Im 17, 5'7 and have a pretty decent physiche, i drink whey and i weigh 65kg (which i think is around 144 pounds) and am looking to build my muscle size up. Ive heard of weight gain but i am a bit clueless about its purpose. I have low bodyfat and have a high protein-low fat diet. Could some one give me a definition of what weight gain is and who should use it i would be very thankful 