Template:Short description Template:Redirect Template:Infobox unit The calorie is a unit of energy that originated from the caloric theory of heat.<ref name="morris1992" /><ref name="marsh2020" /> The large calorie, food calorie, dietary calorie, kilocalorie, or kilogram calorie is defined as the amount of heat needed to raise the temperature of one liter of water by one degree Celsius (or one kelvin).<ref name="morris1992" /><ref name="Webster" /> The small calorie or gram calorie is defined as the amount of heat needed to cause the same increase in one milliliter of water.<ref name="Webster" /><ref name="Cambridge" /><ref name="Oxford" /><ref name="morris1992" /> Thus, 1 large calorie is equal to 1,000 small calories.
In nutrition and food science, the term calorie and the symbol cal may refer to the large unit or to the small unit in different regions of the world. It is generally used in publications and package labels to express the energy value of foods in per serving or per weight, recommended dietary caloric intake,<ref name=FDA2019/><ref name=NHS2019/> metabolic rates, etc. Some authors recommend the spelling Calorie and the symbol Cal (both with a capital C) if the large calorie is meant, to avoid confusion;<ref name=conn2019/> however, this convention is often ignored.<ref name=FDA2019/><ref name=NHS2019/><ref name=conn2019/>
In physics and chemistry, the word calorie and its symbol usually refer to the small unit, the large one being called kilocalorie (kcal). However, the kcal is not officially part of the International System of Units (SI), and is regarded as obsolete,<ref name=marsh2020/> having been replaced in many uses by the SI derived unit of energy, the joule (J),<ref name=BIPM9th/> or the kilojoule (kJ) for 1000 joules.
The precise equivalence between calories and joules has varied over the years, but in thermochemistry and nutrition it is now generally assumed that one (small) calorie (thermochemical calorie) is equal to exactly 4.184 J, and therefore one kilocalorie (one large calorie) is 4184 J or 4.184 kJ.<ref name=BIPM7th/><ref name=FAO2003/>
HistoryEdit
The term "calorie" comes Template:Ety.<ref name="MW_calorie">Template:Cite encyclopedia</ref> It was first introduced by Nicolas Clément, as a unit of heat energy, in lectures on experimental calorimetry during the years 1819–1824. This was the "large" calorie.<ref name=marsh2020/><ref name=harg2007/><ref name=harg2006/> The term (written with lowercase "c") entered French and English dictionaries between 1841 and 1867.
The same term was used for the "small" unit by Pierre Antoine Favre (chemist) and Johann T. Silbermann (physicist) in 1852.
In 1879, Marcellin Berthelot distinguished between gram-calorie and kilogram-calorie, and proposed using "Calorie", with capital "C", for the large unit.<ref name=marsh2020/> This usage was adopted by Wilbur Olin Atwater, a professor at Wesleyan University, in 1887, in an influential article on the energy content of food.<ref name=marsh2020/><ref name=harg2007/>
The smaller unit was used by U.S. physician Joseph Howard Raymond, in his classic 1894 textbook A Manual of Human Physiology.<ref name=raym1894/> He proposed calling the "large" unit "kilocalorie", but the term did not catch on until some years later.
The small calorie (cal) was recognized as a unit of the CGS system in 1896,<ref name=marsh2020/><ref name=harg2006/> alongside the already-existing CGS unit of energy, the erg (first suggested by Clausius in 1864, under the name ergon, and officially adopted in 1882).
In 1928, there were already serious complaints about the possible confusion arising from the two main definitions of the calorie and whether the notion of using the capital letter to distinguish them was sound.<ref name=marks1928/>
The joule was the officially adopted SI unit of energy at the ninth General Conference on Weights and Measures in 1948.<ref name=CIPM1948/><ref name=BIPM9th/> The calorie was mentioned in the 7th edition of the SI brochure as an example of a non-SI unit.<ref name=BIPM7th/>
The alternate spelling Template:Linktext is a less-common, non-standard variant.<ref name="MW_calorie" />
DefinitionsEdit
The "small" calorie is broadly defined as the amount of energy needed to increase the temperature of 1 gram of water by 1 °C (or 1 K, which is the same increment, a gradation of one percent of the interval between the melting point and the boiling point of water).<ref name="Cambridge" /><ref name="Oxford" /> The actual amount of energy required to accomplish this temperature increase depends on the atmospheric pressure and the starting temperature; different choices of these parameters have resulted in several different precise definitions of the unit.
| Name | Symbol | Conversions | Definition and notes |
|---|---|---|---|
| Thermochemical calorie | calth | ≡ Template:Val | The amount of energy equal to exactly 4.184 J (joules) and 1 kJ ≈ 0.239 kcal.<ref name="iso31-4" /><ref name=Rossini/><ref name=lynch1974/><ref name=FAO2003/>Template:Efn |
| 4 °C calorie | cal4 | ≈ 4.204 J | The amount of energy required to warm one gram of air-free water from 3.5 to 4.5 °C at standard atmospheric pressure.Template:Efn |
| 15 °C calorie | cal15 | ≈ 4.1855 J | The amount of energy required to warm one gram of air-free water from 14.5 to 15.5 °C at standard atmospheric pressure.Template:Efn Experimental values of this calorie ranged from 4.1852 to 4.1858 J. The CIPM in 1950 published a mean experimental value of 4.1855 J, noting an uncertainty of 0.0005 J.<ref name="iso31-4" /> |
| 20 °C calorie | cal20 | ≈ 4.182 J | The amount of energy required to warm one gram of air-free water from 19.5 to 20.5 °C at standard atmospheric pressure.Template:Efn |
| Mean calorie | calmean | ≈ 4.190 J | Defined as Template:Frac of the amount of energy required to warm one gram of air-free water from 0 to 100 °C at standard atmospheric pressure.Template:Efn |
| International Steam Table calorie (1929) | ≈ 4.1868 J | Defined as Template:Frac "international" watt hours = Template:Frac "international" joules exactly.Template:Efn | |
| International Steam Table calorie (1956) | calIT | ≡ 4.1868 J | Defined as 1.163 mW⋅h = 4.1868 J exactly. This definition was adopted by the Fifth International Conference on Properties of Steam (London, July 1956).<ref name="iso31-4" /> |
Template:Notelist The two definitions most common in older literature appear to be the 15 °C calorie and the thermochemical calorie. Until 1948, the latter was defined as 4.1833 international joules; the current standard of 4.184 J was chosen to have the new thermochemical calorie represent the same quantity of energy as before.<ref name="Rossini" />
UsageEdit
NutritionEdit
In the United States and Canada, in a nutritional context, the "large" unit is used almost exclusively.<ref>Template:Cite journal</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> It is generally written "calorie" with lowercase "c" and symbol "cal", even in government publications.<ref name=FDA2019/><ref name=NHS2019/> The SI unit kilojoule (kJ) may be used instead, in legal or scientific contexts.<ref name=ukmetric2012/><ref name=NPL2016/> Most American nutritionists prefer the unit kilocalorie to the unit kilojoules, whereas most physiologists prefer to use kilojoules. In the majority of other countries, nutritionists prefer the kilojoule to the kilocalorie.<ref name=patt2017/>
In the European Union, on nutrition facts labels, energy is expressed in both kilojoules and kilocalories, abbreviated as "kJ" and "kcal" respectively.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
In China, only kilojoules are given.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Food energyEdit
The unit is most commonly used to express food energy, namely the specific energy (energy per mass) of metabolizing different types of food. For example, fat (triglyceride lipids) contains 9 kilocalories per gram (kcal/g), while carbohydrates (sugar and starch) and protein contain approximately 4 kcal/g.<ref name=SciAm/> Alcohol in food contains 7 kcal/g.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The "large" unit is also used to express recommended nutritional intake or consumption, as in "calories per day".
Dieting is the practice of eating food in a regulated way to decrease, maintain, or increase body weight, or to prevent and treat diseases such as diabetes and obesity. As weight loss depends on reducing caloric intake, different kinds of calorie-reduced diets have been shown to be generally effective.<ref name="Strychar">Template:Cite journal</ref>
Chemistry and physicsEdit
In other scientific contexts, the term "calorie" and the symbol "cal" almost always refers to the small unit; the "large" unit being generally called "kilocalorie" with symbol "kcal". It is mostly used to express the amount of energy released in a chemical reaction or phase change, typically per mole of substance, as in kilocalories per mole.<ref name=rappo2007/> It is also occasionally used to specify other energy quantities that relate to reaction energy, such as enthalpy of formation and the size of activation barriers.<ref name=bhag2002/> However, it is increasingly being superseded by the SI unit, the joule (J); and metric multiples thereof, such as the kilojoule (kJ).Template:Citation needed
The lingering use in chemistry is largely because the energy released by a reaction in aqueous solution, expressed in kilocalories per mole of reagent, is numerically close to the concentration of the reagent in moles per liter multiplied by the change in the temperature of the solution in kelvins or degrees Celsius. However, this estimate assumes that the volumetric heat capacity of the solution is 1 kcal/(L⋅K), which is not exact even for pure water.Template:Citation needed
See alsoEdit
Template:Portal Template:Div col
- Basal metabolic rate
- Caloric theory
- Conversion of units of energy
- Empty calorie
- Food energy
- A calorie is a calorie
- Nutrition facts label
- British thermal unit
- Satiety value