check out the guy that fasted for a whole year, he lost hundreds of pounds, it was done under medical supervision all he took was water, electrolites and some vitamins I believe.
Why do I bring this up? one small little thing... : he did NOT have loose skin at the end of the fast! Know why? because his body canibalized every source of protein that was available muscle, skin etc and here lies the problem with fasting for jacked dudes and why it is not used, you will preferentially lose more muscle mass than if you were on a normal high protein diet
All the morons that claim fasting is muscle sparing are just that morons, it makes no biological sense especially when you have excess muscle mass on your frame....