Food energy, which is effectively the amount of energy released by a particular quantity of one of the three macronutrients, namely carbohydrate, fat and protein, can be measured using either joules or calories.
A joule is the SI unit of work or energy, equal to the work done by a force of 1 Newton when its point of application moves one meter in the direction of the force. By contrast a calorie is specifically a unit of heat energy and is the energy needed to raise the temperature of one gram of water through one degree centigrade.
There is a direct correlation between joules and calories, namely one calorie is equivalent to approximately 4.2 joules. Note that calories spelled with a small c is equivalent to 4.2 joules but Calories spelled with a capital C is equivalent to 4.2 kilojoules.
When it comes to determining the calorific value of a certain amount of macronutrient, the food is ignited and the amount of energy released is measured simply by determining the rise in temperature of a set amount of water. This value can then be displayed as the energy that is available for utilisation by the body, either in calories or kilojoules.
The absolute value of the calories released by both carbohydrate and protein is similar namely a gram releases 4 calories whilst a gram of fat releases 9 calories. These figures are somewhat academic in that most foods are a mix of more than one macronutrient. Additionally, and importantly, the way the body handles an equivalent number of calories varies with the three macronutrients leading to the concept that not all calories are equal. And this is vital to understanding what we must eat in order to help us lose weight.
The widely-held belief that a calorie is a calorie is a calorie is simply wrong. And this explains, along with other reasons, why individuals should not become obsessed with calorie-counting. It is important to have a basic understanding of how many calories are in a particular type of food but from a practical perspective it isn’t necessary to weigh every piece and then to refer to a table.
Further information about the practical application of calories can be found in the presentation entitled Daily energy requirements and also the ones relating to the causes and management of obesity.