It will always eat a bit of it, even sane muscle, the process is not perfect. But old cells and sick cells are prioritized. That's one reason it has benefits in

It will always eat a bit of it, even sane muscle, the process is not perfect. But old cells and sick cells are prioritized. That's one reason it has benefits in combination with chemo therapy : the body will target the cancer cells. Not only those, but it's enough to help. Same with weak joints, old muscle bruises, etc. But again, eating properly after the fast is important to let the body rebuild what it destroyed.