Personally, I strive to not eat protein on fast days. High protein food like fish certainly makes the fast day easier and I’ve heard all of my life that if you don’t eat protein, you’ll lose muscle. But think about it, if you need to find food, the body isn’t going to go mine for protein in your muscles. You’ll probably need those. What the body will do is tell the dodgy cells it’s time to die. Your cells have programs that tell them to kill themselves if they are malfunctioning (it’s called apoptosis) and this process is probably under overall regulation and would be up-regulated by the need for protein. Some of these cells could be on the road to cancer (once they become cancer they are no longer under regulation).

It’s probable that just fasting up-regulates apoptosis, but it would seem to me that protein deficiency certainly does and probably to a greater extent than just fasting (but I’m just speculating, I wish I had some hard science on this). This process would also improve metabolic efficiency by getting rid of less efficient, malfunctioning cells. That would be another, and a comforting explanation for “plateauing”.