Previous PageTable of ContentsNext Page


prepared by FAO

Since early days the calorie or kilocalorie has been used as a unit of energy. In some circles, however, it is realised that this cannot be continued indefinitely and that in due course the joule is to be substituted for the calorie as the unit of energy in all nutritional work. Calories should then fall into disuse.

In the International System of Units (Système International d'Unités) called S.I., there are 6 basic units as adopted in 1954: the meter (M) for length, the kilogramme (Kg.) for mass, the second (s) for time, the ampere (A) for electric current, the kelvin (K) for thermodynamic temperature and the candela (cd) for luminous intensity (1). All the other units are derived from these 6 basic units such as the unit for force as the newton (N) (Kgm/s2), the unit of energy in any form is the joule (J) (Nm) and the unit of power as the watt (W) (J/s). The joule was adopted as the unit for electrical work, heat, mechanical work and energy in 1948 at the 9th General Conference on Weights and Measures, avoiding the calorie as far as possible. This unit was also formally approved in 1960 in the International System of Units (SI). Also the International Standards Organization (ISO) has recommended its adoption as the preferred unit for energy, as well as the USA National Bureau of Standards, the British Standards Institution and the Royal Society.

Much consideration has already been given on the advisability of substituting joules for calories. The calorie cannot be derived directly from the basic SI units without using an experimentally determined factor, whereas the joule corresponds with the energy measurements in all branches of science on which the nutritional sciences depend. It is well defined in terms of the basic unit of mass, length and time. Efforts to retain the calorie as the unit of energy will tend to isolate the nutritional sciences from the advances in the fields of physics and chemistry. The calorie will disappear in future courses of the basic sciences.

The calorie as used in nutritional science has also been defined in different ways which lends to confusion: The "15° calorie" is defined as the amount of heat required to increase the temperature of water from 14.5 to 15.5 C with the specific heat of the water at 15° C and the constant pressure being defined as unity. The electrical energy required to increase the temperature of water this 1° has been experimentally determined but with a degree of uncertainty. Values for the "15° calorie", therefore, range from 4.1852 J/cal (15° C) to 4.1858 J/cal (15° C), with a mean value recorded by ISO as 4.1855 J. The nutritional calorie is therefore not the 15° calorie. The International Steam Table Calorie (I T calorie) was defined in 1929 at the International Steam Table Conference in terms of electric units namely 1/860 x international watt hour or joule. Because of differences in terms of absolute joule the USA and European International joule differ one I T equals 4.1868 J in European publications and 4.1867 J in American journals.

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 has been redefined as 4.1840 J exactly). The thermochemical calorie actually approximates a "17° calorie" in contrast to the "15° calorie". Since calorimetrical measurements are based on electrical calibration Rossini argued that it would seem logical to calibrate in energy units rather than in thermal units. Another type of calorie, the "Bunsen calorie" also is uncertain about the mean value of the heat capacity of water over the temperature range considered.

While the nutritional calorie has not been defined, basically it is the thermochemical calorie. The standards used in calorimetric work in nutrition is ultimately the heat of combustion of an internationally graded standard benzoic acid. This is primarily expressed as joules per gramme mole and secondarily as thermochemical calories per mole derived by dividing by 4.182, a factor which has been approved by the Committee on Nomenclature of the IUNS.

Although not generally realized nutritionists actually have been measuring energy in joules and thus applying various factors to convert to the various calories. Calorimeters are also calibrated by measuring the amount of electrical energy that is necessary to duplicate the thermal effect (heat of combustion of an amount of standard benzoic acid). The input in electrical energy is measured in the units, volts × amperes × seconds (or watts × seconds). One watt second = 1 Newton meter = 1 J. The specific energy of food whether these are heat of combustion or metabolizable fuels or other physiologically defined entities should be expressed for convenience in terms of kilojoules per kilogrammes (kJ/kg) or megajoules per kilogramme (MJ × kg), a transition from calories and kilocalories to Joules and kiloJoules. Among these lines more accurate information and improved concepts are substituted for the less accurate and less adequate (2).

The Joint FAO/WHO Expert Committee on Nutrition (3) considered the implications of substituting joules for calories in 1966. In view of its importance it was decided to refer this matter to the Committee on Nomenclature of the International Union of Nutritional Sciences (IUNS) and to other appropriate organizations. The conversion from calories to joules was considered by the IUNS Committee at its pre-congress meetings, in Belgrade on 20 to 23 August 1969 and again in Prague on 26 August 1969 (4). It was formally recommended that the change from the calorie to joule should be made, however, such change should be gradual. The conversion factor to be used is debatable; for those engaged in research on energy change involving calorimetry an exact figure of 4.1840 J = 1 calorie is required, whereas for most nutritionists and dietitians a less accurate ratio of 4.19 or even 4.2 kilojoules per kilocalorie will probably suffice.

Also a Working Group of the British National Committee for Nutritional Sciences of the British Royal Society (5) agreed that the calorie should fall into disuse and the joule be adopted as the unity for energy in all nutritional work. To convert calories into joules the conversion factor 4.1840 J = 1 calorie is recommended, which also identifies the calorie used by nutritionists with the thermochemical calorie. It should be expressed as MJ/kg, as a matter of convenience. The pronounciation of the unit joul shall be according to the Concise Oxford Dictionary "Jool". The change over will take a few years to be completed. It has already been started in British schools and should be completed in the early 1970's. International agreement, however, on this matter is important. The Committee of Dietary Allowance of the US Food and Nutrition Board of the National Research Council also proposed to express energy allowances as both kilocalorie and kiloJoules in their next revision.

Conversion Procedures

No doubt the conversion of tables of energy content of foods and of energy allowances and requirements from calories to joules (J) will take time. The Royal Society Conference of Editors proposed that initially journals and other publications should give the nonmetric units plus the metric equivalent in parenthesis, e.g. "55 kcall (230 kJ)" and to reverse the procedure somewhat later as "230 kJ (55 kcal)". Eventually the values will be given only in joules. Initially conversion will be made merely by multiplying the present caloric values by the conversion factor. In the meantime the metabolizable energy values of food can be re-investigated and can be improved. The factors used today to convert food proteins, fats and carboydrates into utilizable calories are based on work done mainly decades ago and different values are also being used in different countries.

Conversion Factor

The Committee on Nomenclature of the International Union of Nutritional Sciences concluded that most nutritionists use the thermochemical calorie which equals 4.1840 J exactly. An exact number cannot really be rounded off especially by those engaged in research on energy changes involving calorimetry. For most nutritional and dietetic requirements a less accurate conversion figure between the Calorie and the kiloJoule will probably be adequate, such as the figure of 4.19 or even 4.2 kiloJoules per kilocalorie. It was, therefore, recommended that as a first step the conversion factor 4.19 between the calorie and the Joule should be included in the literature whatever unit is mentioned - the change over should be gradual.

Physiological Energy Values

In calculating the metabolizable energy value of food the future 4 kcal per gramme for carbohydrate and protein and 9 kcal per gramme of fat are used in rough estimates. If such values are converted from kilocalories to kiloJoules rounded off figures of 17 kJ per gramme for carbohydrate and protein and 38 kJ per gramme for fats are suggested.

Also converting calorie requirements and allowances into joules, figures may be rounded off as has already been done in the latest revision of the Recommended Intakes of Nutrients for the United Kingdom (6) in which energy allowances are given in both kilocalories and megajoules, for example men from 18 to 35 years, sedentary, weighing 65 kg., 2700 kcal or 11.3 MJ (megajoules) and women 18 to 55 years for most occupations, weighing 55 kg., 2200 kcal or 9.2 MJ.

No change is envisaged for terms such as calorimeter, calorimetry and calorific, but "energy value" can be substituted for "calorie value".


(1) Ames, S.R. The Joule - Unit of Energy. J. Amer. Diet. Assoc., 57, 415 (1970).

(2) Harper, A.E. Remarks on the Joule. J. Amer. Diet. Assoc., 57, 416 (1970).

(3) FAO/WHO Expert Committee on Nutrition. Seventh Report, Rome 12 to 20 December 1966. page 8. Food and Agriculture Organization of the United Nations, Rome, 1967.

(4) International Union Nutritional Sciences, Committee on Nomenclature. (1969). The Status of the Joule and the Calorie as Units of Energy. Personal Communication.

(5) Working Party, British National Committee on Nutritional Sciences, Royal Society (1969). The Adoption of the Joule as the Unit of Energy and of the SI Units in Nutritional Sciences. Personal Communication.

(6) Recommended Intakes of Nutrients for the United Kingdom. (1969). Report of the Panel on Recommended Allowances of Nutrients. London, H.M.S.O., 1969.

Previous Page Top of Page Next Page