I've been thinking about this too much

could the theory behind it be that when you're pregnant, your body uses the calories/food more effectively? As in it makes sure the baby gets what it needs before you do. So maybe the thought is that if you're only eating 500cal, your body responds as if it were pregnant and puts those calories and nutrients to better use than it would normally? Although I'm not even sure that's down to hcg, I would have thought it was the placenta? Either way, eating 500 calories a day cannot be healthy! Even on those programmes with the super, super morbidly obese, they only cut them down to about 800!