ENERGY IN, ENERGY OUT

The body needs energy from food and drink to work properly and stay healthy.

Energy input and output

• The amount of energy the body gets from food = energy input.
• The amount of energy the body uses up is = energy output.

The more food a person eats, the larger the input; the more active they are, the bigger the output. It is important the energy input and output is kept in balance.

If you eat too little, the body does not get the energy it needs and there is a negative energy balance. You may lose weight because the body uses up its fat reserves to provide the extra energy.

If you eat too much, more than is needed for the energy your body is using, there is a positive energy balance. Because you are not using up all the food you eat, you may put on weight.

Keeping your energy input and output in balance is not easy. Most people are not active enough. But you don’t have to be very sporty or enjoy the latest fitness fad – simple activities like walking instead of going in the car, climbing stairs instead of taking a lift or escalator, or helping to mow the lawn or do the housework are all ways of being active and keeping your input and output balanced.

Measuring energy

The energy provided by food or drink is usually measured in calories (or joules).

1 calorie = 4.2kJ (kilojoules)

Scientists find out how much energy there is in a particular food by burning a sample of it and measuring how much heat energy is released. The process is called calorimetry.

This chart shows the average energy used in 15 minutes by a child weighing 35kg (5½ stone) doing different activities.

Calories in food

Food and drink provide the calories needed by the body. The table shows the average calories in some everyday foods. The actual number depends on the size of the portion – check the labels to see exactly how many calories they contain. But remember that the type of food you eat is important too (see the Healthy eating, healthy living section).

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