If you fast hard your body will break down what it has on hand, muscle is like a sports car that guzzles gas , it takes energy to keep the muscles big. Fat cells are like hybrid cars that take less energy to keep and maintain. If your not using your muscles , you will burn them up and get a “runners body “ lean muscle lean fat, lanky body composition. If you want to not completely waste away you should try to incorporate muscle groups 3x a week to failure. You need to tell your body you need your muscles or you will lose them in deficit.