On "a calorie is a calorie", a suspicion is that there can be two diets equal in calories per day but very different in how they make the person feel, say, in energy level, and, thus, how many calories they burn or how much weight they gain or lose.

In my own efforts at dieting and weight loss, I'm not sure I've seen any such effect for the range of diet variations I've tried. Still, there are suspicions that some such effect might be true, somewhat common in reality, and significant.