There isn’t a clear answer in that there are no human trials that would tell us with certainty.
The concern I have with heavy weight training to or close to failure is that you are not taking in any amino acids to aid with protein synthesis/muscle repair following that session. Some people will point out that HGH peaks after ~48 hours, but I fail to understand how that overcomes the fact that the essential building block for muscles/protein synthesis (amino acids) are not being consumed.
That said, we know that not lifting likely has deleterious effects as well.
As such, I tend to play it as safe as I can think of, which is to lift light weights not to failure.
For example, if I can bench 185 lbs. for 12 reps, maybe do 135 lbs. for 12 reps, or 185 lbs. but for 6 reps, etc. That way I’m not exhausting the muscle to failure, but still working it in hopes of preservation.
This is all my personal opinion as, again, we don’t have data on this. I know some fasting gurus say it’s perfectly safe, but with all due respect to Jason Fung, I don’t really see him as a scholarly authority on anaerobic training.
>Does someone know if this promotes muscle loss or breakdown?
Absolutely. Unless you’re doing something like a Protein Sparing Modified Fast, about 30% of your weight loss will be lean muscle mass (or organ mass if you don’t have spare muscle…which is extremely dangerous). Light exercise can also help but not entirely. Another portion of your weight loss will be that often mentioned water weight which is glycogen, or glucose and water. Finally, you’ve got your excess body fat. The upper limit for excess fat loss is about .5lb per day.
Note that this sucks. When you’re overweight, you want to see those huge drops on a weight scale. It’s motivating. However, those big drops aren’t improving your overall body composition. You’ll get to your goal weight and likely be “skinny fat” due to a loss of lean mass which you didn’t originally account for.
Bottom line, if you’re fasting to lose actual body fat and look fantastic, you’re better off doing a Protein Sparing Modified Fast for the long term or ensuring you get enough protein during short term fasting refeeds.