Q: How do you work out the products calories if the product available is only giving the kj. Shouldn't we be working on kcal for calories?

A: This can seem confusing but is quite simple when you understand it. Firstly, there are two units for measuring energy, the calorie and the joule. Both can be used to express the amount of energy released by food during metabolism. But we commonly use the kilojoule (kJ) and the kilocalorie (kcal) because the amount of energy involved in the metabolism of food is fairly large.

What makes it confusing is that in everyday language, the word 'calories' is commonly used for kilocalories. Scientifically speaking this is incorrect, because 1 kilocalorie actually equals 1000 calories. In the same way, 1 kilojoule equals 1000 joules.

To convert kilocalories or ‘calories’ to kilojoules, multiply by 4.2 (e.g.100 kcal/'calories’ = 420kJ).

So in a nutshell, kJ and kcal (commonly called calories) are not constituents of food but are different units to measure and express the energy that food provides. This depends on how much fat, carbohydrate and protein the food contains. Practically all foods contain combinations of the energy-providing nutrients (fat, carbohydrate and protein).