After being in a prolonged state of caloric restriction; 0-600, maybe a little more if you're active, the body will begin to burn fatty acids from the body fat stores along with some muscle. Heart rate will slow down, you'll be very tired. In severe cases where the body has been in this state for too long and it has gone through fat stores and muscle, it will begin to consume its own organs.   Fatlogic will have you believe that it means the body holds on to all the fat and you just stay where you're at or even gain weight. It's not possible... You cannot gain weight from air lol. Weight loss will eventually slow down, but it won't stop. If that were the case, underweight people simply would not exist. Anyone who has a better definition, feel free to jump in.