Credentials statements:I am a naturally muscular person, albeit a female one, and now - an unfertile-aged one. Post diagnosis four years ago, I experiment a lot with no-food fasting (longest one 10 days), intermittent fasting (many different regimes) and even (WAY less often, as in rarely) very low calorie dieting, as in Newcastle/Mosley's BS diet, currently second time round.
I have normally a pretty decent blood pressure, heart rate etc. (not so decent - upper end normal - when very low calorie dieting I have discovered, on owning a BP pump this time round.)
I was probably always meant to be lean to be healthy. I have not always been lean! (Ahem. I'm ah, very carb-sensitive.)
My level of fitness is close to excellent, according to health-calc.com self-tests (specifically how fast you can walk a distance test). I do great on very low carb with lots of healthy fats, and for me - plenty of protein. I do not limit protein, but eat what I want of it. (normally.)
My muscles seem unchanged in all those 'famine/food shortage-replicating' regimes. Just get more defined as fat is stripped, if fat is being stripped. As soon as regime is over, or eating normally (very low carb/keto), I am able to do all the physical things I can when eating normally basically immediately upon getting the energy back from food.
I regularly lift heavy things as part of my diabetes treatment, and food-shortage replicating regimes do not affect that. Not a bit, which I merrily call out to Herr Svea immediately after lifting whatever heavy thing I have been lifting.
I believe this is evidence, albeit anecdotal, at least as applies to me, is that no-food regimes do not cannibalise my muscles, if I may use that expression. My body uses fat from fat stores, and ketones, for energy when in semi-starvation mode, I believe. When completely no-food fasting, I cannot do much. As in not much at all. Basically semi-bedridden. I don't like multiple no-food fasting for this reason. (Sorry Dr Fung! I love you!)
When I first got diagnosed I became very interested in ancestral health, and in the Paleo diet, which worked very well for me as a way to reprogram my poor sick body (at HBA1c of 93 I was very sick). The catchphrase from 'The Complete Idiot's Guide to Eating Paleo', was, "Lose Weight, Gain Muscle, Fight Diabetes".
I like that.