What is a calorie?
A calorie is simply a measure of energy (technically the amount of energy needed to heat 1 kg of water by 1 C) that the body uses to drive chemical reactions. Clearly in a system as complex as the human body it is a gross oversimplification to assume that all calories are equal no matter what food they are derived from and that they all elicit identical metabolic responses. This neglects the quality of those calories and, importantly, the hormonal response they provoke in the body. Hormones are the primary driver of not only body weight, but also body weight distribution.