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.
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 |