Casein is a source of protein derived from milk that is slowly absorbed into the body over several hours. Because Casein is pH sensitive, a gel similar consistency is formed in the stomach, which is why it takes time to break down the protein into amino acids – and you get a slow secretion. The important thing for those exercising who are keen to maintain or increase their muscle mass is not to end up in a catabolic state. This can easily happen at night, so a protein with a slow secretion rate is perfect to take as a good shake or pudding in the evening or when you know it will be until the next meal. Protein contributes to normal muscle mass.