There are many definitions for calorie. Originally is a pre-SI metric unit of energy defined by Professor Nicolas Clément in 1824 as a unit of heat. The food calorie known in Europe as kcal, and in the U.S. as cal, is the most common form in usage today, substituting more and more the kilojoule (KJ).
Definitions vary but are all based on the specific heat capacity of water. The gram calorie, approximately 4.2 J, is based on one gram of water. The kilogram calorie, equal to one thousand gram calories, is based on one kilogram of water.
In the context of nutrition, and especially food labelling, a larger unit is used and referred to interchangeably by the terms calorie (or Calorie) and kilocalorie. In the context of food energy the term calorie generally refers to the kilogram calorie. However, the term kilocalorie (kcal), referring to one thousand gram calories, is also in widespread use especially by professional nutritionists (when speaking in terms of calories rather than joules).
To avoid confusion, the prefix kilo- is not used with the kilogram calorie. The kilogram calorie, large calorie, food calorie, Calorie (capital C) or just calorie (lowercase c) is the amount of energy required to raise the temperature of one kilogram of water by one degree Celsius. The conversion factors between joules and calories are: 1 kcal = 4.184 kJ, or conversely, 1 kJ = 0.239 kcal.
The joule (symbol J), named for James Prescott Joule, is the derived unit of energy in the International System of Units. It is the energy exerted by the force of one newton acting to move an object through a distance of one metre.
Carbohydrates, fiber, fats, proteins, organic acids, polyols, and ethanol all release energy during respiration — this is often called 'food energy'. When nutrients react with oxygen in the cells of living things energy is released. A small amount of energy is available through anaerobic respiration.
Fats and ethanol have the greatest amount of food energy per mass, 9 and 7 kcal/g (38 and 30 kJ/g) respectively. Proteins and most carbohydrates have about 4 kcal/g (17 kJ/g). Carbohydrates that are not easily absorbed, such as fiber or lactose in lactose-intolerant individuals, contribute less food energy. Polyols (including sugar alcohols) and organic acids have less than 4 kcal/g.
Calorie restriction, or caloric restriction (CR), is a dietary regimen that restricts calorie intake, where the baseline for the restriction varies, usually being the previous, unrestricted, intake of the subjects. CR when not associated with malnutrition, is thought to improve age-related health and to slow the aging process in some animals and fungi. CR is one of the few dietary interventions that has been documented to increase both the median and maximum lifespan in a variety of species, among them yeast, fish, rodents and dogs.
In human subjects, CR has been shown to lower cholesterol, fasting glucose, and blood pressure. In CR, energy intake is minimized, but sufficient quantities of vitamins, minerals and other important nutrients must be eaten.
CALERIE (Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy) is a trial currently underway in the U.S. to study the effects of prolonged calorie restriction on healthy human subjects. The CALERIE study is being carried out at the Pennington Biomedical Research Center (Baton Rouge, Louisiana), the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University (Boston, Massachusetts) and the Washington University School of Medicine (St. Louis, Missouri).
It is hoped that caloric restriction reduces the incidence of cardiovascular disease and cancer and leads to a longer life, as has been demonstrated previously in numerous animal studies.
CALERIE is the first study to investigate prolonged calorie restriction in healthy humans, which is practiced and advocated for by the members of the Calorie Restriction Society. Study subjects are selected from people who are not obese (because calorie restriction on obese people is already known to lengthen life, but possibly for different reasons).
A smaller predecessor study ended in 2006. Forty-eight subjects were randomly assigned to a control group and a treatment group; those in the treatment group were put on a 25% calorie reduction over a 6 month period. It was found that the treatment group had lower insulin resistance, lower levels of LDL cholesterol, lower body temperature and blood-insulin levels as well as less oxidative damage to their DNA.
- Energy and fuel; Littledyke M., Ross K. A. and Lakin E.
- Nutrient value of foods (PDF); Health Canada.
- Calorie restriction: fountain of youth or dangerous diet?; Yale Daily News.
- Calorie Restriction (CR) Society International
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