Calorie

"kcal" redirects here; this article is about the unit of energy. For other uses, see KCAL (disambiguation) and Calorie (disambiguation).

A 710-millilitre (24 US fl oz) energy drink with 330 kcal, more than a fast-food cheeseburger, and the equivalent of 18 single-serving packets of sugar

The calorie is a unit of energy. The Calorie is 1,000 calories. (The capitalization of this derived second unit's name is a standard, although mention, instead, that "kilocalories" are intended is common, in some consumer-directed contexts.)

The Calorie (large calorie or kilocalorie - symbols: Cal, kcal), also known as the food calorie, is defined as the heat energy involved in warming up one kilogram of water by just one degree Celsius. Note that where the context is clearly about food, nutrition and exercise the term often appears without the capital C.

The small calorie (symbol: cal) was later defined as the heat energy to raise the temperature of one gram of water - rather than a kilogram - by the same amount. (See below for details of the definitions.)

Although both units relate to the metric system, they have been considered obsolete in science since the adoption of the SI system. (The SI unit of energy is the joule.) The small calorie is still often used for measurements in chemistry, although the amounts involved are typically recorded in kilocalories.

History

The (large) calorie was first defined by Nicolas Clément in 1824 as a unit of heat energy. It entered French and English dictionaries between 1841 and 1867. The word comes from Latin calor 'heat'. The small calorie was introduced by Pierre Antoine Favre (Chemist) and Johann T. Silbermann (Physicist) in 1852. In 1879, Marcellin Berthelot introduced the convention of capitalizing the large Calorie to distinguish the senses. The use of the (large) calorie for nutrition was introduced to the American public by Wilbur Olin Atwater, a professor at Wesleyan University, in 1887.

The alternate spelling calory is archaic.

Definitions

The energy needed to increase the temperature of a given mass of water by 1 °C depends on the atmospheric pressure and the starting temperature. Accordingly, several different precise definitions of the calorie have been used.

The pressure is usually taken to be the standard atmospheric pressure (101.325 kPa). The temperature increase can be expressed as one kelvin, which means the same as an increment of one degree Celsius.

Name	Symbol	Conversions	Notes
Thermochemical calorie	calth	≡ 4.184 J ≈ 0.003964 BTU ≈ 1.162×10−6 kWh ≈ 2.611×1019 eV	the amount of energy equal to exactly 4.184 joules
4 °C calorie	cal4	≈ 4.204 J ≈ 0.003985 BTU ≈ 1.168×10−6 kWh ≈ 2.624×1019 eV	the amount of energy required to warm one gram of air-free water from 3.5 to 4.5 °C at standard atmospheric pressure.
15 °C calorie	cal15	≈ 4.1855 J ≈ 0.0039671 BTU ≈ 1.1626×10−6 kWh ≈ 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. Experimental values of this calorie ranged from 4.1852 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	cal20	≈ 4.182 J ≈ 0.003964 BTU ≈ 1.162×10−6 kWh ≈ 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.
Mean calorie	calmean	≈ 4.190 J ≈ 0.003971 BTU ≈ 1.164×10−6 kWh ≈ 2.615×1019 eV	1⁄100 of the amount of energy required to warm one gram of air-free water from 0 to 100 °C at standard atmospheric pressure.
International Steam table calorie (1929)		≈ 4.1868 J ≈ 0.0039683 BTU ≈ 1.1630×10−6 kWh ≈ 2.6132×1019 eV	1⁄860 international watt hours = 180⁄43 international joules exactly.
International Steam Table calorie (1956)	calIT	≡ 4.1868 J ≈ 0.0039683 BTU = 1.1630×10−6 kWh ≈ 2.6132×1019 eV	1.163 mW·h = 4.1868 J exactly. This definition was adopted by the Fifth International Conference on Properties of Steam (London, July 1956).

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.

The calorie was first defined specifically to measure energy in the form of heat, especially in experimental calorimetry.

Nutrition

Main article: Food energy

In a nutritional context, the kilojoule (kJ) is the SI unit of food energy, although the kilocalorie is still in common use. The word calorie is popularly used with the number of kilocalories of nutritional energy measured. As if to avoid confusion, it is sometimes written Calorie (with a capital "C") in an attempt to make the distinction, although this is not widely understood. Capitalization contravenes the rule that the initial letter of a unit name or its derivative shall be lower case in English.

To facilitate comparison, specific energy or energy density figures are often quoted as "calories per serving" or "kilocalories per 100 g". A nutritional requirement or consumption is often expressed in calories per day. One gram of fat in food contains nine calories, while a gram of either a carbohydrate or a protein contains approximately four calories. Alcohol in a food contains seven calories per gram.

Chemistry

In other scientific contexts, the term calorie almost always refers to the small calorie. Even though it is not an SI unit, it is still used in chemistry. For example, the energy released in a chemical reaction per mole of reagent is occasionally expressed in kilocalories per mole. Typically, this use was largely due to the ease with which it could be calculated in laboratory reactions, especially in aqueous solution: a volume of reagent dissolved in water forming a solution, with concentration expressed in moles per liter (1 liter weighing 1 kg), will induce a temperature change in degrees Celsius in the total volume of water solvent, and these quantities (volume, molar concentration and temperature change) can then be used to calculate energy per mole. It is also occasionally used to specify energy quantities that relate to reaction energy, such as enthalpy of formation and the size of activation barriers. However, its use is being superseded by the SI unit, the joule, and multiples thereof such as the kilojoule.

Measurement of energy content of food

In the past a bomb calorimeter was utilised to determine the energy content of food by burning a sample and measuring a temperature change in the surrounding water. Today this method is not commonly used in the USA and has been succeeded by calculating the energy content indirectly from adding up the energy provided by energy-containing nutrients of food (such as protein, carbohydrates and fats). The fibre content is also subtracted to account for the fact fibre is not digested by the body.

See also

• Basal metabolic rate
• Caloric theory
• Conversion of units of energy
• Empty calorie
• Food energy
• Frigorie
• Nutrition facts label