I would make sure that you are not doing very much exercise at all so that you are not breaking down your muscle. Make sure that you are drinking enough water.
Well, believe it or not, though your statement is logical, the truth of the matter is quite the opposite. My friend, who fast for 18 days which he completed just last week, spent the entirety of the fast doing high-intensity exercise and reported significant muscle growth and strength gain. The reason for that is your body releases an excess amount of Human Growth Hormone when you are fasting and will not ony protect from muscle loss, but can even build muscle. I covered this in my blog of this fast on day 2.
Fasting for that long is not an ideal situation.
All throughout humanity, human-beings have been made to survive extended periods of time without food. There are many scientifically studied cases of individuals going on extended periods of fasting, much longer than mine and not only survived, but were found to be in better health than when they began.
Nonethless, thank you for your concern and your generally good advice :)
RE: I Quit Eating for 28 Days [Water Fast] : Day 8 (Focus)