The body will scavenge it's muscle mass in order to get a quick shot of glucose after the glycogen stores are gone.
Probably some muscle mass will be lost the very first day of a fast....since the breakdown net loss of muscle (catabolism) can be repaired ONLY with some new protein....absent in a fast.
I've read that there is a SLIGHT protein storage capacity in the gooey lining of the intestines and that the ebb and flow of this tissue keeps serum proteins on a somewhat level keel....but only for a short while.
Absent food, the body will burn a mix of glycogen, fat and protein.
As the fast wears on the body will get better at saving itself, so protein scavenging will decrease when the blood, heart, brain, liver, kidneys are in danger...but unfortunately the body may not think our hard won abs, biceps, and pecs are are valuable assets