Easy to say, but your body isn't a static system. It is going to respond to the lack of food(or increased exercise). And that response may be the actual problem

Easy to say, but your body isn't a static system. It is going to respond to the lack of food(or increased exercise). And that response may be the actual problem for people. Maybe your body will say "Oh, I'm only getting 1500 calories a day now? Okay, we will just be sleepy, miserable, with no energy or motivation, until we get our calories"