Thecalorie is aunit of energy that originated from thecaloric theory of heat.[1][2] Thelarge calorie,food calorie,dietary calorie,kilocalorie, orkilogram calorie is defined as the amount of heat needed to raise thetemperature of oneliter ofwater by one degreeCelsius (or onekelvin).[1][3] Thesmall calorie orgram calorie is defined as the amount of heat needed to cause the same increase in onemilliliter of water.[3][4][5][1] Thus, 1 large calorie is equal to 1,000 small calories.
Inphysics andchemistry, the wordcalorie and its symbol usually refer to the small unit, the large one being calledkilocalorie (kcal). However, the kcal is not officially part of theInternational System of Units (SI), and is regarded as obsolete,[2] having been replaced in many uses by theSI derived unit ofenergy, thejoule (J),[9] or the kilojoule (kJ) for 1000 joules.
The precise equivalence between calories and joules has varied over the years, but inthermochemistry 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.[10][11]
The term "calorie" comes from Latin calor'heat'.[12] It was first introduced byNicolas Clément, as a unit ofheat energy, in lectures on experimentalcalorimetry during the years 1819–1824. This was the "large" calorie.[2][13][14] The term (written with lowercase "c") entered French and English dictionaries between 1841 and 1867.
The same term was used for the "small" unit byPierre 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.[2] This usage was adopted byWilbur Olin Atwater, a professor atWesleyan University, in 1887, in an influential article on the energy content of food.[2][13]
The smaller unit was used by U.S. physicianJoseph Howard Raymond, in his classic 1894 textbookA Manual of Human Physiology.[15] 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 theCGS system in 1896,[2][14] alongside the already-existing CGS unit of energy, theerg (first suggested by Clausius in 1864, under the nameergon, 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.[16]
The joule was the officially adopted SI unit of energy at the ninthGeneral Conference on Weights and Measures in 1948.[17][9] The calorie was mentioned in the 7th edition of the SI brochure as an example of a non-SI unit.[10]
The alternate spellingcalory is a less-common, non-standard variant.[12]
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).[4][5] 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.
The amount of energy required to warm one gram of air-free water from 3.5 to 4.5 °C at standard atmospheric pressure.[b]
15 °C calorie
cal15
≈ 4.1855 J
≈ 0.0039671 BTU≈ 1.1626×10−6 kW⋅h≈ 2.6124×1019 eV
The amount of energy required to warm one gram of air-free water from 14.5 to 15.5 °C at standard atmospheric pressure.[b] Experimental values of this calorie ranged from 4.1852 to 4.1858 J. TheCIPM in 1950 published a mean experimental value of 4.1855 J, noting an uncertainty of 0.0005 J.[18]
20 °C calorie
cal20
≈ 4.182 J
≈ 0.003964 BTU≈ 1.162×10−6 kW⋅h≈ 2.610×1019 eV
The amount of energy required to warm one gram of air-free water from 19.5 to 20.5 °C at standard atmospheric pressure.[b]
Mean calorie
calmean
≈ 4.190 J
≈ 0.003971 BTU≈ 1.164×10−6 kW⋅h≈ 2.615×1019 eV
Defined as1⁄100 of the amount of energy required to warm one gram of air-free water from 0 to 100 °C at standard atmospheric pressure.[b]
Defined as1⁄860 "international" watt hours =180⁄43 "international" joules exactly.[c]
International Steam Table calorie (1956)
calIT
≡ 4.1868 J
≈ 0.0039683 BTU= 1.1630×10−6 kW⋅h≈ 2.6132×1019 eV
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).[18]
^The 'Thermochemical calorie' was defined by Rossini simply as 4.1833 international joules in order to avoid the difficulties associated with uncertainties about the heat capacity of water. It was later redefined as 4.1840 J exactly.[22]
^The figure depends on the conversion factor between "international joules" and "absolute" (modern, SI) joules. Using the mean international ohm and volt (1.00049 Ω,1.00034 V),[21] the "international joule" is about1.00019 J, using the US international ohm and volt (1.000495 Ω,1.000330 V) it is about1.000165 J, giving4.18684 and4.18674 J, respectively.
The two definitions most common in older literature appear to be the15 °C calorie and thethermochemical 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.[19]
In theUnited States, in a nutritional context, the "large" unit is used almost exclusively.[23] It is generally written "calorie" with lowercase "c" and symbol "cal", even in government publications.[6][7] The SI unitkilojoule (kJ) may be used instead, in legal or scientific contexts.[24][25] 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.[26]
The unit is most commonly used to expressfood energy, namely thespecific energy (energy per mass) of metabolizing different types of food. For example,fat (triglyceride lipids) contains 9 kilocalories per gram (kcal/g), whilecarbohydrates (sugar and starch) andprotein contain approximately 4 kcal/g.[29] Alcohol in food contains 7 kcal/g.[30] The "large" unit is also used to express recommended nutritional intake or consumption, as in "calories per day".
Dieting is the practice ofeating food in a regulated way to decrease, maintain, or increasebody weight, or to prevent and treat diseases such asdiabetes andobesity. As weight loss depends on reducing caloric intake,different kinds ofcalorie-reduced diets have been shown to be generally effective.[31]
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 achemical reaction orphase change, typically permole of substance, as inkilocalories per mole.[32] It is also occasionally used to specify other energy quantities that relate to reaction energy, such asenthalpy of formation and the size ofactivation barriers.[33] However, it is increasingly being superseded by the SI unit, the joule (J); and metric multiples thereof, such as the kilojoule (kJ).[citation needed]
The lingering use in chemistry is largely because the energy released by a reaction inaqueous solution, expressed in kilocalories per mole of reagent, is numerically close to the concentration of the reagent in moles perliter multiplied by the change in the temperature of the solution in kelvins or degrees Celsius. However, this estimate assumes that thevolumetric heat capacity of the solution is 1 kcal/(L⋅K), which is not exact even for pure water.[citation needed]
