No one eats calories; calories are just a measurement (as is an inch) and have no substance. A calorie is a unit of heat equal to the amount of heat required to raise the temperature of one kilogram of water by one degree at one atmosphere pressure (does this sound like a human body?).
The calorie theory is based on the heat engine analogy also known as thermodynamics by engineers. The human body is not a heat engine, as they would like you to believe. The human body is more like a complex chemical factory than it is a heat engine.
Food is converted in to complex substances and structures and not as a machine designed for heat production, which the measurement of a calorie is.