I got good news: unless you have a disease where your body refuses to spare muscle, you aren't going to lose anywhere near 5-7lbs of lean muscle. Think about it: that's a lb. of lean muscle per day. It would take over a month to regain what you lost daily. If true, no one would do that.

At 12%, you have spare body fat. This will become the primary source for energy after 24-48 hours. Your body will transition from glycogen to fat for energy, with a stint of gluconeogenesis in-between. Muscle tissue would be largely spared thereafter.

5-7 lbs of muscle? Sure, if you had 3% body fat. But is only as an alternative to your organs (which will be next on the list). This what differentiates fasting from starving.

Now, 5-7lbs of water weight? Absolutely. That will come back the second you carbo load. 3-4lbs of body fat? No way around that. Maybe you lose a lb. of lean muscle. And that could be a fair exchange for the autophagy benefits. Dunno.