100% Casein Protein where sometimes slower is better. especially when it comes to the rate of protein digestion. While rapid protein absorption is desirable immediately before or after exercise, delayed release is probably more beneficial throughout the remainder of the day. Time release muscle protecting proteins Optimum Nutrition 100% Casein proteins are pH sensitive and gel in the acidic environment of the stomach. As a result, it can take more than twice as long for ON 100% Casein Proteins to be broken down into their amino acid subcomponents than whey and other proteins, because of these unique time-released qualities, casein proteins are aptly described as being anti-catabolic, or muscle-protecting, protein.