"A calorie is a calorie" is a tautology used to convey the speaker's conviction that the concept of the "calorie" is in fact a sufficient way to describe energy content of food. It has been a commonly cited truism since the early 1960s. The tautological phrase means that regardless of the form of food calorie a person consumes (whether a carbohydrate, protein or fat calorie) the energy chemically extracted from the food, or the work necessary to burn such a calorie, is identical to any other. One dietary calorie contains 4184 kilojoules of energy. With this knowledge, it is easy to assume that all calories have equal value.