I'm pro-fasting. I'm also pro-getting the science right. I do think the fear of losing lean mass while fasting is often overblown. But if we say things that are demonstrably untrue, and doctors who advocate fasting say things that are demonstrably untrue, people might be understandably leery when considering our opinions about fasting.

Muscle cells are like fat cells in that they are not usually gained nor lost but can shrink or expand to a point. You don't build muscle you were not born with no mater how much you body-build. The bulking of muscle comes from vascular build up of the existing tissue (neovascularization) stimulated by intense strength training or eccentric contraction. In the military they told us "we will tear you down (muscles) and build you back up" that's kind of what happens.

I have noticed more bulk after fasting doing the same or less strength training as before. I attributed this to upregulation of HGH (growth hormone) and testosterone. HGH can get so high it could lead to failing a blood test in a sports event, but athletes don't compete while fasting.

Good discussion in this thread. When I hear these topics discussed in terms of CICO, protein's impact on metabolism, fasting, etc., it brings to mind this video of Dr. Ben Bikman, who is doing relevant research in this area. His observation that the impact of protein has everything to do with dietary context is very enlightening. I know I've posted this link before, but for those who have not seen it, it's worth watching:

https://www.youtube.com/watch?v=z3fO5aTD6JU

That is not what they meant... They mean the ENTIRE you, mentally and physically...