My experience (from longer fasts - never felt anything like that after 3 days) was that muscles that aren't in heavy use do indeed get metabolized for energy (which makes a lot of sense - maintaining muscles costs energy; if you're not using them and running on fat reserves, why keep them?)
However, if I exercise while fasting, I hardly lose any muscle at all.
One interesting thing is that unaerobic exercise, such as boxing or rope skipping becomes harder for me while fasted; but aerobic exercise, specifically, running - becomes easier and it feels like I can keep running forever (whereas unfasted I usually feel my ability degrading near the end of a 10km run)
My experience (from longer fasts - never felt anything like that after 3 days) was that muscles that aren't in heavy use do indeed get metabolized for energy (which makes a lot of sense - maintaining muscles costs energy; if you're not using them and running on fat reserves, why keep them?)
However, if I exercise while fasting, I hardly lose any muscle at all.
One interesting thing is that unaerobic exercise, such as boxing or rope skipping becomes harder for me while fasted; but aerobic exercise, specifically, running - becomes easier and it feels like I can keep running forever (whereas unfasted I usually feel my ability degrading near the end of a 10km run)