Energy balance
Food gives energy, measured in kilocalories (kcal) or kilojoules (kJ). 1 kcal ≈ 4.2 kJ.
The body uses energy in three ways: to stay alive at rest (the basal metabolic rate, BMR), to digest food, and for movement and exercise.
- Energy in = energy out → weight stays the same.
- Energy in > energy out → weight goes up (extra is stored as fat).
- Energy in < energy out → weight goes down.
A rough rule: energy used in exercise (kcal) ≈ MET × body mass (kg) × time (hours). A MET is how many times harder than resting an activity is: walking ≈ 3.5, football ≈ 7, running ≈ 10.
A balanced diet for sport
A balanced diet gives all nutrients in the right amounts.
- Carbohydrates (rice, roti, bread, pasta, potatoes, fruit) – the main energy source; about 55–60% of energy. Stored as glycogen in muscles and liver.
- Protein (dal, beans, eggs, milk, fish, meat, paneer) – growth and repair of muscle. Power athletes need a little more.
- Fats (nuts, oil, butter) – energy for long, low-intensity exercise; keep saturated fat low.
- Vitamins and minerals – e.g. iron carries oxygen in blood, calcium for bones, vitamin C for healing.
- Fibre – healthy digestion. Water – about 60% of the body.
Hydration
During exercise muscles make heat, and sweat cools you down. But sweat removes water and salts (electrolytes).
Dehydration causes: thicker blood, a faster heart rate, higher body temperature, slower reactions, tiredness and cramps. Even 2% loss of body mass hurts performance.
Good habits: drink before you are thirsty; sip water every 15–20 minutes during long exercise; weigh yourself before and after training – every 1 kg lost ≈ 1 litre of sweat to replace. Pale yellow urine is a good sign. Sports drinks help in exercise longer than about an hour.
Timing, carbo-loading, supplements and doping
- Before: a carbohydrate meal 2–3 hours before, low in fat and fibre so it digests easily.
- During: water; in events over an hour, some carbohydrate (sports drink, banana).
- After: carbs + protein within about an hour to refill glycogen and repair muscle; replace fluid.
Carbohydrate loading: endurance athletes eat extra carbs for 2–3 days before a long event and train less, so glycogen stores are full.
Supplements (protein powder, creatine, caffeine) are mostly unnecessary for young athletes with a good diet; some carry risks or hidden banned substances.
Doping is using banned substances or methods (e.g. anabolic steroids, EPO, stimulants) to gain an unfair advantage. It harms health (heart, liver, hormones) and breaks fair play; testing bodies such as the World Anti-Doping Agency can ban athletes.
Key formulas and definitions
- Energy in = energy out → stable weight
- Exercise energy (kcal) ≈ MET × mass (kg) × time (h)
- 1 kcal ≈ 4.2 kJ
- 1 kg body mass lost in exercise ≈ 1 L of sweat to replace
- 2% body-mass loss through sweat → performance drops
Worked examples
1. A 50 kg student runs (MET 10) for 30 minutes. Estimate the energy used.
Energy ≈ 10 × 50 × 0.5 = 250 kcal.
2. A 60 kg footballer weighs 58.8 kg after a match. How much body mass was lost and how much fluid should she drink?
Loss = 1.2 kg = 2% of 60 kg. She should replace at least 1.2 L (often 1.5 × loss ≈ 1.8 L over the next hours).
3. A swimmer eats 2,800 kcal a day and uses 3,100 kcal. What happens over weeks?
Energy in < energy out by 300 kcal a day, so body weight slowly falls. To keep weight steady he must eat about 300 kcal more.
Common mistakes
- Thinking protein is the main fuel for exercise. Carbohydrate is; protein mainly repairs.
- Drinking only when very thirsty. Thirst comes late; sip regularly.
- Eating a big fatty meal just before a match. It digests slowly and can cause cramps.
- Believing supplements are needed by everyone. A balanced diet is enough for most young athletes.