Energy needs

Each person needs a different amount of energy depending on factors such as:

  • gender (male or female)
  • age
  • amount of daily activity

If you look on the side of food packets you will see the food's energy content. This is usually measured in kilojoules, kJ. For example, a one-year-old baby needs 3850 kJ each day to continue to grow, whereas an adult Olympic swimmer in training needs 15,600 kJ each day. Someone who sits at a desk all day will need less food than their twin who climbs ladders all day to wash windows.

Too thin, too fat

If the amount of energy you get from your food is different from the amount of energy you need, your diet will be imbalanced:

  • too little food may lead to a person being underweight
  • too much food may lead to a person being overweight
Overweight man's stomach being measured
Eating more food than you need can make you overweight

For a given height, there is a range of body weight that is thought to be healthy.

BMI (body mass index) chart for men and women 18 years and over.A weight against height chart for men and women aged 18 and over

Starvation happens if you eat so little food that your body becomes very underweight. This can eventually cause death. Obesity happens when you eat so much food that your body becomes very overweight. This can also cause death, for example through an increased chance of heart disease.

Energy calculations

The average man needs about 10,470 kJ per day, and the average woman needs about 8370 kJ per day. Food labels show how much energy the food provides. The table shows three examples:

Baked beans1395 kJ per can
Bread364 kJ per slice
Grated cheese17.45 kJ per g

Using these numbers, the daily energy needs for a woman could be met from 6 cans of baked beans (8370 ÷ 1395 = 6) or from 23 slices of bread (8370 ÷ 364 = 23). The daily energy needs for a man could be met from 600 g of cheese (10470 ÷ 17.45 = 600).

However, we need to eat a mixture of different foods in the correct proportions to achieve a balanced diet. The amount we need to eat will vary from person to person.

Sport and Exercise Science Professor Lars McNaughton talks about how the daily diet of average healthy people compares to that of athletes