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Kilojoules: what are they really?

Kilojoules are a measure of the amount of energy that food provides. Energy from food used to be measured in calories, but is now more commonly measured in kilojoules (kJ). An easy way to convert calories to kilojoules is to multiply by four. So if you see a food described as containing 100 calories, its kilojoule equivalent will be roughly 400 kJ.

The word ‘energy’ can be confusing as it has a few meanings. While energy can sometimes mean a feeling of vitality, accredited practising dietitians also use it to describe the number of kilojoules in food that can be used by the body as fuel. This is why a statement like “chocolate is high in energy” doesn’t mean eating a chocolate bar will help you run a marathon — it’s just another way of saying chocolate is high in kilojoules.

How much energy is in food?

Different foods yield different amounts of energy. Check out the table below to compare the different energy values of different foods.

One gram of... Energy yield in kJ (Cal)
Carbohydrates 17 kJ (4 Cal)
Proteins 17 kJ (4 Cal)
Fats 38 kJ (9 Cal)
Alcohol 29 kJ (7 Cal)
Water 0 kJ (0 Cal)

The average office worker — aged between 31–50 years old, weighing between 60–70 kilos, with no medical conditions, who sits at their desk all day and does little strenuous physical activity — only needs to eat between 8000–9900 kJ a day to maintain their energy balance.

If you want to lose weight, you need to use up more energy than you eat in your food. Looking at the table above, remember that if you eat lots of fatty foods, which have more than twice the amount of energy as carbohydrates and proteins weight for weight, then you will probably need to do more exercise to be able to burn off those extra kilojoules.

How can we find out the energy or kilojoule content of foods?

Foods can be analysed for energy value, carbohydrate, protein and fat content and micronutrient (vitamin and mineral) content. These food results are then compiled in what we call food composition tables. The food composition tables may be used to calculate dietary intakes. Many companies provide some of this information on their food labels.

By knowing the energy value of foods we can estimate how much energy we’re taking in. While eating enough food to provide energy for body functions is vital, when we consume more energy (kilojoules) than we use, we can gain weight.

Further information

Better Health Channel: Kilojoules and calories explained www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Kilojoules_and_calories-explained

Sources

Harvard School of Public Health. The best diet is one you’ll follow. [online] Boston, MA: The President and Fellows of Harvard College. c2010. [accessed 18 Aug 2010] Available from: http://www.hsph.harvard.edu/nutritionsource/healthy-weight/

National Health and Medical Research Council. Nutrient Reference Values — dietary energy. [online] Canberra, ACT: Commonwealth of Australia. 2009 [accessed 19 Aug 2010] Available from: http://www.nrv.gov.au/energy.htm

SA Health. Reading food labels. [online] Adelaide, SA: Government of South Australia. c2010. [accessed 6 Aug 2010] Available from: http://www.health.sa.gov.au/pehs/srer-award/srer-labelreading-sahs-100419.pdf (PDF, 209Kb)

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Disclaimer
This information has been developed and reviewed for Bupa by health professionals. To the best of their knowledge it is current and based on reputable sources of medical research. It should be used as a guide only and should not be relied upon as a substitute for professional medical or other health professional advice.

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Last published 31 October 2010