Nutrient
- A substance in food that the body needs for energy, growth or to keep working properly.
- Essential nutrient
- A nutrient that the body cannot make (synthesise) for itself, so it MUST be obtained from the diet — for example certain amino acids, certain fatty acids, vitamins and minerals.
- Balanced diet
- A diet that contains all the nutrient groups in the correct proportions to meet the body's needs.
- Malnutrition
- Poor health caused by a diet that has too little, too much, or the wrong balance of nutrients.
- Vitamin
- A nutrient needed in tiny amounts to help body reactions work (for example vitamin C and vitamin D).
| Nutrient group | Main job in the body | Everyday food source |
|---|---|---|
| Carbohydrates | Main supply of energy for respiration | Bread, rice, potatoes |
| Proteins | Building and repairing tissues (growth) | Meat, fish, beans, eggs |
| Lipids (fats) | Energy store, insulation, part of membranes | Oils, butter, nuts |
| Vitamins | Help reactions work (e.g. vitamin C, vitamin D) | Fruit, vegetables, dairy |
| Minerals | Specific roles (e.g. iron in haemoglobin) | Vegetables, dairy, meat |
| Fibre | Keeps food moving through the gut | Wholegrains, vegetables, fruit |
| Water | Solvent and transport medium for the body | Drinks and most foods |
Why 'essential' is the key word: Essential = the body cannot make it. That is the whole definition the exam wants.
Glucose is a nutrient, but the body can make it from other foods — so it is not essential. Vitamin C cannot be made by humans, so it is an essential nutrient and must come from the diet.
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Malnutrition works in two opposite directions, and the exam tests both.
Too little of a nutrient causes a deficiency disease. Too much energy (especially fat and sugar) causes over-nutrition — obesity and the diseases linked to it.
Deficiency — too little of a nutrient: If an essential nutrient is missing, the job it normally does cannot be done, and a specific deficiency disease appears.
Vitamin C is needed to make strong collagen. Without it, connective tissue (gums, skin, blood-vessel walls) is weak — this is scurvy.
Vitamin D is needed to absorb calcium from food. Without it, there is too little calcium for bone, so bones become soft and deformed — this is rickets.
| Missing nutrient | Its job | What goes wrong if it is lacking |
|---|---|---|
| Vitamin C (ascorbic acid) | Needed to make strong collagen for skin, gums and blood-vessel walls | Weak connective tissue → scurvy (bleeding gums, slow wound healing) |
| Vitamin D | Needed to absorb calcium from food into the blood | Too little calcium for bone → soft, weak, deformed bones (rickets) |
| Iron (mineral) | Part of haemoglobin, which carries oxygen | Too little haemoglobin → anaemia, tiredness |
| Protein | Builds and repairs body tissues | Poor growth and muscle wasting in children |
Trace the chain for vitamin D: Examiners love the cause-and-effect chain for vitamin D and bone:
lack of vitamin D → less calcium absorbed from the gut → too little calcium reaches the blood and bone → bones are not hardened (not mineralised) properly → bones are soft and bend, giving abnormal bone development (rickets).
Each arrow is a separate marking point — write the steps, not just the disease name.
Excess — too much energy: Eating more energy than the body uses stores the excess as fat → obesity.
Obesity raises blood pressure (hypertension) and is linked to type-2 diabetes and coronary heart disease (CHD).
So a snack like a large bag of potato chips is a nutritional concern because it is high in fat, salt and energy but low in vitamins, minerals and fibre.
Under-nutrition (too little)
- Too little energy or a missing nutrient
- Causes deficiency diseases (scurvy, rickets, anaemia)
- Poor growth and weight loss
- Treated by adding the missing nutrient
Over-nutrition (too much)
- Too much energy, especially fat and sugar
- Causes obesity and high blood pressure
- Linked to CHD and type-2 diabetes
- Treated by reducing energy intake
Both ends are malnutrition: A common slip is to think malnutrition only means starvation.
In the IB, malnutrition means any unbalanced diet — too little or too much. A 'discuss malnutrition' answer should cover both deficiency and obesity.
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How this is tested: This micro is a Paper 2 / Paper 3 explain-and-discuss favourite, and it is often paired with data.
A short question may ask you to outline what an essential nutrient is (1 mark) or to state a role of vitamin C (1 mark).
A longer, higher-mark question asks you to explain how a high-fat diet leads to poor health — here you must give the full cause-and-effect chain down to coronary heart disease, with each arrow scoring a mark.
You may also be handed a table or graph (for example CHD death rate by sex, or fat intake versus CHD across countries) and asked to describe the trend and then suggest a biological reason for it.
IB-style question — explain how a high-fat diet damages health
Explain how a diet high in saturated fat can lead to coronary heart disease. [5]
How to score all five marks
- Start with the blood cholesterol. A diet high in saturated fat raises the level of LDL cholesterol in the blood.
- Deposits build up. Cholesterol is deposited in the walls of arteries, forming fatty plaques — this is atherosclerosis.
- Arteries narrow. The plaques make the coronary arteries narrower (and less elastic), so less blood can flow through them.
- Heart muscle is starved of oxygen. With reduced blood flow, the heart muscle receives less oxygen, so it cannot respire enough — this causes chest pain.
- A blockage causes a heart attack. If a coronary artery becomes fully blocked, part of the heart muscle dies — a heart attack (coronary heart disease). (Award 1 mark per linked step in the chain, up to 5.)
Final answer
Saturated fat raises LDL cholesterol → cholesterol is deposited in artery walls (atherosclerosis) → coronary arteries narrow → less blood and oxygen reach the heart muscle → a blockage kills heart muscle, causing CHD / a heart attack.
Why this scores full marks: Each sentence is one link in a chain, joined by 'so' or 'therefore' — fat → cholesterol → plaque → narrowing → less oxygen → heart attack.
A 'how-it-leads-to' explanation scores on the logical links, not on naming the disease. Listing facts without connecting them loses marks.
| Step | What happens | Why it matters |
|---|---|---|
| 1. Diet high in saturated fat | Raises the level of LDL cholesterol in the blood | Cholesterol is deposited in artery walls |
| 2. Fatty deposits (plaques) build up | Atherosclerosis — artery walls thicken and harden | The space inside the coronary arteries narrows |
| 3. Coronary arteries narrow | Less blood (and oxygen) can reach the heart muscle | The heart muscle is starved of oxygen |
| 4. Reduced oxygen to the heart | Heart muscle cells cannot respire enough | Chest pain, and risk of a blocked artery |
| 5. An artery becomes blocked | Part of the heart muscle dies — a heart attack | Coronary heart disease (CHD) — a leading cause of death |