Joules and calories

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Joules and calories in food and exercise.
General background.

We need to understand how energy is measured - both energy we use up in living and energy we derive from our food.

Joules.

In science, the basic unit in the International System of Units used for measuring energy is called the joule. It is defined as being the amount of energy transferred to an object when a force of one newton (about 100 grams of force on Earth) acts to move that object one metre.

For example:

if we hold a 100g object a metre off the floor and drop it, that object will have a Joule of kinetic energy when it hits the floor;
it takes about 4 joules to warm 1 ml of water by 1°C.

We do not need to know these things - but one joule is a small amount of energy.

The important thing to appreciate is that energy is measured in joules. When we exercise, we measure the energy we use up in joules or in kilojoules.

Sometimes energy is expressed in kilojoules (written as kJ).

Calories.

A CALORIE is also a measure of energy in or applied to something. In our usual discussions, a Calorie (with a big C) is the amount of energy required to raise the temperature of 1 kg of water by 1° C.

Some scientists working in physics and chemistry situations use the unit of calorie (with a small c) to mean the amount of energy required to raise the temperature of 1 gram of water by 1°C.

So 1 Calorie = 1,000 calories.

In our discussions (especially related to food), we use the big C idea and write the units as Kcal or just calories.

Calories are the basic measure of energy in foods and drinks. They are also used as a measure of how much energy we use up when exercising (or even just sitting around or sleeping).

As a very general guide, women need about 2,000 calories per day while men need about 2.500 calories. The needs of young people and children vary widely depending on lifestyle.

Joules or calories.

We therefore have two measures for energy to analyse food and exercise. Clearly these two measures are interchangeable.

The common applications are that:

joules are used to measure the energy used when we exert ourselves in exercise (or sleeping);
calories (Kcal) are used to measure the amount of energy we derive from food and drinks.

To convert from one measure to another, we use the equation 1 calorie (or kilocalorie) = 4.184 Joules.

The use of one measure rather than the other is far from fixed however. Different countries use differing labels for energy derived from food. For example:

Australia, New Zealand and the UK use kJ or both kJ and kCal;
Canada and the USA use calories;
China uses kJ;

The food we eat has three different macronutrients:

carbohydrates (with 4 calories per gram);
protein (with 4 calories per gram);
fat (9 calories per gram).

A very good summary of this topic with an analysis of issues and guidelines is available through the Victorian Department of Health and