A calorie is a unit of measurement for energy. The unit's name is French and derives from the Latin calor (heat). In most fields, it has been replaced by the joule, the SI unit of energy. However, it remains in common use for the amount of energy obtained from food. Many different definitions for the calorie emerged during the 19th and 20th centuries. They fall into two classes:

• The small calorie or gram calorie approximates the energy needed to increase the temperature of 1 g of water by 1 °C. This is about 4.185 J.
• The large calorie or kilogram calorie approximates the energy needed to increase the temperature of 1 kg of water by 1 °C. This is about 4.185 kJ, and exactly 1000 small calories.

In scientific contexts, the name "calorie" refers strictly to the gram calorie, and this unit has the symbol cal. SI prefixes are used with this name and symbol, so that the kilogram calorie is known as the "kilocalorie" and has the symbol kcal.

Colloquially, and in nutrition and food labelling, the term "calorie" almost always refers to the kilogram calorie. This applies only to English text; if an energy measurement is given using a unit symbol then the scientific practice prevails there. A convention of capitalising "Calorie" to refer to the kilogram calorie, with uncapitalised "calorie" referring to the gram calorie, is sometimes proposed, but neither recognized in any official standards, nor commonly followed.

The conversion factor between calories and joules is numerically equivalent to the specific heat capacity of liquid water (in SI units).

Nutritional and food labels

The "calorie" has become a common household term as dietitians recommend in cases of obesity to reduce body weight by increasing exercise (energy expenditure) and reducing energy intake. Many governments require food manufacturers to label the energy content of their products, to help consumers control their energy intake. In Europe, manufacturers of prepackaged food must label the nutritional energy of their products in both kilocalories ("kcal") and kilojoules ("kJ"). In the United States, the equivalent mandatory labels display only "Calories" (meaning kilocalories); an additional kilojoules figure is optional. The energy content of food is usually given on labels for 100 g and for a typical serving size.

The amount of food energy in a particular food could be measured by completely burning the dried food in a bomb calorimeter, a method known as direct calorimetry [1]. However, the values given on food labels are not determined this way, because it overestimates the amount of energy that the human digestive system can extract, by also burning dietary fiber. Instead, standardized chemical tests and an analysis of the recipe are used to estimate the product's digestable constitutents (protein, carbohydrate, fat, etc.). These results are then converted into an equivalent energy value based on a standardized table of energy densities:

Other substances found in food (water, non-digestable fibre, minerals, vitamins) do not contribute to this calculated energy density.

As a rough guideline, recommended daily energy intake values for young adults are: 2500 kcal/d (10 MJ/d, 120 W) for men and 2000 kcal/d (8 MJ/d, 100 W) for women. Children, sedentary and older people require less energy, physically active people more.

Versions

The energy needed to increase the temperature of 1 g of water by 1 °C varies depending on the starting temperature, and is in any case difficult to measure precisely. Accordingly there have been several definitions of the calorie:

• 15 °C calorie: the amount of energy required to warm 1 g of air-free water from 14.5 °C to 15.5 °C at a constant pressure of 101.325 kPa (1 atm). Experimental values of this calorie ranged from 4.1852 J to 4.1858 J. The CIPM in 1950 published a mean experimental value of 4.1855 J, noting an uncertainty of 0.0005 J.
• 20 °C calorie: the amount of energy required to warm 1 g of air-free water from 19.5 °C to 20.5 °C at a constant pressure of 101.325 kPa (1 atm). This is about 4.182 J.
• 4 °C calorie: the amount of energy required to warm 1 g of air-free water from 3.5 °C to 4.5 °C at a constant pressure of 101.325 kPa (1 atm).
• Mean calorie: 1/100 of the amount of energy required to warm 1 g of air-free water from 0 °C to 100 °C at a constant pressure of 101.325 kPa (1 atm). This is about 4.190 J
• International Steam Table Calorie (1929): (1/860) W h = (180/43) J exactly. This is approximately 4.1860 J.
• International Steam Table Calorie (1956): 1.163 mW h = 4.1868 J exactly. This definition was adopted by the Fifth International Conference on Properties of Steam (London, July 1956).
• Thermochemical calorie: 4.184 J exactly.
• IUNS calorie: 4.182 J exactly. This is a definition implied by the Committee on Nomenclature of the International Union of Nutritional Sciences (date and reference needed).

The two perhaps most popular definitions used in older literature are the "15 °C calorie" and the "thermochemical calorie". Since the many different definitions are a source of confusion and error, all calories are now deprecated in favour of the SI unit for heat and energy: the joule (J).

In nutrition, the difference between these calorie definitions is of no practical relevance. This is, because nutritional calories are not measured amounts of energy, but are calculated from food composition. Such calculations use internationally agreed conventional conversion factors, which are generously rounded values that roughly approximate the average energy density of a large number of different food samples. The exact composition of agricultural products varies far more than the less than 0.1% difference between the above definitions of the calorie as a physical energy measure.

Trivia

• Unicode has a symbol for "cal": (㎈), but this is just a legacy compatibility code to accommodate old code pages in certain Asian languages, and it is not recommended for use in any language today.

• The conventional value chosen to define a ton of TNT is equal to 1 billion thermochemical calories: 1 t_TNT ≡ 1 × 10⁹ cal_th. The actual energy liberated from the explosion is somewhat more; see megaton.

• Human fat tissue contains about 87% lipids, so that 1 kg of body-fat tissue has roughly the caloric energy of 870 g of pure fat, or 7800 kcal. Therefore one has to create a −7800 kcal deficit between energy intake and use to lose 1 kg of body-fat. (In U.S. customary units, that is about 3500 kcal per pound.) ^{citation needed]}

See also

• Food energy
• Empty calorie
• ISO 31-4

External links

Primary sources

• USDA National Nutrient Database for Standard Reference - Release 18 Official, publicly available reference database and online search site. Includes 7,146 foods and is free to download and use. This database is the one used by most websites that provide calorie information, and forms the basis of the Canadian national nutrient database and others

Web-based calorie sites

• Calorie Counter Database
• CalorieKing Nutrition Database - Calories for generic, fast food, and brand name packaged foods, based on the popular book. Includes Australian foods.
• CalorieLab - Calories and basic macronutrients displayed in a multifood table format.
• CaloriesPerHour.com - Calories for generic, chains (about 100), and brand name packaged foods. Chain and brand data is updated on a regular schedule.
• Calorie counter and calories burned database Searchable database of over 7,000 foods and calories burned for over 600 different activities.
• Diet Facts - Calories for about 300 chains and several hundred brands, with ingredients.
• MAVIs Calory Calculators Calculate the calories you need or the kCal you burn from certain activities.
• NutritionData's Nutrition Facts Calorie Counter - The USDA database supplemented with data for a few dozen chains, showing all USDA nutrients with interpretation.
• TheDailyPlate.com - Calorie details for over 90,000 foods, including home recipes.
• Conversion of cal in other units

References

• European Union regulations on nutrition labeling
• United Kingdom Food Labelling Regulations 1996 – Schedule 7: Nutrition labelling
• United States federal food-labeling regulations 21CFR101.9
• NIST Special Publication 811, Appendix B8: calorie.
• Donatelle, Rebecca J. Health: The Basics. 6th ed. San Francisco: Pearson Education, Inc. 2005.
• The adoption of joules as units of energy, FAO/WHO Ad Hoc Committee of Experts on Energy and Protein, 1971.
• Methods used in measuring rate of burn in humans [2]