Energy in food

Energy stored in food can be released by combustion(burning) or by respiration in our cells. The labels on packets of food show how much energy is available from the food.

Nutritional information label on packet of naan bread.
A food label on a packet of naan bread. 215 calories is the same as 900 kJ.

The amount of energy available may be shown in a unit called the calorie, as in the photograph. However, the scientific unit for energy is the joule, which has the symbol J.

A lot of energy is available from most foods, so food labels usually show kJ (kilojoules) instead of J:

1 kJ = 1000 J

For example, 2000 J = 2000 ÷ 1000 = 2 kJ.

To give you an idea of what 2 kJ can do, it is the energy needed to lift a 100 kg mass by 2 m, or to keep a 20 W electric lamp alight for 100 seconds.

The table shows the typical amount of energy available from 100 g of several different foods. The larger the number, the more energy is available.

FoodEnergy in kJ per 100 g
Egg omelette690
Chocolate cake1920
Potato crisps2240

Brisk walking typically uses 14 kJ per minute. This means that:

  • 100 g of apple provides enough energy for a 10 minute walk (140 ÷ 14)
  • 100 g of potato crisps provides enough energy for a 160 minute walk (2240 ÷ 14)

A typical adult woman needs around 8400 kJ per day. You could get this from just 375 g of potato crisps (fifteen 25 g bags).

Jon Chase demonstrates how food can be used as fuel