Nutrients and what they do
| Nutrient | Main job | Good sources |
|---|---|---|
| Carbohydrates (sugars, starch) | Quick energy | Rice, wheat, millets, potatoes, fruit |
| Proteins (made of amino acids) | Growth, repair, enzymes, hormones | Dal, beans, eggs, fish, meat, milk, nuts |
| Fats/lipids | Stored energy, warmth, cell membranes, carry vitamins A, D, E, K | Oil, ghee, butter, nuts, seeds |
| Vitamins | Keep processes working (tiny amounts) | Fruit, vegetables, milk, eggs, sunlight (vit D) |
| Minerals | Bones (calcium), blood (iron), nerves (sodium, potassium) | Milk, green leaves, pulses, salt |
| Water | Transport, body temperature, reactions | Drinks, fruit, vegetables |
| Fibre | Keeps food moving through the gut | Whole grains, vegetables, fruit |
Carbohydrates, proteins and fats are macronutrients (needed in grams). Vitamins and minerals are micronutrients (needed in milligrams or micrograms).
Deficiency and excess
- Too little protein and energy → poor growth, weak muscles (malnutrition).
- Vitamin C → scurvy (bleeding gums). Vitamin D or calcium → weak, bent bones (rickets). Vitamin A → night blindness.
- Iron → anaemia (tired, pale). Iodine → goitre.
- Too much energy, sugar, salt or saturated fat → obesity, type 2 diabetes, high blood pressure and heart disease.
Energy in food and energy balance
Food energy is measured in kilojoules (kJ) (1 kcal = 4.18 kJ). Approximate values: carbohydrate 17 kJ/g, protein 17 kJ/g, fat 37 kJ/g.
Cells release this energy in respiration. Needs depend on age, sex, body size, growth and activity. A teenager needs very roughly 9 000–12 000 kJ a day. If energy in > energy out, extra is stored as fat.
Try it: look at a snack packet. Multiply its grams of carbohydrate, protein and fat by 17, 17 and 37 and add. Compare with the kJ printed on the label.
A balanced diet and healthy choices
A balanced diet gives every nutrient in the right amount. A simple plate guide: about half vegetables and fruit, a quarter whole grains, a quarter protein foods, a little dairy, and small amounts of oil, salt and sugar. Drink water.
What we eat is shaped by physical factors (hunger, age, health, allergies), emotional factors (stress, mood, boredom), and social factors (family, culture, religion, cost, advertising, friends). As consumers we can also choose local, seasonal food and cut food waste.
Reading a food label
Check the serving size, energy (kJ), total fat and saturated fat, sugars, salt (sodium) and the ingredients list (largest amount first). Compare per 100 g to judge between products.
Digestion and food tests
Big food molecules must be broken down by enzymes before they can be absorbed: starch → sugars (amylase), proteins → amino acids (protease), fats → fatty acids and glycerol (lipase). They are absorbed in the small intestine.
Simple lab tests (with a teacher):
- Starch: iodine turns blue-black.
- Reducing sugar: Benedict's solution, heated, turns brick-red.
- Protein: Biuret solution turns purple.
- Fat: rub on paper → a see-through greasy spot; or the ethanol emulsion test turns cloudy white.
Key formulas and definitions
- Energy (kJ) ≈ 17 × carbohydrate (g) + 17 × protein (g) + 37 × fat (g)
- 1 kcal = 4.18 kJ
- Energy in = energy out → steady body mass
- Macronutrients: carbohydrates, proteins, fats
- Micronutrients: vitamins and minerals
Worked examples
1. A snack has 30 g carbohydrate, 5 g protein and 10 g fat. Find its energy.
30 × 17 = 510 kJ; 5 × 17 = 85 kJ; 10 × 37 = 370 kJ. Total = 965 kJ.
2. A child eats almost no fruit or vegetables and has bleeding gums. What is likely missing, and what should be added?
Vitamin C (scurvy). Add citrus fruit, amla, guava, tomatoes and green vegetables.
3. Two cereals: A has 22 g sugar per 100 g, B has 6 g sugar and 8 g fibre per 100 g. Which is healthier and why?
B: much less sugar and more fibre. Comparing per 100 g makes the products fair to compare.
Common mistakes
- Thinking all fat is bad. The body needs some fat for energy, cell membranes and vitamins A, D, E, K; the problem is too much, especially saturated and trans fat.
- Thinking protein is the main energy food. Carbohydrates and fats are the main fuels; protein is mainly for growth and repair.
- Comparing labels 'per serving' when serving sizes differ. Use 'per 100 g'.
- Thinking vitamins give energy. Vitamins help reactions that release energy but have no energy value themselves.