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Kilojoules and calories explained

We eat food to fuel our bodies for energy, growth and repair. Carbohydrates, proteins and fats are broken down by the digestive system into their simplest components: simple sugars, amino acids and fatty acids. Carbohydrates are the body’s preferred fuel, although proteins and fats can be converted into energy too. This food energy is measured in ‘kilojoules’. The common term for this used to be ‘calorie’ (or ‘kilocalorie’), but ‘kilojoule’ is the term now accepted internationally. This unit of measurement allows us to talk about how much energy a food contains and how many kilojoules are burned up during a particular exercise.

Energy value
A calorie is the amount of energy or heat needed to increase the temperature of one gram of water by 1 Celsius. One calorie has the same energy value as 4.186 kilojoules (kJ). This doesn’t mean that kilojoules are four times more fattening than calories. Remember that these terms are units of measurement only, similar to the terms ‘kilometre’ and ‘mile’: the unit of measurement might be different, but it is still the same distance from point A to point B.

Kilojoules and calories in food
The foods we eat provide kilojoules. Just how many depends on the amounts of carbohydrate, protein and fat they contain. Fats and alcohol are by far the most energy dense foods. This is why they should only be consumed in moderation, particularly if you are overweight or obese. The energy value per gram of various food components includes:

  • Fat - 37 kJ (9 calories) (not all fatty acids may provide the same amount of energy)
  • Alcohol - 29 kJ (7 calories)
  • Carbohydrates - 16 kJ (4 calories) (not all carbohydrates may provide the same amount of energy)
  • Protein - 17 kJ (4 calories)
  • Dietary fibre - 13 kJ (3 calories) (if fermented by bacteria in the large intestine)
  • Water - 0 kJ (0 calories).
  • Continuing research in the field of fat and carbohydrate metabolism is changing our thinking and understanding about the energy values of different types of fats and carbohydrates. The more that is known about the different roles of fatty acids and carbohydrates in the body, the greater the understanding that not all fatty acids or carbohydrates have the same energy value as determined by chemical analysis in the test tube.
Animal studies now show that polyunsaturated, mono-unsaturated and saturated fatty acids are handled differently metabolically and, therefore, may not have equivalent availability for use as fuel. The biological function of fatty acids, especially those that are polyunsaturated, are not only for energy utilisation, but they also form part of cell membranes and play a role in cell regulation. Polyunsaturated fats, especially omega 3 fatty acids from fish oils, may be more readily mobilised from fat stores, especially during exercise. This suggests that saturated fat is more likely to go into and stay in fat cells than some forms of polyunsaturated fat and possibly mono-unsaturated fat.

In the case of carbohydrates, simple sugars in foods such as fruit juices or candy are absorbed more efficiently (higher glycaemic index) than the carbohydrate in foods high in unrefined carbohydrates (such as grainy bread), which have a lower glycaemic index. The energy value of carbohydrates in foods of low glycaemic index can therefore be less than 16kJ/g once consumed. There is also evidence that a diet dominated by carbohydrate foods with a high glycaemic index is associated with greater body fatness.

Our energy requirements are variable
Energy requirements differ from one person to the next because of genetic predisposition, build, gender, age, metabolism, environment and amount of regular physical activity. An individual’s energy requirements can also differ from one day to the next and as we grow older. Young children and adolescents require high amounts of energy to fuel their growth and development.

Women have increased energy needs during certain stages of their reproductive lives, such as pregnancy and lactation. It is thought that daily energy needs increase on average by about 1,800 kJ for pregnant women and around 2,500kJ during lactation.

Muscle tissue has a voracious appetite for kilojoules. The more muscle mass you have, the more kilojoules you will burn. Men generally have higher energy requirements than women because of their ratio of lean muscle to fat. As we age, our energy requirements tend to decrease; activity is reduced, which causes loss of muscle tissue. Various other age-related changes to the metabolism also contribute to the reduced energy requirements seen with ageing. It is not known how much of the muscle lost during ageing is a result of the ageing process itself or just due to reduced activity levels. Strength and resistance training in older adults (even the very old and frail) seems promising in reducing or preventing the decline in muscle mass generally observed with ageing.

Too many kilojoules
When we regularly eat more energy than our body needs, the excess is stored inside the fat cells. Just 1kg of body fat contains the equivalent of 37,000 kJ. To lose 1kg of body fat in a week, you would need to burn an additional 37,000 kJ, or around 5,000 kJ a day. The best way to lose excess weight is to switch to a high fibre, low fat diet and, most importantly, to exercise regularly. Exercise helps to stimulate muscle development. Remember, the more muscle tissue you have, the more kilojoules you can burn. If you are over 40, have a pre-existing medical condition or you haven’t exercised for some time, see your doctor before embarking on any new fitness program.

Where to get help
  • Your doctor
  • Dietitian.
Things to remember
  • A kilojoule (or calorie) is a unit of energy.
  • The kilojoule content of foods depends on the amounts of carbohydrates, fats and proteins present in the food.
  • If we regularly eat more kilojoules than our body needs, the excess will be stored as body fat.
    Better Health Channel - Related Quiz.

    Related articles:

Carbohydrates explained.
Food variety and a healthy diet.
Healthy eating for children and adults.
Weight loss - common misconceptions.


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Article publication date: 22/06/2000
Last reviewed: 30/07/2004

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