This is a unit of heat energy, not fat. It is the amount of heat required to raise 1 gram of water by 1 degree Celsius at sea level. The calories in food are also units of heat, which the body can use as energy to do work or, if no word is done, which the body will store as fat for future use. Since the physicist’s calorie is inconveniently, small for measuring the energy content of food, nutritionist’s use a larger unit called the Calorie (with a capital C), which is equal to 1,000 calories (with a lower-case c). The Calorie content of a food is determined by literally burning the food and measuring the amount of heat put out.
From How The World Works by Boyce Rensberger
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