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The calorie is a
unit of energy Energy is defined via work, so the SI unit of energy is the same as the unit of work – the joule (J), named in honour of James Prescott Joule and his experiments on the mechanical equivalent of heat. In slightly more fundamental terms, is ...
. For historical reasons, two main definitions of "calorie" are in wide use. The large calorie, food calorie, or kilogram calorie was originally defined as the amount of heat needed to raise the
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
of one
kilogram The kilogram (also kilogramme) is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquially ...
of
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
by one degree
Celsius The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The ...
(or one
kelvin The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phys ...
). The small calorie or gram calorie was defined as the amount of heat needed to cause the same increase in one
gram The gram (originally gramme; SI unit symbol g) is a Physical unit, unit of mass in the International System of Units (SI) equal to one one thousandth of a kilogram. Originally defined as of 1795 as "the absolute weight of a volume of pure wate ...
of water. Thus, 1 large calorie is equal to 1000 small calories. In
nutrition Nutrition is the biochemical and physiological process by which an organism uses food to support its life. It provides organisms with nutrients, which can be metabolized to create energy and chemical structures. Failure to obtain sufficient n ...
and
food science Food science is the basic science and applied science of food; its scope starts at overlap with agricultural science and nutritional science and leads through the scientific aspects of food safety and food processing, informing the developm ...
, the term ''calorie'' and the symbol ''cal'' almost always refers to the large unit. 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,
metabolic rate Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
s, etc. Some authors recommend the spelling ''Calorie'' and the symbol ''Cal'' (both with a capital C) to avoid confusion; however, this convention is often ignored. In
physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...
and
chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...
the word ''calorie'' and its symbol usually refer to the small unit; the large one being called ''kilocalorie''. However, this unit is not officially part of the metric system (SI), and is regarded as obsolete, having been replaced in many uses by the SI unit of energy, the
joule The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applied ...
(J). The precise equivalence between calories and joules has varied over the years, but in
thermochemistry Thermochemistry is the study of the heat energy which is associated with chemical reactions and/or phase changes such as melting and boiling. A reaction may release or absorb energy, and a phase change may do the same. Thermochemistry focuses on ...
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.


History

The term "calorie" was first introduced by
Nicolas Clément Nicolas Clément (12 January 1779 – 21 November 1841) was a French physicist and chemist. He was a colleague of Charles Desormes, with whom he conducted the Clément-Desormes experiment. The two chemists are also credited with determining a ...
, as a unit of
heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is al ...
energy, in lectures on experimental
calorimetry In chemistry and thermodynamics, calorimetry () is the science or act of measuring changes in ''state variables'' of a body for the purpose of deriving the heat transfer associated with changes of its state due, for example, to chemical reacti ...
during the years 1819–1824. This was the "large" calorie. The term (written with lowercase "c") entered French and English dictionaries between 1841 and 1867. It comes . The same term was used for the "small" unit by Pierre Antoine Favre (Chemist) and Johann T. Silbermann (Physicist) in 1852. This unit was used by U.S. physician
Joseph Howard Raymond Joseph is a common male given name, derived from the Hebrew Yosef (יוֹסֵף). "Joseph" is used, along with "Josef", mostly in English, French and partially German languages. This spelling is also found as a variant in the languages of the mo ...
, in his classic 1894 textbook ''A Manual of Human Physiology''. He proposed calling the "large" unit "kilocalorie", but the term didn't catch on until some years later. In 1879,
Marcellin Berthelot Pierre Eugène Marcellin Berthelot (; 25 October 1827 – 18 March 1907) was a French chemist and Republican politician noted for the ThomsenBerthelot principle of thermochemistry. He synthesized many organic compounds from inorganic substance ...
distinguished between gram-calorie and kilogram-calorie, and proposed using "Calorie", with capital "C", for the large unit. This usage was adopted by
Wilbur Olin Atwater Wilbur Olin Atwater (May 3, 1844 – September 22, 1907) was an American chemist known for his studies of human nutrition and metabolism, and is considered the father of modern nutrition research and education. He is credited with developing t ...
, a professor at
Wesleyan University Wesleyan University ( ) is a Private university, private liberal arts college, liberal arts university in Middletown, Connecticut. Founded in 1831 as a Men's colleges in the United States, men's college under the auspices of the Methodist Epis ...
, in 1887, in an influential article on the energy content of food. The small calorie (cal) was recognized as a unit of the CGS system in 1896, alongside the already-existing CGS unit of energy, the
erg The erg is a unit of energy equal to 10−7joules (100 nJ). It originated in the Centimetre–gram–second system of units (CGS). It has the symbol ''erg''. The erg is not an SI unit. Its name is derived from (), a Greek word meaning 'work' o ...
(first suggested by Clausius in 1864, under the name ''ergon'', and officially adopted in 1882). Already in 1928 there were 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. The joule was the officially adopted SI unit of energy at the ninth
General Conference on Weights and Measures The General Conference on Weights and Measures (GCWM; french: Conférence générale des poids et mesures, CGPM) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established i ...
in 1948. The calorie was mentioned in the 7th edition of the SI brochure as an example of a non-SI unit. According to Wiktionary the alternate spelling is dated.


Definitions

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). The amount depends on the atmospheric pressure and the starting temperature, and different choices of these parameters have resulted in several different precise definitions of the unit. 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.


Usage


Nutrition

In a nutritional context, the "large" unit is used almost exclusively. It is generally written "calorie" with lowercase "c" and symbol "cal", even in government publications, although the capitalized form "Calorie" (with symbol "Cal") or the name "kilocalorie" (with symbol "kcal") are also used, and may be mandated by law. The SI unit of energy
kilojoule The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applied. ...
(kJ) may be used instead, in legal or scientific contexts. In the United States, most 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. On nutrition facts labels in the European Union, energy is expressed in both kilojoules and kilocalories. In the United States and Canada, labels use "Calories", referring to the large unit. In China, only kilojoules are given.


Food energy

The unit is most commonly used to express
food energy Food energy is chemical energy that animals (including humans) derive from their food to sustain their metabolism, including their muscle, muscular activity. Most animals derive most of their energy from aerobic respiration, namely combining the ...
, namely the
specific energy Specific energy or massic energy is energy per unit mass. It is also sometimes called gravimetric energy density, which is not to be confused with energy density, which is defined as energy per unit volume. It is used to quantify, for example, sto ...
(energy per mass) of metabolizing different types of food. For example,
fat In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food. The term often refers spec ...
(lipids) contains 9 kilocalories per gram (kcal/g), while
carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or ma ...
s (sugar and starch) and
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
contain approximately 4 kcal/g. Alcohol in food contains 7 kcal/g. The "large" unit is also used to express recommended nutritional intake or consumption, as in "calories per day".
Dieting 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 calorie intake, different kinds of calorie-redu ...
is the practice of
eating Eating (also known as consuming) is the ingestion of food, typically to provide a heterotrophic organism with energy and to allow for growth. Animals and other heterotrophs must eat in order to survive — carnivores eat other animals, herbi ...
food in a regulated way to decrease, maintain, or increase
body weight Human body weight is a person's mass or weight. Strictly speaking, body weight is the measurement of weight without items located on the person. Practically though, body weight may be measured with clothes on, but without shoes or heavy accessor ...
, or to prevent and treat diseases such as
diabetes Diabetes, also known as diabetes mellitus, is a group of metabolic disorders characterized by a high blood sugar level ( hyperglycemia) over a prolonged period of time. Symptoms often include frequent urination, increased thirst and increased ap ...
and
obesity Obesity is a medical condition, sometimes considered a disease, in which excess body fat has accumulated to such an extent that it may negatively affect health. People are classified as obese when their body mass index (BMI)—a person's we ...
. As weight loss depends on reducing caloric intake, different kinds of
calorie-reduced Diet food (or dietetic food) refers to any food or beverage whose recipe is altered to reduce fat, carbohydrates, and/or sugar in order to make it part of a weight loss program or diet. Such foods are usually intended to assist in weight loss or ...
diets have been shown to be generally effective.


Chemistry and physics

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 A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
or
phase change In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic State of ...
, typically per
mole Mole (or Molé) may refer to: Animals * Mole (animal) or "true mole", mammals in the family Talpidae, found in Eurasia and North America * Golden moles, southern African mammals in the family Chrysochloridae, similar to but unrelated to Talpida ...
of substance, as in kilocalories per mole. It is also occasionally used to specify other energy quantities that relate to reaction energy, such as
enthalpy of formation Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant ...
and the size of
activation barrier In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules p ...
s. However, it is increasingly being superseded by the SI unit, the joule (J); and metric multiples thereof, such as the kilojoule (kJ). The lingering use in chemistry is largely due to the fact that the energy released by a reaction in
aqueous solution An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be re ...
, expressed in kilocalories per mole of reagent, is numerically close to the concentration of the reagent, in moles per
liter The litre (international spelling) or liter (American English spelling) (SI symbols L and l, other symbol used: ℓ) is a metric unit of volume. It is equal to 1 cubic decimetre (dm3), 1000 cubic centimetres (cm3) or 0.001 cubic metre (m3). ...
, multiplied by the change in the temperature of the solution, in kelvin or degrees Celsius. However, this estimate assumes that the
volumetric heat capacity The volumetric heat capacity of a material is the heat capacity of a sample of the substance divided by the volume of the sample. It is the amount of energy that must be added, in the form of heat, to one unit of volume of the material in order ...
of the solution is 1 kcal/ L/ K, which is not exact even for pure water.


See also

*
Basal metabolic rate Basal metabolic rate (BMR) is the rate of food energy, energy expenditure per unit time by endotherm, endothermic animals at rest. It is reported in energy units per unit time ranging from watt (joule/second) to ml O2/min or joule per hour per kg b ...
*
Caloric theory The caloric theory is an obsolete scientific theory that heat consists of a self-repellent fluid called caloric that flows from hotter bodies to colder bodies. Caloric was also thought of as a weightless gas that could pass in and out of pores i ...
*
Conversion of units of energy Conversion of units is the conversion between different units of measurement for the same quantity, typically through multiplicative conversion factors which change the measured quantity value without changing its effects. Overview The process ...
*
Empty calorie In human nutrition, empty calories are those calories found in beverages (including alcoholic) and foods composed primarily or solely of sugars and/or certain fats and oils such as cholesterol, Saturated fat, saturated or trans fats, that provide l ...
*
Food energy Food energy is chemical energy that animals (including humans) derive from their food to sustain their metabolism, including their muscle, muscular activity. Most animals derive most of their energy from aerobic respiration, namely combining the ...
*
A calorie is a calorie "A calorie is a calorie" is a tautology used to convey the thermodynamic concept that a "calorie" is a sufficient way to describe the energy content of food. History In 1878, German nutritionist Max Rubner crafted what he called the "isodynamic ...
*
Nutrition facts label The nutrition facts label (also known as the nutrition information panel, and other slight variations) is a label required on most packaged food in many countries, showing what nutrients and other ingredients (to limit and get enough of) are in t ...
*
British Thermal Unit The British thermal unit (BTU or Btu) is a unit of heat; it is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. It is also part of the United States customary units. The modern SI ...


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International Organization for Standardization The International Organization for Standardization (ISO ) is an international standard development organization composed of representatives from the national standards organizations of member countries. Membership requirements are given in Ar ...
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