All units of measurement are human “inventions.” That is, at a point in history a person or team of scientists decided to make a certain amount of something (length, volume, weight, etc.) the unit of measurement for that thing. The calorie is no exception. In 1824 a man by the name of Nicolas Clement decided that amounts of heat energy could be measured in calories, one calorie being the amount of heat energy required to raise the temperature of one gram of water one degree Celsius. Since this is a very tiny amount of heat energy, one food calorie is defined as one kilocalorie (one thousand calories). Sometimes these two units and referred to a “little calories” and “big calories”. See further at:
https://en.wikipedia.org/wiki/Calorie