DietCambridge IGCSE Biology: Revision notes
Section 1
What is meant by a balanced diet?
A balanced diet is one that contains the right proportions and amounts of different nutrients needed to maintain health, provide energy, and support growth and repair. It should include adequate amounts of:
- Carbohydrates (for energy)
- Fats and oils (for energy and essential fatty acids)
- Proteins (for growth and repair)
- Vitamins (for various metabolic processes)
- Mineral ions (for specific physiological functions)
- Fibre (for digestive health)
- Water (for all body processes)
The proportions vary depending on age, activity level, and individual circumstances, but all components must be present for the diet to be truly balanced.
When answering questions about balanced diets, always mention that it must contain ALL nutrient types, not just some. State that proportions matter as much as the presence of nutrients.
Section 2
What are carbohydrates and what roles do they play?
Carbohydrates are a major energy source and should form the largest proportion of a balanced diet (approximately 50-60% of total energy intake).
Principal dietary sources:
- Bread
- Cereals
- Rice
- Pasta
- Potatoes
- Fruits
Importance:
- Primary energy source for respiration in all cells
- Glucose is essential for the brain and nervous system
- Provide quick, accessible energy for physical activity
- Some carbohydrates (starch) are stored in the liver and muscles as glycogen for later use
Types: Carbohydrates include sugars (simple carbohydrates) and starch (complex carbohydrates). Complex carbohydrates are preferred as they release energy more slowly.
Think of carbohydrates as fuel for a car—they provide the energy needed to keep the body running smoothly, especially the brain and muscles.
Section 3
What are fats and oils, and why are they needed?
Fats and oils are energy-rich nutrients that provide more than twice the energy per gram compared to carbohydrates (9 kJ/g versus 17 kJ/g for carbohydrates).
Principal dietary sources:
- Butter and margarine
- Oils (olive, sunflower, vegetable)
- Meat and meat products
- Dairy products (cheese, milk, cream)
- Nuts and seeds
- Fatty fish (salmon, mackerel)
Importance:
- Long-term energy storage (stored as fat under the skin and around organs)
- Insulation—subcutaneous fat provides thermal insulation
- Protection—fat cushions vital organs
- Essential fatty acids that the body cannot synthesise
- Absorption of fat-soluble vitamins (A, D, E, K)
- Production of cell membranes and hormones
Fats should constitute approximately 30-35% of total daily energy intake, with emphasis on unsaturated fats.
Students often think all fat is 'bad' and should be avoided. In reality, fats are essential for survival; the diet should simply limit saturated fats and favour unsaturated fats.
Section 4
Why are proteins, vitamins, and mineral ions essential?
PROTEINS
Principal dietary sources:
- Meat, poultry, and fish
- Eggs
- Beans and lentils
- Nuts and seeds
- Dairy products (milk, cheese, yoghurt)
Importance:
- Essential for growth and repair of tissues
- Build enzymes and antibodies
- Provide energy (secondary source)
- Formation of hair, skin, and muscle
VITAMIN C (Ascorbic acid)
Principal dietary sources:
- Citrus fruits (oranges, lemons, limes)
- Berries
- Kiwifruit
- Peppers
- Broccoli and cabbage
Importance:
- Production of collagen (connective tissue)
- Immune system function
- Antioxidant—prevents cell damage
- Aids iron absorption
VITAMIN D (Calciferol)
Principal dietary sources:
- Fatty fish (salmon, mackerel, sardines)
- Egg yolks
- Fortified milk
- Manufactured by skin when exposed to sunlight (UVB rays)
Importance:
- Calcium absorption in the small intestine
- Bone mineralisation and strength
- Muscle and nerve function
CALCIUM (Mineral ion)
Principal dietary sources:
- Dairy products (milk, cheese, yoghurt)
- Leafy green vegetables
- Fortified plant-based milks
- Nuts and seeds
Importance:
- Bone and teeth formation and maintenance
- Blood clotting
- Muscle contraction
- Nerve impulse transmission
IRON (Mineral ion)
Principal dietary sources:
- Red meat and offal
- Beans and lentils
- Fortified cereals
- Dark leafy vegetables
- Dried fruits
Importance:
- Production of haemoglobin in red blood cells
- Oxygen transport throughout the body
- Energy production in cells
- Immune function
Remember the relationship between vitamin D and calcium: vitamin D enables the body to absorb calcium. Without sufficient vitamin D, calcium absorption is impaired even if dietary intake is adequate.
Section 5
What are scurvy and rickets, and how are they caused?
SCURVY
Cause: Deficiency of vitamin C (ascorbic acid) over an extended period.
Symptoms and effects:
- Bleeding gums and tooth loss
- Poor wound healing (lack of collagen formation)
- Anaemia
- Fatigue and weakness
- Bruising easily
- Inflamed joints
Historical context: Scurvy was common among sailors on long voyages who had limited access to fresh fruits and vegetables. The disease became rare once vitamin C's importance was understood and supplies of citrus fruits became standard on ships.
RICKETS
Cause: Deficiency of vitamin D or calcium (or both), leading to impaired bone mineralisation. Vitamin D deficiency is the primary cause as it prevents calcium absorption.
Risk factors:
- Limited sun exposure (vitamin D synthesis in skin)
- Strict vegan diet without fortified foods
- Malabsorption disorders
- Dietary calcium deficiency
Symptoms and effects:
- Weak, soft bones that become deformed
- Bowing of the legs
- Delayed tooth eruption
- Poor growth and stunted development
- Muscle weakness
- Delayed motor milestones (e.g., walking)
- Swollen wrists and ankles
Note: Rickets most commonly affects children, as their bones are still developing and require adequate mineralisation. Adults with similar deficiencies develop osteomalacia (soft bones).
A child eating a diet low in vitamin D and calcium with minimal sun exposure will have impaired calcium absorption due to lack of vitamin D. This prevents proper bone mineralisation, causing rickets with symptoms like bowing of the legs and delayed growth.
For exam questions, always explicitly state the nutrient deficiency (vitamin C for scurvy; vitamin D or calcium for rickets) and link it to the specific symptoms. Show understanding of the mechanism, not just the disease name.
Section 6
What is fibre and water, and why are they necessary?
FIBRE (Roughage)
Principal dietary sources:
- Wholemeal bread and cereals
- Fruit and vegetables (especially with skins)
- Beans and lentils
- Brown rice
- Nuts and seeds
Importance:
- Provides bulk to faeces, promoting peristalsis (muscle contractions in the intestine)
- Aids regular bowel movement and prevents constipation
- Reduces risk of bowel cancer and other intestinal disorders
- Slows digestion and glucose absorption, helping regulate blood sugar levels
- Feeds beneficial gut bacteria (prebiotics)
- Promotes feeling of fullness, aiding weight management
Note: Fibre is not digested by human enzymes but is essential for digestive health.
WATER
Principal sources:
- Drinking water
- Beverages (tea, milk, juice)
- Food (fruits and vegetables have high water content)
Importance:
- Universal solvent for all metabolic reactions
- Transport medium for nutrients and waste products
- Temperature regulation through sweating
- Lubrication of joints and protection of organs
- Component of blood and lymph
- Maintains blood volume and pressure
- Enables digestion and absorption of nutrients
- Essential for kidney function and urine production
Water intake should be adjusted based on activity level and climate; dehydration impairs all body functions.
Water is like the transport system of the body—without it, nutrients cannot be delivered to cells, waste cannot be removed, and reactions cannot occur efficiently.
Must Know
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A balanced diet contains all seven nutrient types in the correct proportions: carbohydrates (50-60%), fats and oils (30-35%), proteins, vitamins, mineral ions, fibre, and water.
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Carbohydrates (bread, cereals, rice, pasta, potatoes) provide rapid energy for cells and the brain; fats and oils (butter, oils, meat, nuts) provide long-term energy storage, insulation, and protection.
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Proteins (meat, fish, eggs, beans, dairy) are essential for growth and repair of tissues and enzyme synthesis.
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Vitamin C (citrus fruits, berries, peppers, broccoli) is needed for collagen production and immune function; vitamin D (fatty fish, egg yolks, sunlight) is essential for calcium absorption and bone strength.
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Calcium (dairy, leafy greens, nuts) and iron (red meat, beans, fortified cereals) are vital mineral ions: calcium for bones and muscle contraction; iron for haemoglobin and oxygen transport.
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Scurvy is caused by vitamin C deficiency and results in bleeding gums, poor wound healing, and anaemia; rickets is caused by vitamin D or calcium deficiency and results in weak, deformed bones and impaired growth.
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Fibre (wholemeal bread, fruits, vegetables, beans) promotes peristalsis and prevents constipation; water is essential as a solvent and transport medium for all metabolic processes.
That's the notes covered.
Carry on to the next subtopic.