I'm currently in the middle of a hardcore body recomposition routine that involves eating 200g of protein a day on a 1400-1600 calorie budget (so ~60% protein, but not keto)
Workouts are mentally draining, and obviously muscle gain is slow (some people think you can't gain muscle and lose weight... unless you're a competition prepped body builder with 6% body fat you don't need to worry about that).
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Before toning things down a bit I had a few episodes where I'd enter ketosis, not necessarily from avoiding carbs, but from completely depleting my body's glycogen stores during a workout (cyclists and runners know it as bonking).
It can be a horrible feeling because a truly deep part of your mind is convinced it's trying to preserve you, and it deeply affects your emotional state. Each time it happened I'd spend the next day feeling like crap despite upping my carbs again
I am a 60 year old lifelong cyclist, ectomorphic build. I have fasted for 24 hours (dinner time to next day's dinnertime) once weekly for about 5 years now. When fasting, I incorporate moderate exercise throughout the day, usually periodic sets of pushups and pullups, sometimes a 30 or 45 minute ride.
I can report I have no problem maintaining muscle or body mass with this regimen.
Further I can recall a few minor hangry episodes while fasting, early on. But like fasting itself, it became progressively easier for me to fast over time.
You would have a difficult time maintaining and gaining muscle mass with frequent, long fasts. Your muscles need protein to repair/recover/grow. If you don't provide it from food, your body will cannibalize it from wherever it can. This is one of the reasons it is recommended to increase protein intake, distributed throughout the day, when in a caloric deficit.
Honestly I was more wondering about the 48 hour fasts. At first glance, I'm assuming it might impair recovery over the long term if the regimen is high enough intensity. I would assume if you're in more of a maintenance mode it could work better or even in a deficit
I don't train particularly hard and mostly just do cardio stuff, like lots of pushups daily.
Based on what I have experienced I think it's fine provided you're still maintaining an overall balance in the long-term. As in, on the non-fasting days you're eating enough of (if not more than) everything your body needs for your training regimen.
