Understanding calories, kilocalories, and kilojoules

This is the fifth post in the dieting basics series.

What does a calorie measure? What does a joule measure?

Calories measure energy, particularly heat energy. One calorie is the energy required to raise the temperature of one gram of water by one degree Celsius. Nowadays, the calorie is only used for food energy – it’s considered archaic in other contexts.

The standard, metric, equivalent of the calorie is the joule. This is used in scientific contexts and is defined as the energy expended by a force of one Newton moving one metre in the direction of the force. (If that gives you scary flashbacks to school physics lessons, don’t worry too much about it!)

There are 4.2 joules to every calorie.

So why do we have “calories” and “kilojoules”? Shouldn’t it be “kilocalories”?

There are two units defined as “calories”, one a thousand times large than the other:

  • The small calorie (with a lower-case c) is the amount of heat needed to raise the temperature of one gram of water by one degree Celsius.
  • The large Calorie (usually with a capital C) is the amount of heat needed to raise the temperature of one kilogram of water by one degree Celsius.

Particularly when talking about food energy, the large Calorie is used. Sometimes they are called kilocalories, particularly in a more scientific or specialised health context – for example, if you use a heart rate monitor when working out, your energy expenditure may be listed in kilocalories or Kcals. Each kilocalorie is exactly the same as one Calorie.

When were calories invented? Where are they used today?

The calorie was first defined by the French professor Nicolas Clément in 1824 as a kilogram-calorie, and was not initially applied to food – it was used in lectures on heat engines. The definition entered French and English dictionaries between 1841 and 1867. The word calorie is French, from the Latin calor (heat).

Nowadays, the US and UK use “Calories” or “kcals” on food labelling and recipes – in the UK, kilojoules are also listed on food labels, though not always used in dieting advice. In Australia, kilojoules are the unit of food energy in standard use.

Why not look up your recommended daily calorie allowance (another post in this series!)

How do I convert between kilocalories, Calories and kilojoules?

Remember that (when talking about food and exercise), one Calorie = one kilocalorie

And one Calorie = 4.2 kilojoules.

So, if you burned 300 kcals in the gym, and your post-gym snack of an apple contains 50 calories, and your cereal bar contains 315 kilojoules…

Used: 300 Calories

  • 50 Calories + (315 kilojoules/4.2)
  • = 50 Calories + 75 Calories
  • = 125 Calories

So you still have 175 “extra” exercise Calories that you can safely eat and not gain weight.

Healthy eating is about so much more than just counting calories. Check out the previous posts in the dieting basics series for the facts about carbohydrates, fat, protein and fibre. And subscribe to the RSS feed to ensure you don’t miss out on any of the series!

(Image above by jobeone)


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