A calorie refers to a non SI unit of energy. 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" (usually capitalized
and abbreviated as Cal or sometimes C), 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 more accurately be called a kilocalorie. However, in chemistry
calories have been deprecated as a scientific unit of measure in favour of joules,
and therefore in common modern usage the word "calorie" will usually refer 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 9 kcal per gram, while proteins and carbohydrates
have 4 kcal per gram, or, one may say that fat has 9 Calories per gram while
carbohydrates and proteins have 4 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
(http://www.merck.com/mmhe/sec12/ch152/ch152e.html).
Dieticians recommend counting calories to avoid obesity. The
government of the United Kingdom recommends consumption of 2000 Calories (2000 kcal)
by women each day and 2500 Calories (2500 kcal) by men each day.
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