Calorie comes from the Latin, calor, meaning heat. A Calorie is a measure of heat energy. In many countries, Calories are a measure of the energy stored in the foods we eat. A definition for the Calorie is-- the amount of energy required to raise the temperature of one kilogram of water, one degree Celsius. A Calorimeter is a device that can determine how many Calories of stored energy a food has. If one compares the Calories in an equal amount of carbohydrates, proteins and fats, fats contain the most Calories.
The calorie is a unit of Energy which can be used instead of the SI unit of Energy which is the Joule. It is the Energy required to raise the temperature of 1 gram of water by 1 degree Celsius. This is approximately 4.2 Joules.
The calorie associated with nutritional labeling is actually the food calorie which is equal to 4200 Joule or 1 kiloJoule approximately. For example carbohydrates have 4 cal/ gram and fat has 9 cal/g. This means that 1 gram of carbohydrate and fat can give the body 4 and 9 kilocalories of energy respectively, if all the energy contained in them is completely utilized by the body.
Calorie is a unit for measurement of heat energy in metric system of measurement. One calories is defined as the amount of heat required to increase the temperature of one gram of water by one degree Celsius. Heat energy can be expressed in equivalent amount of mechanical energy by using a suitable conversion factor. For example, one joule is equal to 0.24 calorie.
It is important to note that though the unit used for measuring the energy in food is also called calorie, its vale is equal to 1000 calories as the unit is used for all other applications.