Energy from food
- Energy is measured in kilocalories (kcal) or kilojoules (kJ). 1 kcal = 4.2 kJ.
- 1 g of carbohydrate gives about 4 kcal (3.75), 1 g of protein gives 4 kcal, and 1 g of fat gives 9 kcal.
- Recommended share of energy: about 50% from carbohydrate, no more than 35% from fat, and about 15% from protein.
How much energy we need
- Basal metabolic rate (BMR) is the energy needed to keep the body alive at rest: breathing, heartbeat and keeping warm.
- Physical activity level (PAL) is the extra energy used through movement. Energy needs = BMR × PAL.
- Needs depend on age, sex, body size, activity level, and life stage (such as growing children and pregnancy).
- As a guide, adult women need about 2,000 kcal and adult men about 2,500 kcal a day.
Energy balance
- If energy in equals energy out, weight stays the same.
- If energy in is more than energy out, the extra is stored as fat and weight goes up.
- If energy in is less than energy out, stored energy is used and weight goes down.
Worked example
- A snack contains 20 g of carbohydrate, 5 g of protein and 10 g of fat. Energy = (20 × 4) + (5 × 4) + (10 × 9) = 80 + 20 + 90 = 190 kcal.
Key terms
- Kilocalorie
- A unit of energy in food (kcal).
- Basal metabolic rate
- The energy needed to keep the body alive at rest.
- Physical activity level
- A measure of how active a person is, multiplied by BMR to find energy needs.
- Energy balance
- When energy taken in equals energy used.
- Kilojoule
- A unit of energy: 1 kcal is 4.2 kJ.