The major catch there is that, if some common hypotheses hold, then where a calorie comes from has a direct affect on how your body burns it. The example commonly used is to say you need and consume 3500 calories to support your current level of activity. If you consume 600 that your body will only store, then your body will behave as though you still need an additional 600 to maintain your energy level. Likely leading to you being lethargic until you get additional food. (Which, as stipulated, you don't actually want/need to consume.)