Calorie (Cal for short), transliterated from English Calorie, is defined as the heat required to raise 1 gram of water to 1 degree Celsius at 1 atmospheric pressure; Calorie is an energy unit, which is still widely used in nutrition measurement and fitness manual. The international standard unit of energy is joule. As the legal unit of food calories, joules are widely used in Europe, while calories are used in the United States.
The calorie content in food is a measure of how much potential energy food produces. Our body "burns" calories in food through metabolism, which breaks down carbohydrates into glucose and other sugars through enzymes, fats into glycerol and fatty acids, and protein into amino acids. Then, these molecules are transported to cells through the blood stream, where they can be absorbed and utilized immediately or enter the final metabolic stage.