Calorie (symbol cal) is a unit of energy originally defined in terms of heat: the energy required to raise the temperature of a specified mass of water by a specified amount. In everyday usage, especially on food nutrition labels, “calorie” usually refers to a unit 1,000 times larger—the kilocalorie (kcal, often written as Calorie with a capital C in nutrition). In the International System of Units (SI), the standard unit of energy is the joule; the calorie is not an SI unit.
Definitions and Units
Two definitions of the calorie have historically coexisted for many years. The small calorie (also called the gram calorie, symbol cal) is the amount of heat needed to raise the temperature of 1 gram of water by 1 °C; the large calorie (also called the kilogram calorie, symbol kcal or Cal) is the amount of heat needed to raise the temperature of 1 kilogram of water by 1 °C. The two differ by a factor of 1,000.
Because the specific heat capacity of water varies slightly with temperature, a precise definition of the small calorie must also specify the temperature interval. A common convention is to specify heating water from 14.5 °C to 15.5 °C. Several slightly different variants therefore emerged historically, such as the International Steam Table calorie (cal_IT, approximately 4.1868 J), the thermochemical calorie (cal_th, 4.184 J), and the 15 °C calorie (cal_15, approximately 4.1858 J). For practical conversions, 1 kcal ≈ 4.184 kJ is generally used.
History
The word “calorie” comes from the Latin calor (“heat”). According to research on the history of nutrition, the French chemist and engineer Nicolas Clément introduced the concept in lectures in Paris between 1819 and 1824, and provided its earliest technical definition around 1824: the amount of heat needed to raise the temperature of 1 kilogram of water by 1 °C. His lectures were closely connected with Sadi Carnot’s research on heat engines. Both men accepted the caloric theory prevalent at the time, which held that heat was a substance that could be conserved. Some popular accounts attribute the term to Antoine Lavoisier, but historical scholarship generally credits Clément.
In the mid-19th century, James Prescott Joule and others established the concept of the mechanical equivalent of heat. Heat consequently came to be understood as a form of energy, and the calorie gradually became associated with units of work and energy. In 1948, the General Conference on Weights and Measures recommended abandoning the calorie because it could not be derived directly from base units. Following the establishment of the SI base units in 1954, the joule became the standard unit for measuring thermal energy. In 1970, the nomenclature committee of the American Institute of Nutrition recommended replacing kilocalories with kilojoules (kJ) in scientific publications.
In the field of food science, the American nutritionist Wilbur Olin Atwater used the calorie as a unit of food energy in his research in the late 19th century and developed a system for converting macronutrient quantities into energy values, ensuring the calorie’s continued use in nutrition.
Food Energy and Atwater Factors
In nutrition, calories express the chemical energy contained in food—that is, the energy released when food is metabolized in the human body.
The Atwater system is known for its “4-9-4” (or 4-4-9) rule: approximately 4 kcal per gram of protein, 4 kcal per gram of carbohydrate, and 9 kcal per gram of fat. This system forms the basis for calculating energy values on food labels. These figures are averages, not exact physical constants. Atwater himself also proposed “specific factors” for different foods. For example, the value for egg protein is approximately 4.36 kcal/g, whereas that for potato protein is approximately 2.78 kcal/g—a substantial difference. Thus, even when the same macronutrient data are used, different calculation methods may yield slightly different energy values.
The energy content of food is generally determined by measuring its heat of combustion. In experiments, food is completely burned in a calorimeter, and the heat released is measured as the basis for its energy value, with adjustments for digestibility and other factors.
Energy Balance in the Human Body
The human body obtains energy from food and beverages to maintain body temperature, support basal metabolism, and fuel physical activity. In nutrition, weight changes are commonly described in terms of the balance between energy intake and expenditure: when intake consistently exceeds expenditure, body weight increases; when expenditure exceeds intake, it decreases. The specific processes involving Energy and Metabolism fall within the field of physiology, with Cellular Respiration serving as the principal pathway for releasing chemical energy.
Limitations and Controversies
The main controversies surrounding the calorie fall into three areas.
The first is confusion over the unit itself. The same word, “calorie,” can refer to either cal or kcal, even though the two differ by a factor of a thousand. This is often called “calorie confusion.” Some studies argue that the scientific community should adopt joules universally and stop using kilocalories. However, US food labels still express energy in Calories, and public usage is firmly entrenched.
The second is the approximate nature of the conversion factors. The 4-4-9 rule provides only average values, and the actual available energy from the same type of nutrient can vary considerably between foods. Conversion errors are particularly pronounced for fiber, alcohol, sugar alcohols, and certain processed foods, so label values should be understood as estimates.
The third concerns limitations in measurement and application. Calorimetry measures the heat released when food is completely burned, whereas the human body does not fully digest and absorb food. Individual differences in metabolism, activity levels, and gut microorganisms also affect the amount of energy actually obtained. Calorie values are therefore better used as a guide than as a precise measure of an individual’s energy needs.
Related articles
The definition of the calorie depends on the properties of Heat, Temperature, and Water, and its conceptual foundation lies in Thermodynamics. In nutritional applications, it is closely related to Nutrition, Carbohydrate, and Protein. The relationship between excess dietary energy intake and Obesity is an important topic in public health research, but it falls outside the scope of a discussion of the unit itself.
References
- Calorieen.wikipedia.org
- calorie - wikidocwikidoc.org
- Nicolas Cl%C3%A9menten.wikipedia.org
- History of the Calorie in Nutritionjn.nutrition.org
- Does the history of food energy units suggest a solution to "Calorie confusion"?ncbi.nlm.nih.gov
- Atwater systemen.wikipedia.org
- Why are calories not adding up with the 4-4-9 rule?nutriadmin.com
- How Are Calories Calculated on Food Labels? (Free Calorie Calculator and PDF handout) — Food and Health Communicationsfoodandhealth.com
- Calorie - an overviewsciencedirect.com
- 3.5: Energy And Caloriescourses.lumenlearning.com