Protein is essential for the building and repair of every part of the body, especially muscles. Milk contains two very good sources of protein, namely whey and casein. These two proteins together contain all the essential amino acids. These are the building blocks of protein and the body cannot make them, so they have to be obtained by means of the food we eat. Whey and casein are readily broken down into the individual amino acids, making it easy for the body to utilize them efficiently in repairing and rebuilding, which is a daily process.