A "calorie" is a form of energy that is expended through respiration by consuming food. Also known as "food energy". Food energy was discovered in 1824 as a unit of heat and was described as the approximate energy required to increase 1 gram of water by 1 degree celsius. There are two ways to define calories, a small calorie (gram calorie) or a large calorie (kilogram calorie). The small calorie is equal to about 4.2 joules of energy while the large calorie is about 4.2 kilojoules.