I agree with the protein shakes, yes you need alot of protein if you want to gain muscle mass and not fat, but you also need do weight trainer exercise in order to gain the muscle or else one of two things will happen, 1. if you eat alot of protein and not exercise you will gain weight in fat since protein consumed without exercise in the end will be no more than extra calories, or 2. if you have a high metabolism and consume excess protein and no exercise you will not gain weight,you will stay the same. This has happen to me and my boyfriend. I insist on taking my whey protein shakes everyday since I know i need it to maintain my muscle so I can get my metabolism up so I can lose my extra weight or keep it off but on weeks that I do not do any weight exercise I put on some extra pounds and my body fat goes up. My boyfriend on the other hand is on the skinny side and has a very high metabolism, he tried to gain weight using protein gainers along with making sure he was taking in 4000 to 5000 calories daily, but since he never got up to do any heavy exercise like I had insisted, He stayed the same he did not gain one pound nor did his mass or fat percentages changed. If you are naturally very thin you have to put in alot of effort to gaining the good type of weight making sure you are getting 3500+ calories (not empty calories aka, junk food or fats), atleast 1 gram of protein per pound you have, and plenty of weight training. In the end if you can't gain weight just behappy being thin, trust me you dont want to be on the opposite side of the spectrum.