on March 22, 2013
at 08:43 PM
I've fasted up to 40 hours more than once. I've seen no measurable loss in muscle mass or loss in strength based on major lifts. I'd go with protein, but that's far from optimal. If you're looking to retain muscle you shouldn't be focusing on one specific macronutrient. But now I'm moving away from your question :)
on April 05, 2013
at 10:44 PM
(muscle loss?) In a word, no.
I tend to feel really good from 18 hours on, so I tend to be relatively active during and after a fast.
When I finally eat, I have a complete meal with meat, fruit and vegetables and muscle size/tone either stays the same or improves. The only difference from regular eating is that I take my time eating so I don't shock my system with a whole bunch of food at once.