I’m the book The Obesity Code by Jason Fung, he says fasting will generally not cause muscle loss. His explanation (As far as I remember) was that your body specially stores fat to use as fuel when you have no food so it’s evolutionarily designed to be consumed by your body and therefore easier and more beneficial for your body to use as fuel. Muscle is a less helpful fuel source for your body and harder to process so it won’t start eating muscle until the fat is no longer sustaining you. (He’s an interesting guy, but he’s a doctor who monitors people who are fasting and he had a patient who fasted for over 400 days, so I take his word for it.)
I’ve been trying to figure this out as well. I starting lifting again recently after doing IF for over a year. I was definitely worried about being able to gain any muscle and still fast but six weeks in and after a little trial and error I can confidently say I’m gaining more muscle mass and definition.
I first tried OMAD with weightlifting but was almost making myself sick trying to eat all the calories I needed within an hour. I’ve gone back to a 16/8 schedule and think that will work better for me.
One thing I think that has really helped is I also take creatine. I’d heard good things about it and can really tell a difference in muscle size, strength, and recovery time.
Background - I’m a 60 tr old guy who lifted a lot when I was younger but got heavier and lazier as I got older. I wasn’t expecting much beyond just getting a little more fit but am surprised and happy with my results.
My understanding is that your impact will be minimal/negligent, depending on your body fat percentage. Muscle tissue takes a long time to build and break down. The first 24-48 hours your body will consume it’s glucose & glycogen stores. At this point it will start leveraging fat to make ketones. Yes, muscle synthesis pathways will be affected, but that means no new muscle growth. I think especially if you work out during the fast, you’re good. My understanding is related to normal body fat though. I imagine if you’re close to essential body fat it would be highly dependent on your genetics.