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Kilojoules and calories both define the energy value of food. Calorie is still used in the USA to define energy, but Australia now uses kilojoule and you'll find this term shortened to kJ on food labels. The measurement of 'calories' signifies the amount of chemical energy that may be released as heat when food is metabolised. One calorie is equivalent to 4.2 kilojoules.
The balance between our energy intake versus the energy we expend is the main reason for weight loss or weight gain. Too many kJ eaten spells too much body fat on you. Too few kilojoules causes body fat loss.
Approximately two-thirds of your daily kJ needs are needed to just run your body as though you were asleep - to keep the blood pumping, your heart beating, your brain and nerves firing, your lungs breathing, and for the repair and maintenance of all your body's cells. This is called the "basal metabolic rate".
Those who exercise, or have a job requiring physical energy, have a higher basal metabolic rate and thus burn more kilojoules. For example a sedentary office worker may use about 9000kj per day, a manual worker lifting materials may burn approximately 12500kj per day, and a labourer or elite athlete may use up to 17000kj per day.
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