How many kilojoules do we need?

The average adult needs about 8700 kilojoules (kJ) a day to maintain a healthy weight. But it varies quite a bit - some people need more and others less.

Many factors influence how much energy you need, such as:

• how active you are
• how much muscle you have
• whether or not you are growing
• your age, gender, height and weight

How active you are

Movement burns energy. Movement includes exercise, manual work, walking around and small movements like fidgeting. There are many ways to influence how much energy you use. For example, if you use public transport for a day, you are likely to burn more kilojoules than if you use a car, because you probably have a walk a distance at each end of the trip.

How much muscle you have

More muscle needs more energy. How much muscle you have is influenced by:

• age – most people have less muscle as they age
• gender – most men have more muscle than most women
• height – tall people generally have more muscle
• body build – some people have genes for a more muscular body

In general, older people, women and shorter people, need fewer kilojoules than those who are young, male, tall and muscular.

Growing

Children and adolescents need plenty of energy in their food, as they are growing. Adults aren’t growing, so don’t need to take in extra food and drink to allow for growth.

About energy

The language around energy can be confusing. Many people think that foods high in kilojoules are bad and many people think that foods high in energy are good. This is often the case in advertising about kids’ food, where a food high in kilojoules can be presented as ‘giving you energy’.

However, energy and kilojoules are the same thing – kilojoules are simply the way energy is measured. Foods high in energy, which means they’re high in kilojoules, are not always what a child needs.

Kilojoules or calories?

The standard measure of energy is kilojoules – abbreviated to kJ. Australia officially dropped the use of calories by 1988.

If you think in calories, then 1 calorie = 4.2 kilojoules. To do a rough conversion when reading a food label, divide kilojoules by 4 to get to calories.

How many kilojoules in foods?

Food and drinks provide the energy you need to stay alive and active. You should be able to work out how many kilojoules are in foods you buy by looking at the food labels.

By law, food labels must list the kilojoules in 100g (or 100mL) of a product and in some states and territories, certain food retailers are required to display the kilojoule content of their food and drinks at point of sale.

Some also include the kilojoules in a ‘serving’. Be aware that what the manufacturers say is a serving might be different from what you eat.

The number of kilojoules in fresh foods is on a list developed by Food Standards Australia and New Zealand.

In general, vegetables and most fruits are quite low in energy. Fast foods, fried foods and products such as cakes, desserts and pastries have much more energy.

Balancing kilojoule intake

If you take in more energy than you use, you store the extra as fat. To lose excess fat means you need to take in less energy (fewer kilojoules) or use more through exercise, or preferably both.