Energy can be measured in two types of units: calories and joules. Because the amounts involved in the measurement of energy provided by foodstuffs are so large, kilocalories (1 000 calories) and kilojoules (1 000 joule) are mostly used as units of energy.

On a scientific level, a calorie is the amount of heat energy required to raise the temperature of 1mL of water at 15 degrees Celsius by 1 degree Celsius. In other words, if you take 2 slices of salami (the equivalent of 145 kilocalories) and set them on fire, the reaction would theoretically produce 145 kilocalories of energy.

The term "kilocalorie" is, however, popularly referred to as "calorie". In literature or on food labels, when one reads, for example, that a mango contains 135 calories, it actually means that the mango has the potential to provide 135 kilocalories of energy.

A multiplier of 4,2 must be used to convert kilocalories (or just "calories") to kilojoules.

Energy supplied by macronutrients
The number of calories in a food product is an indication of the potential energy the food possesses. The energy value of the macronutrients can be calculated as follows:

• 1 g protein provides 4 calories
• 1 g carbohydrate provides 4 calories (dietary fibre excluded)
• 1 g fat provides 9 calories

If you know, for example, that an egg weighing 50 grams consists of 6 g of protein, 1 g of carbohydrate and 6 g of fat it is easy to calculate the energy value of the egg:

(6 g protein x 4 calories) + (1 g carbohydrate x 4 calories) + (6 g fat x 9 calories) = 82 calories

All calories are the same

It is important to note that a fat calorie is exactly the same as a carbohydrate calorie. The only difference is that one gram of fat provides more then double the amount of calories than one gram of carbohydrates or proteins. And the consumption of fat has several other nutritional disadvantages, of which high cholesterol levels is just one.

Read more:
Calories explained