Calorie

Information and facts about Nutrition.

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.

Nutrition
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 4,184 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 favour 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 [1].

Dieticians recommend counting calories to avoid obesity. The government of the United Kingdom recommends consumption of no more than 2000 Calories (2000 kcal) by women each day and 2500 Calories (2500 kcal) by men each day.

Physics and chemistry
In physics or chemistry, a calorie (abbreviated cal) is a unit of energy that equals the amount of heat necessary to raise the temperature of one gram of water by 1 degree Celsius, at a pressure of 1 atm. This amount of heat depends somewhat on the initial temperature of the water, which results in various different units sharing the name of "calorie" but having slightly different energy values:

the 15 °C calorie,
the 4 °C calorie,
the mean 0 °C to 100 °C calorie,
the International Steam Table calorie,
the thermochemical calorie,
The slight variations in these units can be seen if you convert them to joules. For example, one 15 °C calorie is the amount of heat necessary to raise the temperature of 1 g of water from 14.5 °C to 15.5 °C. This is approximately equal to 4.1855 J or 3.968×10-3 Btu. The International Steam Table calorie is approximately equal to 4.1868 J and the thermochemical calorie is defined to be precisely 4.184 J.

Of these various units, what is most commonly meant by calorie in contemporary English text is the 15 °C calorie. The nutritional Calorie represents 1000 of these 15 °C calories.

Since this could be a source of confusion and error, these units are now deprecated. The International System of Units (SI) unit for heat (and for all other forms of energy) is the joule (J), while the (obsolete) cgs system uses the erg.

Trivia
Unicode has a symbol for "cal": (㎈), but this is just a legacy code to accommodate old code pages in certain Asian languages, and it is not recommended for use in any language today.
The conventional value chosen to define a ton of TNT is equal to 1 billion thermochemical calories. (The actual energy liberated from the explosion is less; see the megaton entry.)

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