I’ve done many 4.5-5.5 day fasts, many 3.5 day fasts, many, many 36 hour fasts, and was able to gain muscle mass and strength while doing this. And I’ve been fasting, including skipping breakfast and/or lunch, since June 2016. So 4+ years.
It IS possible to overdo it. If you begin to get really cold, as in you can’t touch your own hand to your body, that’s not good. But all you do is back off, stop longer fasts, and eat more. You feast and fast, feast and fast.
If you find you can’t feast enough, then delay starting fasting.
I find I cannot eat breakfast, unless I’m really hungry. But then what happens is I’ll eat “breakfast” around 9-10am, but I won’t eat again until dinner. On rare occasions, I’ll eat three times in one day. But it’s very rare. If, however, I’ve eaten my first meal and am still hungry, I’ll eat again.
Today, I exercised about 53 minutes, body weight training to failure. Only coffee before. Then I ate “breakfast” at about 10:20 am. It’s 3:47pm, and I’m still not hungry. When I get hungry (or get home and the family is having dinner), I’ll eat.