Calorie
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 In most fields, it has been replaced by the joule, the SI unit of energy.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.
Definitions for calorie fall into 2 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, and exactly 0.001 large calories.
- 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.
In scientific contexts, 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 non-scientific contexts the kilocalorie is often referred to as a Calorie (capital "C"), or just a calorie, and it has to be inferred from the context that the small calorie is not intended.
The conversion factor between calories and joules is numerically equivalent to the specific heat capacity of liquid water (in SI units).
1 cal<sub>INT</sub> = 4.1867 J (1 J = 0.23885 cal<sub>IT</sub>)
1 cal<sub>th</sub> = 4.184 J (1 J = 0.23901 cal<sub>th</sub>)
1 cal<sub>15</sub> = 4.18580 J (1 J = 0.23890 cal<sub>15</sub>)
Versions
The energy needed to increase the temperature of 1 g of water by 1 °C 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: 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): 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: 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).
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.
Trivia
- Unicode has a symbol for "cal": (㎈), but this is just a legacy compatibility code to accommodate the old code pages in certain Asian languages.
- The conventional value chosen to define one ton of TNT is equal to 1 billion thermochemical calories: 1 t<sub>TNT</sub> ≡ 1 × 10<sup>9</sup> cal<sub>th</sub>. The actual energy liberated from the explosion is somewhat more; see megaton.
- 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. Therefore one has to create a 7800 kcal deficit between energy intake and use to lose 1 kg of body-fat. (In U.S. customary units, that is about 3500 kcal per pound.) [1] In other words, if you eat 3,500 kilocalories more than your body needs, you will put on about 1 pound of fat. If you use up 3,500 kilocalories more than you eat, you will lose about 1 pound of fat. These approximations assume that there is no net gain or loss in muscle, which can also be built using food energy, or metabolized as a source of energy. This also assumes that there is a direct relationship between calories of food consumed and that stored in body fat, which is not proven or likely.
See also
References
External links
- USDA National Nutrient Database for Standard Reference - Release 19 Official, publicly available reference database and online search site. Includes 7,293 foods and is free to download and use. This database is the one used by most websites that provide calorie information, and forms the basis of the Canadian national nutrient database and others