i think the last reports is that if you dont eat protein and do not exercise, 1/3 of every pound you lose is muscle.
Protein is also vital for an easier transition as well as healthy fats.
Protein is responsible for skin health and hair as well as the facilitation of breast development. Also, protein is very important for nail development (as finger and toe nails are pure protein).
If you are about 160 lbs you should be eating about 70 grams of protein a day. The heavier you are, the more protein you need to take in. For example, at 200 lbs you need 90 grams of protein per day to keep up with vital functions.
If you do not eat a lot of protein, your skin, hair, nails, breasts, muscle tone will suffer greatly.