Kilojoules and kilocalories

Kilojoules (kJ) and kilocalories (kcal) are ultimately the same thing; they are both a unit used to express energy expenditure.

In Australia it is more common to use kilojoules. It forms the basis of the most simple equation to understand how the body uses fuel – the amount of kilojoules you consume through food has to equal that which is used by the body.

Heading back to year 10 science, matter or energy can’t be created or destroyed without a reaction, which means every kilojoule you eat needs to be accounted for. Most will be used for bodily functions and maintaining muscle mass and good health (even while you sleep).

Activity requires more fuel on top of that and more activity you do the more kilojoules you need to consume to keep fuelling things like muscle contraction and temperature control.

Either unit can be used, as long as any comparisons are done using the same units. If you are trying to compare kilojoules and kilocalories you can multiply kilocalories by 4.2 and you will get kilojoules. For example 100kcal = 420kJ.