Is a Calorie a Calorie?

“Is a calorie a calorie? A calorie may be a calorie when people talk about weight loss and nothing else, there are other factors involved. And once you get past my perhaps lame “Is a calorie a calorie” question, you can begin to see something approaching the truth.
For one thing, says Nestle: “There are dozens of factors involved in weight regulation. It’s hard to lose weight, because the body is set up to defend fat, so you don’t starve to death; the body doesn’t work as well to tell people to stop eating as when to tell them when to start.”