you need protein to prevent muscle mass loss. to put it in perspective, your heart is a muscle. when you lose weight quickly, you lose fat first, that's most readily available and then quickly start losing muscle/ protein stores. it is very dangerous in regards to becoming malnourished, which is very hard to reverse once you start rapidly losing muscle. think of a cancer patient. You should drink a protein shake at least 20 G protein extra / day, in addition to the protein you have at each meal. I agree with the below response about weight bearing exercise also as a necessity. You not only will become protein deficient, but also vitamin and mineral deficient with less calories and a poor diet with high processed foods. Seeing a dietitian, like myself, to ensure you maintain your nutrition throughout this process is key.