Regulating food intake to improve physical condition, especially to lose weight. Examples include diets low in fat for weight loss, low in saturated fat and cholesterol to prevent or help treat coronary heart disease, or high in carbohydrates and protein to build muscle. Weight-loss diets are based on reducing calorie intake in different proportions of fat, carbohydrate, or protein; most result in some weight loss, but often the weight is gained back within a few years. Diets must include adequate nutrition and are most effective combined with exercise. Appetite suppressants may have dangerous side effects. Excessive weight loss may be a sign of anorexia nervosa.
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Unit of energy or heat. Various precise definitions are used for different purposes (physical chemistry measurements, engineering steam tables, and thermochemistry), but in all cases the calorie is about 4.2 joules, the amount of heat needed to raise the temperature of 1 g of water by 1 °C (1.8 °F) at normal atmospheric pressure. The calorie used by dietitians and food scientists and found on food labels is actually the kilocalorie (also called Calorie and abbreviated kcal or Cal), or 1,000 calories. It is a measure of the amount of heat energy or metabolic energy contained in the chemical bonds (see bonding) of a food.
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The calorie is a pre-SI unit of energy, in particular, heat. In most fields, its use is archaic, and the SI unit of energy, the joule, has become accepted. However, it remains in common use as a unit of food energy. It was first defined by Professor Nicolas Clément in 1824 as a kilogram-calorie, and this definition entered French and English dictionaries between 1841 and 1867. Etymology: French calorie, from Latin calor (heat).
The unit calorie has historically been used in two major alternate definitions that differ by a factor of 1000:
The second definition is the one commonly used to express food energy, e.g. when discussing dieting or nutrition plans. Under this definition, 1 g of pure carbohydrate yields about 4 Calories of energy, and the recommended intake for an adult person is about 2,000 - 2,500 Calories/day. It is alternatively referred to as Calorie (Cal), kilocalorie (kcal) or even calorie with lowercase 'c'. The potential for confusion can be avoided by using the SI units (joules or kilojoules).
Apart from these two major alternate definitions, there exist also minor variants of the definition of this unit, which differ in the exact experimental conditions used, most notably the start temperature of the water (see section below).
The factors used to convert measurements in calories to their equivalents in joules are numerically equivalent to expressions of the specific heat capacity of water in SI units. See "Versions" below for an explanation of the units.
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).