Calorie Totally Explained

A calorie is a unit of measurement for energy. Calorie is French and derives from the Latin calor (heat). In most fields, it has been replaced by the joule, the SI unit of energy. However, the kilocalorie or Calorie (capital "C") remains in common use for the amount of food energy. The calorie was first defined by Professor Nicolas Clément in 1824 as a kilogram-calorie and this definition entered French and English dictionaries between 1842 and 1867. The calorie was never an SI unit. Modern definitions for calorie fall into 3 classes:

The small calorie or gram calorie approximates the energy needed to increase the temperature of 1 gram of water by 1 °C. This is about 4.184 joules.
The large calorie or kilogram calorie approximates the energy needed to increase the temperature of 1 kg of water by 1 °C. This is about 4.184 kJ, and exactly 1000 small calories.
The megacalorie or ton calorie approximates the energy needed to increase the temperature of 1 tonne of water by 1 °C. This is about 4.184 MJ, and exactly 1000 large calories.

In some scientific contexts such as physics and chemistry, the name "calorie" refers strictly to the gram calorie, and this unit has the symbol cal. SI prefixes are used with this name and symbol, so that the kilogram calorie is known as the "kilocalorie" and has the symbol kcal.
   In the medical sciences and non-scientific contexts the calorie is equal to a kilocalorie in the physics or chemistry sense, and is occasionally referred to as a Calorie (capital "C") in an unsuccessful and rather ineffective—because not adopted by any significant groups of people, and partly because it has no effect when the word appears at the beginning of a list item or of a sentence—attempt to distinguish it, and it has to be inferred from the context that the small calorie isn't intended.
   The conversion factor among calories and joules is numerically equivalent to the specific heat capacity of liquid water (in SI units). See "Versions" below for explanation of units. ť 1 cal_IT = 4.1868 J (1 J = 0.23885 cal_IT) (International Steam Table calorie, 1956)

   1 cal_th = 4.184 J (1 J = 0.23901 cal_th) (Thermochemical calorie) ť 1 cal₁₅ = 4.18580 J (1 J = 0.23890 cal₁₅) (15°C calorie)

Versions

The energy needed to increase the temperature of 1 g of water by 1 Celsius varies depending on the starting temperature, and is in any case difficult to measure precisely. Accordingly there have been several definitions of the calorie:

15 °C calorie (cal₁₅): the amount of energy required to warm 1 g of air-free water from 14.5 °C to 15.5 °C at a constant pressure of 101.325 kPa (1 atm). Experimental values of this calorie ranged from 4.1852 J to 4.1858 J. The CIPM in 1950 published a mean experimental value of 4.1855 J, noting an uncertainty of 0.0005 J.

20 °C calorie: the amount of energy required to warm 1 g of air-free water from 19.5 °C to 20.5 °C at a constant pressure of 101.325 kPa (1 atm). This is about 4.182 J.

4 °C calorie: the amount of energy required to warm 1 g of air-free water from 3.5 °C to 4.5 °C at a constant pressure of 101.325 kPa (1 atm).

Mean calorie: 1/100 of the amount of energy required to warm 1 g of air-free water from 0 °C to 100 °C at a constant pressure of 101.325 kPa (1 atm). This is about 4.190 J

International Steam Table Calorie (1929): (1/860) W h = (180/43) J exactly. This is approximately 4.1860 J.

International Steam Table Calorie (1956) (cal_IT): 1.163 mW h = 4.1868 J exactly. This definition was adopted by the Fifth International Conference on Properties of Steam (London, July 1956).

Thermochemical calorie (cal_th): 4.184 J exactly.

IUNS calorie: 4.182 J exactly. This is a definition implied by the Committee on Nomenclature of the International Union of Nutritional Sciences (date and reference needed). The two perhaps most popular definitions used in older literature are the "15 °C calorie" and the "thermochemical calorie". Since the many different definitions are a source of confusion and error, all calories are now deprecated in favour of the SI unit for heat and energy: the joule (J).

Nutrition

In nutrition, the difference between these calorie definitions is of no practical relevance. This is because nutritional calories are not measured amounts of energy, but are calculated from food composition. Such calculations use internationally agreed conventional conversion factors, which are generously rounded values that roughly approximate the average energy density of a large number of different food samples. The exact composition of agricultural products varies far more than the 0.1% difference between the above definitions of the calorie as a physical energy measure.
Human fat tissue contains about 87% lipids, so that 1 kg of body-fat tissue has roughly the caloric energy of 870 g of pure fat, or 7800 kcal. In principle one has to create a 7800 kcal deficit or surplus between energy intake and use to lose or gain 1 kg of body-fat. (or 3500 kcal per pound). However, if one eats 7800 kcal more than the body needs, one won't necessarily gain 1 kg of fat, since muscle and other tissues may be built. The same way, if one eats 7800 kcal less than their maintenance level, they may not lose 1 kg of fat, since muscle and sugars may be metabolized to generate energy.

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