You may need to speak to a fasting doctor about adjusting your meds (if necessary) during a fast, but the body has protein and to spare while fasting. Fasting is not a diet and it isn't starving yourself. Unless you are skin and bones already you have enough stores on you to provide all your nutrients throughout your fast. When you run out of protein or anything else you need to live there is a pleasant feeling called hunger that returns to let you know.
So don't worry about the protein, it's covered. :)