MET-Rx Ready To Drinks are made from milk protein. Milk protein comes from two main sources, casein & whey. Whey is the most bio-available protein in the world. It's fast acting & supplies the most important aminos in large amounts. Casein on the other hand is a slower acting protein that contains other essential aminos, providing the base of your body's amino acid pool. MET-Rx RTDs provide a balance of the two. The casein influences the whey to act slower & stay active in the body longer. MET-Rx RTDs are enriched with vitamins & minerals.