[rhinoceraptor]

I haven't seen any convincing evidence that significant muscle loss occurs with a 7 day fast. If you get a DEXA scan before and after, you will have lost a lot of "lean body mass" and your muscles will be smaller, but it's only because you lose about 5-10lbs of water weight.

The only major health risk is refeeding syndrome, and even then it's only really notable if you're an alcoholic or have anorexia. If you're otherwise healthy with normal sodium/potassium/etc. intake and you take it slow when refeeding, I don't think it's particularly dangerous. Obviously, talk to your doctor, and it's probably good to get blood work before doing something like this.