Calorie
From Wikipedia, the free encyclopedia
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 In most fields, it has been replaced by the joule, the SI unit of energy.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, it remains in common use for the amount of food energy. The word "Calorie" is often mis-used to mean "energy", for example people speak of "high-calorie foods" when they mean foods high in energy.
Definitions for calorie fall into two classes:
- The small calorie or gram calorie approximates the energy needed to increase the temperature of 1 gram of distilled water by 1 °C, from 14,5°C to 15,5°C. This is 4,1855 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 distilled water by 1 °C, from 14,5°C to 15,5°C. This is 4,1855 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).
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[edit] Nutritional and food labels
The calorie has become a common household term as dietitians recommend in cases of obesity to reduce body weight by increasing exercise (energy expenditure) and reducing energy intake. Many governments require food manufacturers to label the energy content of their products, to help consumers control their energy intake. In the countries of the European Union, manufacturers of prepackaged food must label the nutritional energy of their products in both kilocalories ("kcal") and kilojoules ("kJ"). In the United States, the equivalent mandatory labels display only Calories (with a capital “C,” meaning kilocalories); an additional kilojoules figure is optional. The energy content of food is usually given on labels for 100 g and for a typical serving size.
The amount of food energy in a particular food could be measured by completely burning the dried food in a bomb calorimeter, a method known as direct calorimetry [1]. However, the values given on food labels are not determined this way, because it overestimates the amount of energy that the human digestive system can extract, by also burning dietary fibre. Instead, standardized chemical tests and an analysis of the recipe are used to estimate the product's digestible constituents (protein, carbohydrate, fat, etc.). These results are then converted into an equivalent energy value based on a standardized table of energy densities:
| food component | energy density | |
|---|---|---|
| kcal/g | kJ/g | |
| fat | 9 | 37 |
| ethanol (alcohol) | 7 | 29 |
| proteins | 4 | 17 |
| carbohydrates | 4 | 17 |
| organic acids | 3 | 13 |
| polyols (sugar-free sweeteners) | 2.4 | 10 |
Other substances found in food (water, non-digestible fibre, minerals, vitamins) do not contribute to this calculated energy density.
As a rough guideline recommended daily energy intake values for young adults are: 2500 kcal/d (10 MJ/d, 120 W) for men and 2000 kcal/d (8 MJ/d, 100 W) for women. Children, sedentary, and older people require less energy; physically active people, more.
[edit] 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.
[edit] 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, and it is not recommended for use in any language today.
- The conventional value chosen to define one ton of TNT is equal to 1 billion thermochemical calories: 1 tTNT ≡ 1 × 109 calth. 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.) [2] 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.
[edit] See also
[edit] References
- European Union regulations on nutrition labeling
- United Kingdom Food Labelling Regulations 1996 – Schedule 7: Nutrition labelling
- United States federal food-labeling regulations 21CFR101.9
- NIST Special Publication 811, Appendix B8: calorie.
- Donatelle, Rebecca J. Health: The Basics. 6th ed. San Francisco: Pearson Education, Inc. 2005.
- The adoption of joules as units of energy, FAO/WHO Ad Hoc Committee of Experts on Energy and Protein, 1971.
- Methods used in measuring rate of burn in humans [3]
[edit] 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
