An article in the NY Times today on The Biggest Loser contestants explains that their metabolism slows to bring them back to a set weight. This disproves the "calories in calories out" idea since calories out is not the same for everyone. Two otherwise identical people eating the same thing and exercising the same amount will not have the same resulting weight change. There is more to it that has to do with how the body processes food. Of course, on the other hand, going along with the idea that all calories are equal, the article does not discuss whether there are differences in the type of calories eaten as to what the consequences are for metabolism.