NutritionOxford AQA IGCSE Biology: Revision notes
Section 1
What are the components of a balanced diet?
A balanced diet contains the correct proportions of all nutrients needed for health and growth. The main components are:
| Component | Function | Example Food Sources |
|---|---|---|
| Carbohydrates | Provide energy for respiration | Bread, rice, pasta, potatoes |
| Proteins | Build and repair tissues; make enzymes and hormones | Meat, fish, eggs, beans, lentils |
| Lipids | Store energy; insulate the body | Oils, butter, nuts, fatty fish |
| Vitamin C | Collagen formation; immune function | Citrus fruits, berries, peppers |
| Vitamin D | Calcium absorption; bone health | Oily fish, egg yolks, sunlight exposure |
| Calcium | Build and maintain bones and teeth | Milk, cheese, yoghurt, leafy greens |
| Iron | Oxygen transport in haemoglobin | Red meat, beans, fortified cereals |
| Water | Transport medium; temperature regulation; chemical reactions | Drinking water, juices, fruits |
| Dietary fibre | Aids digestion; prevents constipation | Wholegrain cereals, vegetables, fruit |
Each nutrient has a specific role, and deficiency or excess of any component can lead to health problems.
Examiners expect you to name specific food sources for each nutrient. Learn at least one example for each component—this demonstrates applied understanding.
Think of a balanced diet like a car engine: you need the right fuel (carbohydrates for energy), oil (lipids for lubrication), spare parts (proteins for repair), and maintenance fluid (water). Without any one, the engine breaks down.
Section 2
What are the consequences of an unbalanced diet?
An unbalanced diet lacks the correct proportions of nutrients, leading to serious health conditions:
Obesity
- Caused by excessive energy intake (too many calories) relative to energy expenditure
- Results in accumulation of excess lipid stores
- Increases risk of heart disease, type 2 diabetes, and joint problems
Type 2 Diabetes
- Develops when the body cannot regulate blood glucose properly (usually due to insulin resistance)
- Often linked to obesity and excessive refined carbohydrate intake
- Cells fail to respond adequately to insulin, causing blood glucose to remain elevated
Malnutrition
- General term for inadequate nutrient intake in terms of quantity or quality
- Can result from insufficient calorie intake or lack of specific nutrients
- Leads to poor growth, weakened immunity, and lack of energy
Deficiency Diseases
- Occur when a specific nutrient is absent or insufficient over a long period
- Examples include scurvy (vitamin C deficiency), rickets (vitamin D and calcium deficiency), and anaemia (iron deficiency)
- Symptoms vary depending on the missing nutrient
When answering questions about consequences, always link the cause (unbalanced diet) to the specific effect. For example: 'Excessive energy intake → excess lipids stored → obesity → increased disease risk.'
Students often confuse malnutrition with undernutrition. Malnutrition includes both insufficient and excess nutrients; you can be obese and malnourished simultaneously if your diet lacks specific vitamins or minerals.
Section 3
How does vitamin C prevent scurvy?
Vitamin C (ascorbic acid) is essential for preventing scurvy, a deficiency disease with serious consequences.
Role of Vitamin C:
- Required as a coenzyme in collagen synthesis
- Collagen is a structural protein found in connective tissue, blood vessel walls, and gums
- Vitamin C maintains collagen stability and cross-linking, keeping tissues strong and flexible
Consequences of Vitamin C Deficiency (Scurvy):
- Collagen cannot be properly synthesised without vitamin C
- Connective tissues become weak and fragile
- Symptoms include:
- Bleeding gums and loose teeth
- Poor wound healing
- Joint pain and swelling
- Fatigue and weakness
- In severe cases, reopening of old wounds
Prevention and Treatment:
- Consuming foods rich in vitamin C (citrus fruits, berries, peppers, leafy greens)
- Vitamin C is water-soluble, so it cannot be stored—regular intake is essential
- Scurvy was historically common among sailors with limited access to fresh produce during long voyages
Examiners want you to explain the mechanism: vitamin C is needed for collagen synthesis, and without it, collagen is weak, leading to bleeding gums and poor wound healing. Don't just list symptoms—show the biological link.
A sailor on a long voyage eats only dried biscuits and salted meat (no fresh produce). After 3 months, his gums bleed and his old wounds reopen. This is scurvy. Why? His vitamin C intake was zero, so collagen synthesis stopped, and existing collagen degraded, weakening blood vessels and connective tissue.
Section 4
How do vitamin D and calcium prevent rickets?
Rickets is a deficiency disease affecting bone development, prevented by adequate vitamin D and calcium intake.
Role of Vitamin D and Calcium:
- Vitamin D enables intestinal absorption of dietary calcium
- Calcium is essential for bone mineralisation and hardening
- Without sufficient vitamin D, calcium cannot be absorbed effectively, even if dietary intake is adequate
- Both minerals work together: vitamin D is the transporter, calcium is the building material
Consequences of Deficiency (Rickets):
- Bones fail to mineralise properly, remaining soft and weak
- Bones bend and deform under body weight
- Symptoms include:
- Bowed legs (most visible in children)
- Delayed bone growth and development
- Soft skull bones; delayed closure of fontanelle in infants
- Increased susceptibility to respiratory infections
- Muscle weakness and pain
Prevention and Treatment:
- Consuming oily fish (salmon, mackerel), egg yolks, and fortified milk
- Natural vitamin D synthesis through skin exposure to sunlight
- Supplementation if dietary intake or sun exposure is insufficient
- Rickets is now rare in developed countries due to food fortification but remains prevalent in populations with limited sun exposure or dietary diversity
Remember that vitamin D and calcium must work together. Examiners expect you to explain why: vitamin D increases intestinal calcium absorption, and calcium hardens bones. Either alone is insufficient.
Vitamin D is like a key that unlocks the door for calcium to enter the bloodstream from the intestines. Without the key, calcium cannot get in, even if there's plenty of calcium food available.
Section 5
What is the role of iron and why is iron deficiency dangerous?
Iron is a critical mineral for oxygen transport in the blood, and deficiency leads to serious health consequences.
Role of Iron:
- Iron is a component of haemoglobin, the protein in red blood cells that binds oxygen
- Each haemoglobin molecule contains four iron atoms, each capable of binding one oxygen molecule
- Iron enables efficient oxygen transport from the lungs to respiring tissues
- Without adequate iron, fewer functional haemoglobin molecules can be made
Consequences of Iron Deficiency:
- Reduced haemoglobin production leads to iron deficiency anaemia
- Symptoms include:
- Fatigue and weakness (insufficient oxygen delivery to muscles and brain)
- Pale skin and pale mucous membranes
- Shortness of breath, even during light activity
- Difficulty concentrating and poor cognitive performance
- Headaches and dizziness
- Reduced immune function, increasing infection risk
Risk Groups and Prevention:
- Particularly affects women of reproductive age (due to menstrual blood loss), infants, and children
- Food sources: red meat, poultry, fish, beans, lentils, fortified cereals
- Absorption is enhanced by vitamin C, which keeps iron in a form that intestines can absorb
- Iron supplements may be required in cases of severe deficiency
Examiners test whether you understand the mechanism: iron is in haemoglobin, haemoglobin carries oxygen, so without iron, oxygen transport fails. Always link iron deficiency to reduced oxygen delivery and its effects on cells.
A teenage girl with heavy periods eats little meat and few fortified foods. After months, her iron intake is insufficient, so her bone marrow cannot make enough haemoglobin. Her red blood cells carry less oxygen, so her muscles and brain receive less oxygen, causing fatigue and poor concentration.
Section 6
How is Body Mass Index calculated and interpreted?
Body Mass Index (BMI) is a simple numerical measure used to assess whether a person's body weight is appropriate for their height. This is a higher tier requirement.
BMI Calculation:
BMI = mass (kg) ÷ [height (m)]²
Or: BMI = mass (kg) ÷ height (m) ÷ height (m)
Important: Height must be converted to metres before squaring.
Example Calculation: A person weighs 70 kg and is 1.75 m tall.
- BMI = 70 ÷ (1.75 × 1.75)
- BMI = 70 ÷ 3.0625
- BMI = 22.9 kg/m²
BMI Categories and Interpretation:
| BMI Range | Classification | Health Status |
|---|---|---|
| Below 18.5 | Underweight | Insufficient body mass; risk of malnutrition |
| 18.5–24.9 | Normal/Healthy weight | Optimal health risk profile |
| 25.0–29.9 | Overweight | Increased risk of health problems |
| 30.0 and above | Obese | Significantly increased risk of obesity-related diseases |
Limitations of BMI:
- Does not distinguish between muscle and fat (athletes may have high BMI despite low body fat)
- Does not account for age, sex, or bone density
- Varies by ethnicity and population
- Does not reflect visceral fat distribution (fat around organs is more dangerous than subcutaneous fat)
Use in Nutrition Assessment:
- BMI is a screening tool to identify individuals at risk of weight-related health problems
- Must be combined with other health markers (blood pressure, cholesterol, fitness level) for comprehensive assessment
Examiners often provide a BMI value and ask for interpretation. Always state the category (e.g. 'overweight') and link it to health consequences (e.g. 'increased risk of type 2 diabetes'). Show your calculation steps if you're asked to calculate BMI.
A student is asked: 'Calculate BMI for a person of mass 85 kg and height 1.80 m. Is this healthy?' Step 1: BMI = 85 ÷ (1.80)² = 85 ÷ 3.24 = 26.2 kg/m². Step 2: This falls in the 'overweight' category (25.0–29.9). Step 3: This person has increased risk of obesity-related diseases and should modify diet and exercise.
Students often forget to convert height to metres or square it correctly. Common error: using height in cm (180) instead of metres (1.80), or forgetting to square the height entirely. Always convert and square height first.
Must Know
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A balanced diet contains carbohydrates (energy), proteins (growth/repair), lipids (energy/insulation), vitamins (specific functions), minerals (specific functions), water (transport/reactions), and dietary fibre (digestion). Each nutrient has a distinct role; deficiency or excess causes disease.
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An unbalanced diet causes obesity (excess energy intake), type 2 diabetes (insulin resistance due to high refined carbohydrate intake), malnutrition (inadequate overall nutrition), and deficiency diseases (specific nutrient absence). Link cause to effect: excessive calories → obesity → increased disease risk.
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Vitamin C is essential for collagen synthesis; without it, collagen is weak, causing scurvy (bleeding gums, poor wound healing). Vitamin D enables calcium absorption; both together mineralise bone; deficiency causes rickets (soft, bent bones).
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Iron is in haemoglobin, which carries oxygen; iron deficiency anaemia reduces oxygen transport, causing fatigue, weakness, and poor concentration. Vitamin C enhances iron absorption.
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BMI = mass (kg) ÷ [height (m)]². Interpret results: <18.5 = underweight; 18.5–24.9 = healthy; 25.0–29.9 = overweight; ≥30 = obese. BMI is a screening tool; high BMI indicates increased disease risk but does not distinguish muscle from fat.
That's the notes covered.
Carry on to the next subtopic.