1Food is an energy store
Food is a store of energy. Your body uses it to keep warm, keep your organs working, move your muscles and grow. Energy is measured in joules (J).
storeenergyjoule
Lesson 12.1 notes
Energy in food
Food labels in kilojoules, per 100 g and per portion, using the energy, and measuring it by burning food.
2Reading a food label
Food labels give the energy in kilojoules (kJ) and kilocalories (kcal). 1 kJ = 1000 J. One column shows the energy in 100 g, the other in one portion.
kilojoulekilocalorie
3Per 100 g or per portion?
Per 100 g compares foods fairly: crisps store more than chips. But a portion of chips is 180 g and a bag of crisps 25 g, so the chips give more.
portion
4Fat stores the most
Each gram of fat stores about 37 kJ, more than twice as much as carbohydrate or protein (about 17 kJ). Fatty foods like crisps and chocolate store the most.
fatcarbohydrate
5Using the energy
The more active you are, the faster you use energy: about 3.5 kJ a minute asleep, 12 kJ walking, 21 kJ cycling and 32 kJ running.
6Energy in, energy out
Eat more energy than you use and the extra is stored, mostly as fat; eat less and your body uses its stores. Young teenagers need about 8500 to 10 500 kJ a day.
7Measuring the energy in food
Burn food under a tube of water and measure the temperature rise. Energy (J) = 4.2 × mass of water (g) × temperature rise (°C). Much of the energy escapes to the air.
temperature rise
Explore: Snack power
Choose a snack and an activity, and see how long the energy stored in the snack would keep you going.
Common mistake: Per 100 g is not the same as per portion
Per 100 g compares foods fairly, mass for mass. But a bag of crisps is only 25 g and a portion of chips is 180 g, so check the per-portion column to see what you actually eat.
Remember
- Energy is measured in joules (J). Food labels use kilojoules (kJ): 1 kJ = 1000 J.
- Fatty foods store the most energy per gram. The more active you are, the faster you use energy.
- Burning food under a tube of water: energy (J) = 4.2 × mass of water (g) × temperature rise (°C). Much of the energy escapes.
Going further at GCSE
GCSE: Energy stores and transfers → GCSE: Aerobic respiration → GCSE: Exercise →