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In physics, 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 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 4.184 J.

Of these various units, what is most commonly meant by calorie in contemporary English text is the 15 °C calorie.

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.


Nutritionists, when describing the energy content of food, typically refer to Calories (capitalized and abbreviated as C); one food Calorie equals 1000 calories (the 15 C variety), or about 4,186 J. A food calorie is more precisely abbreviated as kcalcal for kilocalorie.

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.

This thousand-fold difference has led to a joking calculation, sometimes taken seriously, which "shows" that warming cold beer (or ice cream) in the stomach requires more energy than present in the refreshment, and thus a cold beer or a frozen dessert can be used to lose weight. The error lies in the conversion of nutritional beer Calories to thermochemical calories on a one-to-one basis instead of properly converting a food Calorie to a thousand thermochemical calories. Once that mistake is corrected, it becomes clear that cold or even frozen beer contains much more nutritional energy than required to warm it to body temperature.

(see Beer and Ice Cream Diet)

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