Diet
Section: Biology | Syllabus: Cambridge IGCSE Combined Science (0653)
Balanced Diet
A balanced diet contains all the necessary nutrients in the correct proportions to maintain health - providing enough energy for daily activities, and the materials needed for growth and repair.
A balanced diet must contain the right amounts of carbohydrates, fats, proteins, vitamins, mineral ions, fibre and water. The ideal proportions vary with age, sex, activity level and health status.
Dietary Components - Sources and Importance
| Nutrient | Principal dietary sources | Importance / function |
|---|---|---|
| Carbohydrates | Bread, rice, pasta, potatoes, sugar, fruits, cereals | Main source of energy; used in respiration; excess is stored as glycogen (liver and muscle) or converted to fat |
| Fats and oils | Butter, oil, cheese, meat, nuts, fish, avocado | Energy store (more than twice the energy per gram of carbohydrates); insulation; protection of organs; component of cell membranes |
| Proteins | Meat, fish, eggs, milk, cheese, beans, lentils, nuts | Growth and repair of tissues; making enzymes, antibodies and hormones; used as an energy source only when other nutrients are scarce |
| Vitamin C | Citrus fruits (oranges, lemons), kiwi, peppers, tomatoes, strawberries | Needed to make collagen, a protein that holds cells together, keeping skin, gums and blood vessels healthy; deficiency causes scurvy |
| Vitamin D | Oily fish, eggs, dairy products, margarine; also made in the skin on exposure to sunlight | Needed for the absorption of calcium from the gut, and for correctly depositing calcium in bones and teeth; deficiency causes rickets |
| Calcium | Dairy products (milk, cheese, yoghurt), leafy green vegetables, fish with bones, fortified foods | Essential for the formation and maintenance of strong bones and teeth; needed for muscle contraction and blood clotting |
| Iron | Red meat, liver, leafy green vegetables, beans, fortified cereals | Component of haemoglobin in red blood cells, which carries oxygen in the blood; deficiency causes anaemia |
| Fibre (roughage) | Wholegrain bread, vegetables, fruits, legumes, bran | Indigestible plant material (mainly cellulose); adds bulk to faeces, stimulating peristalsis in the intestines and helping to prevent constipation |
| Water | Drinks, fruits, vegetables, most foods | Major component of cells and blood plasma; medium for chemical reactions and transport of substances; needed for temperature regulation and digestion |
Deficiency Diseases
- Scurvy is caused by a deficiency of Vitamin C
- Symptoms: swollen, bleeding gums; poor wound healing; skin rashes; fatigue; painful joints - because collagen cannot be maintained
- Rickets is caused by a deficiency of Vitamin D
- Symptoms: soft, weakened bones that bow outward under body weight; delayed tooth development; bone pain; poor growth in children
Common Mistakes
- ❌ Muddling which vitamin deficiency causes which disease
- ✓ scurvy = lack of Vitamin C → lack of collagen → gums and tissues break down. Rickets = lack of Vitamin D → calcium not absorbed properly → bones stay soft and deform
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