I workout 3-4 times a week with adequate rest in between sessions. I lift as heavily as possible and utilize "reverse pyramid training". I would also like to mention -- I always train fasted.
Within these three years i have steadily gained muscle mass and strength while maintaining a lean physique. I use NO supplements and have never lost muscle mass following my IF routine.
With that being said -- once again, I am on my phone so i cannot post links. Medically speaking, the human body is incredibly efficient and adaptable. The human body goes through several phases of energy restoration. The last line of defense, yes -- is the body utilizing muscle for energy, however, it would take several weeks for your body to actually break down muscle tissue for energy. This, of-course, also depends on your body fat percentage as well. Your bodies ability to utilize other forms of energy before muscle, is nothing short of amazing. I also follow the ketogenic diet, so i consume little to zero carbs at most. Im talking no greater than 50 grams a day. Once again -- i have lost no muscle (do periodic DEXA scans) while following this regime.
Now, every human body is different and reacts to stimuli differently. What i would recommend is "experimenting" with what works best for you, maybe even with a doctors advice if you have any medical conditions. For me, this is more of a lifestyle and not i diet. I believe that is the ultimate goal and one should discover what works best and is most comfortable to their lifestyle.
This rapid change in diet and philosophy has given me absolute control of my mind and body. I was able to quit alcohol and drugs in the process and feel so much healthier because of it.
Good luck and I am more than happy to help with any questions you may have!
(I am not an MD or registered nutritionist/dietician. These are only my opinions and should be researched by you and confirmed through facts -- medicinal or scientific.)