The term calorie refers to a non-SI unit of energy, where SI is “The International System of Units (abbreviated SI from the French phrase, Système International d’Unités”). There are two common but different meanings: one is used in food and nutrition, the other was formerly widely used in chemistry, and a food calorie represents 1000 chemistry calories. The food calorie is sometimes capitalized as Calorie to distinguish it from the chemistry calorie; however, this capitalization is rarely observed in practice.
Nutritionists measure the energy content of food in “calories” (sometimes capitalized and abbreviated as Cal or sometimes C, or abbreviated kcal as if small calories were being used), where each food calorie represents 4,186 joules. This is equivalent to 1000 of the calories used in chemistry, and thus the food Calorie would be called a kilocalorie if small calories were being used. However, in chemistry calories have been deprecated as a scientific unit of measure in favor of joules, and therefore in common modern usage the word “calorie” usually refers to a food calorie.
This situation provides two ways of talking about the amount of calories in food which look quite different but that express the exact same amount of energy. One may say that dietary fat has nine kilocalories (kcal) per gram, while proteins and carbohydrates have four kcal per gram, or, one may say that fat has nine Calories per gram while carbohydrates and proteins have four Calories per gram.
The amount of food energy in a particular food is measured by completely burning the food in a bomb calorimeter, a method known as direct calorimetry.
Dieticians recommend counting calories to avoid obesity. An average recommendation for the maximum daily consumption of Calories is:
For Women: 2000 Calories (2000 kcal)
For Men: 2500 Calories (2500 kcal)
Of course, these figures are only averages, i.e. depending on the type of work and physical exercise you have each day, you will need more or less Calories.
