This was interesting info - from the Fast-5 E-book:
Basically, anyone worried about losing muscle should include adequate protein in the meal or meals eaten during the eating window (as many hours as you feel comfortable with). Also, balanced nutrition is important. Including light exercise along with dietary protein will preserve muscle. If one is going to lose muscle it will only be about 5% of the weight lost and that can easily be regained with exercise, plus exercise will be so much easier with less weight on one.