I have not said that a lack of activity/exercise will spare more protein; what I HAVE said is that exercise/activity will use-up and deplete nutrients more quickly when compared to being in a resting state and this includes muscle tissue in the burning of amino acids for fuel.
Write to or contact Dr Fuhrman and or the Doctors who supervise fasts at the Truenorth center, and you may learn something of value instead of debating and arguing about this. Don't take my word for this by any means.
However, Intermittent Fasting or IF can result in muscle gain, but what we are referring to here (or at least I am) is that prolonged fasting where the BMR is reduced, exercise will undoubtedly lead to muscle loss well over and above what would have occurred in the resting state during abstinence.
The need for exercise whether eating or fasting is ultimately decided by the body itself, just as when anyone would feel the need to rest or sleep, or drink when thirsty. Have you ever tried to do any strenuous activity/exercise immediately after a large meal? if you haven't, then you could try this and witness the results for yourself. Digestion is known to be suspended altogether if exercising this way.
Hygienists have always stressed the need for a period of rest after a meal to allow for the efficient digestion of foods, and if you tried exercise after this period of time it would be conducive to health.
The lesson here is I think you have not learnt to listen to your own body, as this is the central tenet of health as taught by Hygiene based on physiological principles and laws of the vital economy.