The Journal of Nutrition

History of Nutrition

History of the Calorie in Nutrition

James L. Hargrove*

Department of Foods and Nutrition, University of Georgia, Athens, GA 30602

Abstract

The calorie was not a unit of heat in the original metric system. Some histories state that a defined Calorie (modern kcal)

originated with Favre and Silbermann in 1852 or Mayer in 1848. However, Nicholas Clement introduced Calories in lectures

on heat engines that were given in Paris between 1819 and 1824. The Calorie was already defined in Bescherelle’s 1845

Dictionnaire National. In 1863, the word entered the English language through translation of Ganot’s popular French

physics text, which defined a Calorie as the heat needed to raise the temperature of 1 kg of water from 0 to 1�C. Berthelot

distinguished between g- and kg-calories by 1879, and Raymond used the kcal in a discussion of human energy needs in an

1894 medical physiology text. The capitalized Calorie as used to indicate 1 kcal on U.S. food labels derives from Atwater’s

1887 article on food energy in Century magazine and Farmers’ Bulletin 23 in 1894. Formal recognition began in 1896 when

the g-calorie was defined as a secondary unit of energy in the cm-g-s measurement system. The thermal calorie was not

fully defined until the 20th century, by which time the nutritional Calorie was embedded in U.S. popular culture and

nutritional policy. J. Nutr. 136: 2957–2961, 2006.

Introduction

The thermal units of g-calorie (4.184 J) and kg-calorie (kcal orcapitalized Calorie; 4.184 kJ; Calorie is capitalized when theoriginal text refers to a kg-calorie; the lower case word denotes ag-calorie) are so familiar in nutrition that one tends not to askwhen they were first defined or how they entered common usage.The hypothesis of this paper is that the French word, calorie,had been coined and defined by 1824 and was originally used inthe sense of a 0–1�C kcal (1–4). The Dictionnaire de l’AcademieFrancxaise lists ‘‘CALORIE n. f. XIXe siecle.’’ If so, this con-tradicts the statement that ‘‘Lavoisier served on the committeethat developed the cm-g-s measurement system (CGS) whichdefined the ‘‘calorie’’ used today’’ (5). There is no documentationthat the original metric commission defined a unit of heat. IfLavoisier’s work is too early, other attributions of 1848–53(3,6,7) are clearly too late, because the Calorie had already beendefined in the 1845 edition of Bescherelle’s dictionary of theFrench language (8).

Prior histories of the Calorie in nutrition do not discuss theorigin of the calorie or kcal as units of heat (9,10). This article willshow that use of the Calorie as a defined unit of heat developedconcurrently with the metric system but not as a recognizedmetric unit and dates to work in chemistry or engineering nolater than 1819–1824. A timeline of some key events is shown inFigure 1.

In addition to its technical usage by scientists, the wordCalorie entered the popular vocabulary across Europe and theUnited States by the late 19th century. For example, the Ox-ford English dictionary cites E. Atkinson’s 1863 translation ofAdolphe Ganot’s French physics text (11) as the first occurrenceof the Calorie in English. However, this physics text is not the

source for the Calorie of nutrition. This article will outline theearly usage and spread of the calorie as a heat unit from Franceto other countries during the 19th century.

Origin and usage of the calorie in France

The word calorie as a unit of heat seems to have been coinedsometime between 1787 (1) and 1824 (12). Lavoisier studiedspecific heats of water and other materials and conducted someof the earliest experiments involving direct and indirect calo-rimetry (13). He named the calorimeter (calorimetre) by 1789(14). Although Lavoisier was credited with coining ‘‘oxygen’’and many new chemical terms, he did not include the calorieon his list of new words. Lavoisier’s papers refer to calorique(caloric) and chaleur (heat) but not to the calorie as a thermalunit. At that time, ‘‘caloric’’ was regarded as a substance ratherthan a unit of heat. Lavoisier served on the 1791 Commissionon Weights and Measures of the French Academy of Scienceand helped define the kg (5,15). A strict definition of the caloriewould require metric units and Lavoisier used a quantity calledthe livre (about a pound) rather than the kg (16). He was ex-ecuted by French revolutionaries in 1794 before the metric sys-tem was officially adopted in 1799 (13).

The original metric system of 1795–9 defined the base unitsfor length, area, volume, capacity, weight (not mass), and moneyalong with various prefixes (13). It was intended as a meansof simplifying trade and did not include derived scientific unitsfor energy, electricity, or magnetism. These secondary units werenot defined until after the Metric Convention of 1875, when theBureau International des Poids et Mesures (BIPM) was formed.

Several histories attribute the first usage of the calorie tosources that are too late. For example, Taton (7) indicates thatFavre and Silbermann coined the term in 1852 (17). Others(6,18) also credit Favre for defining the calorie. However, the*To whom correspondence should be addressed. E-mail: jhargrov@fcs.uga.edu.

0022-3166/06 $8.00 ª 2006 American Society for Nutrition. 2957Manuscript received 21 August 2006.

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original publication states that the calorie was a well-knownunit of physics (digital copies of L.N. Bescherelle’s DictionnaireNational, Adolphe Ganot’s Traite Elementaire de Physiques, andthe original text from Favre and Silbermann’s article on thermo-chemistry are available at the Gallica internet site: http://gallica.bnf.fr/ark:/12148/bpt6k34775c.table.):

Nous repetons que l�unite que nous avons adoptee estcelle adoptee par tous les physiciens (physicists), c�est-a-direla quantite de chaleur necessaire pour elever 1 grammed�eau de 1 degre, et que l’on appelle unite de chaleur oucalorie.

It is hard to assign priority to workers who state, ‘‘We repeatthat the unit that we adopted is that adopted by all the physicists.’’This is confirmed in the 1855 version (4th edition) of Ganot’sbasic physics text. This text contains a modern definition (11):

.c’est pourquoi on est convenu de prendre pour unitede chaleur, ou calorie, la quantite de chaleur necessairepour elever de zero a 1 degre la temperature d’unkilogramme d’eau.

Because Ganot defines the calorie (not capitalized) as a 0–1�Ckcal in relation to the heating of water without providing areference, it seems evident that the calorie was well known. AFrench etymological dictionary lists the first occurrence of calorieas the 1842–3 volumes of Bescherelle’s Dictionnaire national(19,20). The listing in the 1845 edition defines the calorie as Phys.Quantite de chaleur necessaire pour elever un kilogr. d’eau un degredu thermometre centigrade. The definition is similar to Ganot’smore precise usage. Furthermore, the Dictionnaire historique de lalangue francxaise states that the word calorie was coined about1819–24 and usage had become widespread by 1845 (12). Thus,French sources do not indicate that Lavoisier defined the calorie.

By 1824, the word calorie was being used as a unit of heat(1,3,4,12). In 1819, Nicholas Clement began giving lectures onthe theory of heat and steam engines at the Conservatoire desArts et Metiers in Paris (21). Clement was regarded as an in-dustrial chemist or chemical engineer. Among the students wasSadi Carnot (3) and class notes are available that were taken byL.B. Francoeur and J.M. Baudot (2). Both Clement and Carnotsubscribed to the caloric hypothesis, which stated that heatbehaved as a material substance and that its total amount wasalways conserved (2). The notes show that Clement defined alarge (grande) and small (petite) calorie by 1823–4. A definitionis written in Baudot’s notes of 23 December 1824:

La petite Calorie est la quantite de chaleur qu’il fautpour elever d’un degre la temperature d’un K.me d’eau.

Assuming that the mass abbreviation refers to a kg, thisdefines a modern kilocalorie. Clement sometimes referred to a‘‘grande calorie’’ as the heat needed to melt a kg of ice, which is;334 kJ/kg. This is different usage from the modern kcal, butthe passage indicates that the calorie was known to engineers bythis time (12).

Medard (4) suggests that Clement may have coined the wordcalorie around 1820 but agrees that this is not certain, becauseClement did not publish the definition. Other than the hand-written course notes, the first published use of the calorie wasprobably in 1825 in an anonymous description of Clement’scourse in a local journal called Producteur (4). Carnot usedClement’s definition of heat units but not the name calorie inReflexions On The Motive Power of Fire (3,22). Other engineersbegan using the calorie by 1829 (4), but the word apparently didnot enter physics texts until much later.

It is true that some scientists used heat units that would nowbe called g-calories in the 1850s. However, it was not until1877–9 that Marcellin Berthelot stated that the ‘‘large’’ calorieequalled 1000 ‘‘small’’ 0–1�C g-calories and distinguished be-tween them by capitalizing the abbreviation for the large Calorie(4,23). Although Medard states that the kcal was not introduceduntil 1935 (4), it was used in the context of daily energy expen-diture in a U.S. medical physiology text from 1894 (24). A14.5�C kcal was defined in German law in 1924 (3). It is notcertain who first named the kcal.

The metric system In France

From 1795–9, a committee of the Academie des Sciences soughtto define the meter as a unit of length related to an arc of theearth’s circumference. The calorie could not be strictly definedwithout knowing the mass of water contained in a liter, whichwas based on a 10-cm cube. It is germane that the law of 1795defined the g as ‘‘the absolute weight of a volume of pure watercontained in a cube one-hundredth of a meter on a side atthe temperature of melting ice’’ (25). This is important becausethe g-calorie eventually became a practical unit of heat, and thespecific heat and mass of a volume of water depend on its tem-perature (26).

Because the kg, g, m, and cm were all defined from the start,it is ambiguous whether the original metric system should becalled m-kg-s or cm-g-s. However, the kg was considered tobe the base unit of weight in the 1795 metric system. This is be-cause the charge to the commission was to define standards forweights and measures used in commerce, and scientific concernswere secondary. Note that the original definition specified a weightrather than a mass, meaning that the original kg was defined askg-force, similarly to a pound. Not until 1904 was the newtondefined as a unit of force so that the kg became a mass unit.

Because the g was not considered to be a base unit in themetric system (27) when the Calorie was defined, there was noneed to add the ‘‘kilo-’’ prefix. The definition of a Calorie as theheat needed to raise the temperature of 1 kg of water by 1�C wasthe original usage and the kcal as 103 g-calories was not intro-duced until sometime between 1877 and 1894 (23,24). Indeed,the thermal g-calorie was not defined as 4.184 joules until 1902and it was a 17�C (not 15�C) unit (28).

The original metric system was short-lived as a French na-tional standard. In 1812, Napoleon Bonaparte issued a decree toestablish a ‘‘usual system’’ that changed metric values to conformto earlier weights and measures. Not until 1840 was a lawpassed to reinstate a metric system based on scientific standards(25). During this same period, European interest in thermody-namics, fuels, and electricity was high. Indeed, one of the reasons

Figure 1 Timeline of 19th century events in the history of the metric system

and calorie. BAAS, British Association for Advancement of Science.

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that Joule began his experiments on heat was a desire to reducethe cost of running equipment in the family brewery (29).

Metric advocates worldwide recognized the simplicity ofbasing units of measure on the decimal system, and the metricsystem was promoted at world expositions in London (1851)and Paris (1867). Also in 1867, the International Geodetic Asso-ciation formed a committee to investigate the use of the metricsystem in scientific measurements. The system was made legalfor commerce in Britain (1864) and the United States (1866) andbecame compulsory in Germany (1868) (15). The InternationalMetric Convention of 1875 gave scientists impetus to define ac-curate standards in electricity, magnetism, and thermodynamics.This led to the introduction of the CGS in 1896 in which the erg,dyne, and joule were defined. In 1918, the newton was added asa unit of force to the m-kg-s system and the joule was defined as1 N-m of work. The ability to define the joule unambiguously inbase units (amp-s) led to its adoption as the SI (Systeme Inter-national) unit of energy (30).

During the 1930s, the BIPM convened the Consultative Com-mittee on Thermometry to clarify standards of heat and W.H.Keesom served as president. In addition to reviewing the historyof the calorie, Keesom summarized a proposal that the calorieshould equal 1/860 watt-hours or 3600/860 joules (4.186 J) (3).From then on, any secondary thermal unit was to be definedrelative to the joule rather than to the heating of water at anytemperature. The 1948 General Conference also recommendeddiscarding the calorie, because it cannot be derived directly frombasic units. In 1954, the SI base units were adopted, and in 1970,the Committee on Nomenclature of the American Institute ofNutrition advised that the kilocalorie should be replaced by thekilojoule (kJ) in scientific publications (30,31).

The Calorie in German Physiology

The Calorie probably entered U.S. English because W.O. Atwaterlearned the term during studies in Germany, and not because itwas defined in a newly translated physics text. Justus Liebig didnot mention the calorie as such in his 1842 book on animalchemistry (32) or his paper on energy production from food-stuffs (33). However, he published J.R. Mayer’s first scientificpaper (34), which defined a mechanical equivalent of heat. Mayerself-published two intriguing papers that dealt partly with theefficiency of energy metabolism, which he estimated to be 15–20% (35,36). It was in the context of relating physical workagainst gravity (Fallkraft or potential energy; Fig. 2) to theenergy supplied by foods that Mayer defined a kg-cal in 1846–8(3,4,36). As translated by Lindsay (37) the quotation is:

When substances endowed with considerable chemicalaffinity for each other combine chemically, much heat isdeveloped during the process. We shall estimate thequantity of heat thus set free by the number of kilo-grammes of water which it would heat 1�C. The quantityof heat necessary to raise 1 kilogramme of water 1 degreeis called a unit of heat, Calorie.

The German text reads in part, ‘‘die Warmemenge. nenntman Warmeeinheit, Calorie.’’ Note that Mayer’s definition,which occurred 48 y before the joule was introduced as a unit ofenergy, is essentially identical to the Calorie that is still used onU.S. food labels.

Mayer’s views became known due to his dispute with Joulefor priority in the discovery of the caloric equivalent of physicalwork (37), for which he was awarded the Copley medal in 1871,1 y after Joule. Presently, it seems that an amateur has claim tothe first definition of the Calorie in German science! Mayer mayhave learned of the Calorie because he studied in Paris andpurchased a library that included French and German physicsand chemistry texts of the period (38).

In 1860, Carl Voit visited Edward Frankland in Englandand brought a Thomson calorimeter back to his laboratory inMunich. The British Thermal Unit employed with the Thomsoncalorimeter was defined as the heat needed to raise the temper-ature of 1 pound of water by 1�F. Despite this, Voit began usingthe g-calorie in lectures on human calorimetry in 1866 andstated that daily metabolism of 1 male subject was 2.25 to 2.4 3

106 g-calories, depending on prior diet (39). At that time,Pettenkofer and Voit (40) were using indirect calorimetry tostudy human and animal respiration and Voit was conductingnutritional balance studies (41). It was this juxtaposition be-tween measuring human energy output and the energy contentof foods by bomb calorimetry that gave the German schoolimpetus to define daily energy intake and output in terms ofcalories.

By 1883–5, Max Rubner had published papers using theg-calorie to define heats of combustion for food and heat pro-duced in respiration studies (42–44). In the same period,Henneberg and Stohmann were developing methods for theproximate analysis of livestock feeds at the Weende ExperimentStation (45). Stohmann initiated calorimetric work in 1880 andconferred with Rubner concerning experiments in which theenergy content of sugars, fats, and egg white was measured ing-calories (46). Note that different German scientists varied inusing either the g-calorie or the Calorie.

The German school of physiology introduced proximateanalysis and calorimetry to determine the least expensive sourcesof nutritionally balanced foods and animal feeds (39). Thisapproach appealed to the American Wilbur O. Atwater, whovisited Voit’s laboratory in 1882–3 and conducted postdoctoralstudies along with Rubner (47). In their 1896 report on nutritivevalues of foods, Atwater and Woods (48) specifically credit theWeende method of proximate analysis including fuel analysis asdeveloped by Henneberg and Stohmann (45).

Atwater returned to the US and gave a presentation oneconomic aspects of the protein and energy content of foods(49). In 1887, his series of articles in Century magazine includeda description of the energy content of foods that defined the0–1�C Calorie (kcal) for American audiences (50). Neglectingmuscular efficiency, he noted that 1 Calorie equaled ;1.53 foot-tons of work. It is not evident why Atwater selected the Calorierather than the g-calorie as used in publications from colleaguesin Voit’s laboratory.

Figure 2 Caloric equivalent of work done against gravity. A mass of 427 kg

falling 1 m against a 1 g gravitational field would increase the temperature of 1 kg

of water by 1�C [redrawn from (57) with permission of the publisher].

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The articles on nutrition were published the same year thatthe U.S. Congress enacted the Hatch Act, and Atwater wasoffered a position as director of the Storrs Agricultural Exper-iment Station. His interest in establishing a rational basis forfood selection led to work on the proximate analysis of foodsand their physiological energy value (48). Graham Lusk alsovisited the Voit laboratory and set the tone for nutrition edu-cation in the United States with his text, The Science of Nutrition,which emphasized energetics (39).

Formal definition of the Calorie in the United Kingdom

James P. Joule did not use the calorie as a unit of heat in hispublished papers until 1878 (29,51). In describing the mechan-ical equivalent of heat, he monitored the heating of water in�F/pound and expressed work in ft-lb (52). He also discoveredthe phenomenon of ‘‘Joule heating’’ in which electricity passingthrough a resistor releases heat and determined the mechanicalequivalent of heat in electrical units. Calorimetric studies bySmith (53) measured energy usage in terms of the carbon dioxideformed in metabolism but did not refer to calories. Frankland(54) conducted calorimetry on foods and employed unnamed‘‘heat units’’ or work units of kg-m (Fig. 2) in the same sense asMayer (55). Frankland’s heat units were identical to g-calories(9) and he stated that his work measured calorific values offoods. He referred to Joule’s mechanical equivalent of heat instating that the heat needed to raise the temperature of 1 kg ofwater by 1�C equals 423 kg-m of work. In 1865, Fick andWislicenus observed that protein metabolism accounted for onlya minor part of the energy required by muscular exertion duringa mountain climb (56). They calculated the energy content ofprotein (albumen) in heat units that equal modern kcal. In theensuing international discussion of the study, the kg-m wasconsistently used as a unit of energy or work rather than thecalorie (Fig. 2). Kleiber’s book explained that energy expendi-ture can be estimated by multiplying men’s body masses in kgtimes the vertical distance of the climb and adjusting for me-chanical efficiency (57). The usage of kg-m or ft-lbs as units ofenergy persisted until the joule was adopted as a standard forall energy.

From 1873 to 1913, a committee of the British Associationfor Advancement of Science met concerning standards forelectrical measurements. Among other tasks, the committeesolicited international opinion concerning heat units. In 1873, itdefined the dyne and erg and recommended that the CGS beadopted (58). In 1888, the joule was named and defined as thework done in 1 s by the power of 1 W (59). The committee alsoadopted the name ‘‘Therm’’ for ‘‘the quantity of heat required toraise one gramme of water at its maximum density by one degreeCentigrade’’ (59). The Therm was discarded in 1896, when thecommittee discussed whether an international consensus couldbe determined concerning units of heat (60). A theoretical unit ofheat was proposed to be 42 3 106 ergs and a practical unit of heatwas proposed to be the heat required to raise the temperature of1 g of water by 1�C at some temperature between that of ice and20�C. The report stated:

There is fairly general agreement in the view that as apractical unit the heat required to raise 1 gramme of water1�C at some fixed temperature must be taken, but viewsdiffer as to the temperature which it is most convenient tochoose. (60)

In this regard, the committee recommended using the joule, or107 ergs, as the theoretical unit of heat. It advised that a 10�C

thermal unit based on the heating of 1 g of water should equal 4.2J. The committee indicated that ‘‘this secondary thermal unit maybe called a Calorie’’ (60). The word was capitalized although itmeant g-calorie, indicating that there was no internationalconvention of capitalizing Calorie to indicate a kcal even thoughAtwater had used Calorie in that sense in 1887 (50) and his earlycompendia (48,61,62). In their calorimetric work, Atwater andRosa (63) defined the Calorie at 20�C and noted that it wasslightly larger than the 18�C unit employed by Armsby (64).

Conclusions

This review raises several questions that should be answerable.First, Marcellin Berthelot defined a large calorie as 1000 g-cal(23) but did not call it a kilocalorie. A review of publicationsin engineering, chemistry, and physics between 1877 and 1894should disclose a source for the name. Second, one does notknow why Voit adopted the g-calorie in 1866 or why Atwaterdecided to use the Calorie (kcal) after visiting the Munichlaboratory. It is likely that both heat units were being used inresearch publications and engineering handbooks. A review ofFrench and German sources would clarify this question. Third,Nicholas Clement may have been first to coin and define thecalorie, but it is possible that the term was already being used byother engineers and chemists. New evidence could falsify thehypothesis that Clement was first. In a larger context, the caloriemade its way from France into international scientific literatureduring the 19th century and important work that is not men-tioned here was being done in many other countries.

The Calorie began to enter popular American vocabularyafter Atwater explained the unit in his 1887 article in Centurymagazine. The most important avenue was probably the USDAFarmers’ Bulletins (61,62), which provided the first U.S. fooddatabases to be used in dietetics. Then, as now, American audi-ences were interested in managing weight, and the Calorie wassoon introduced in articles and books. For example, Dr. LuluHunt Peters’ best-selling ‘‘Diet and Health with Key to theCalories’’ specifically cited Farmers’ Bulletin 142 as a source ofinformation (65). Eventually, the Calorie was adopted for thenutrition facts panels on U.S. food labels. At present, there doesnot seem to be a movement by policy makers in the US to replacethe Calorie with the kJ on nutrition information panels.

Acknowledgments

J.L.H. thanks Dr. Patrick Reidenbaugh (University of GeorgiaLibraries), the staff of the BIPM, the Bibliotheque Nationale deFrance, and the Library of Congress for bibliographic assistance.Thanks also to Pat Naughtin of Geelong, Australia, for corre-spondence about the history of the Calorie.

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