The problem with the 'a calorie is a calorie' theory (which I used to subscribe to as well) is that is that your body doesn't burn food like a calorimeter. Different types of foods require very different amounts of energy to digest; some are only broken down with help from intestinal flora (or not at all)..while others are digested quickly and completely. Combine this with insulin's role in triggering fat storage, and you should be able to see that _what_ you eat, and not just how much, absolutely matters.