Most of the protein lost during fasting is from autophagy. This is scar tissue, loose skin, and dead cells. I believe Dr. Fung has some studies in his book on this. If not it’s one of the presentations he has on YouTube. Proteins were lost but lean muscles stayed the same or improved because of the increased production of HGH during ketosis.
I recommend atleast maintaining some light exercise like the other poster suggested.
It’s dependent on your current mm, and the amount of time spent fasting. You won’t have decrease in your core muscle mass until 15-20 days into a fast(assuming you aren’t doing anything to retain muscle like exercise, or eating lol). A water fast multiplies the autophagy response by day 3 exponentially, and research groups in intermittent fasting studies(not pure water fasts) experienced increased pituitary gland hormone secretion(HGH was given special attention in these studies) by over 1200%. I’d expect the hormone response to be more aggressive in pure fasts like dry, water, or juice fasts. This is where it gets tricky, so bear with me.
In comparison, the average HGH injection for replacement therapy on a non weight-based regimen will be at a higher dose is.3mg(less than 1IU), whereas performance oriented individuals use up to 10IU(a little over 3mg, permanent side affects seen at doses over 8IU, stay below 6IU). Average male HGH levels in the body are between 0.4-10ng/ml, so multiplying that by the increased HGH secretion during a fast would put a male at peak HGH levels at an elevated 48-51ng/ml using the lowest number to be considered within the normal range(0.4ng/ml). The highest possible secretion for HGH would be around 1200ng/ml in a fasted state of a normal male. 1200ng/ml is 1.2mg. So based off the math you’d be getting the more than the equivalent of prescription dose of HGH replacement therapy in an intermittent-fasted state in a male after 5 days; however, I’m optimistic that those numbers will be higher in the purer fasting states on the 5th day.