A calorie (cal) is a unit of measurement that indicates the amount of energy we obtain from a particular food. It is a unit of thermal energy equivalent to about 4.185 J.
By definition, one calorie is the energy needed to increase the temperature of 1 g of water by 1 °C at 15 °C under normal atmospheric pressure (76 cmHg). In nutrition, it is used to quantify the energy of food and the defining quantity of water is 1 kilogram, making the "food calorie" 1000 times as large as the other calorie. Hence, 1 calorie (nutrition) = 1 kcal = 1000 cal, and it is also called the large calorie.
The calorie is not an SI unit where the joule is the only unit of energy. The kilocalorie is frequently used in chemistry because it is a convenient measure of molar free energy: for example, 1.4 kcal/mol engenders a change in equilibrium by a factor of 10.
Nutritionists measure the energy content of food today in both "calories" (large calories) or "kilojoules" (kJ), a usage widely known in the general population and (confusingly) often referred to as a "kilocalorie" as though small calories were being used. Sporadic attempts have been made, particularly in the United States, to write the kilogram calorie capitalized as Calorie and abbreviated as Cal, to alleviate the confusion with the chemical definition of the term "calorie" as a small calorie. This alternate notation has no legal standing, and is not endorsed by any standards body.
The amount of food energy in a particular food is measured by completely burning the food in a bomb calorimeter, a method known as direct calorimetry . For example, dietary fat has about 9 kcal/g, while proteins and carbohydrates have about 4 kcal/g. Recommended daily energy intake values for average adults are 2500 kcal/d (10 MJ/d) for men and 2000 kcal/d (8 MJ/d) for women. We consume calories in the form proteins, fats, and carbohydrates, three basic nutrients that are necessary for life. There are three other nutrients, vitamins, minerals, and water, are necessary for bodily function but do not contribute any calories to our daily diet.
The "calorie" has become a common household term, because dietitians recommend in cases of obesity to reduce body weight by increasing exercise (energy expenditure) and reducing energy intake.
Three definitions of the calorie are today recognized internationally:
Other historic definitions include
* the 4 °C calorie, and
Of all these, what is most commonly meant by calorie in contemporary English text is the 15 °C calorie. The nutritional kilocalorie or Calorie equals 1000 cal15.
Since these many definitions are a source of confusion and error, all calories are now deprecated. The International System of Units (SI) unit for heat (and for all other forms of energy) is the joule (J), while the (obsolete) CGS system used the erg.