The calorie is a pre-SI metric unit of energy. The unit was first defined by Professor Nicolas Clément in 1824 as a unit of heat. This definition entered French and English dictionaries between 1841 and 1867.[1] In most fields its use is archaic, having been replaced by the SI unit of energy, the joule. However, in many countries it remains in common use as a unit of food energy. The kilocalorie per mole remains in use in computational chemistry and molecular spectroscopy.
Definitions vary but are all based on the specific heat capacity of water. The gram calorie, approximately 4.2 J, is based on one gram of water. The kilogram calorie, equal to one thousand gram calories, is based on one kilogram of water. In the context of nutrition, and especially food labelling, a larger unit is used and referred to interchangeably by the terms calorie (or Calorie) and kilocalorie.
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