well, I've never viewed the intestinal lining as something that renders slow and fast absorbing protein sources equal in terms of how they act once theyre through. theres a lot going on with a slower-digesting protein after it reaches plasma. this study shows pretty clearly how casein made it through and behaved very differently from whey, stating that when the casein was studied from the plasma, they observed its breakdown was markedly inhibited compared to whey:
"For example, one study looked at both casein and whey protein absorption and the subsequent metabolic effects.iii In this study two labeled milk proteins, casein (CAS) and whey protein (WP), of different physicochemical properties were ingested as one single meal by healthy adults and postprandial whole body leucine kinetics were assessed.
WP induced a dramatic but short increase of plasma amino acids. CAS induced a prolonged plateau of moderate hyperaminoacidemia, probably because of a slow gastric emptying. Whole body protein breakdown was inhibited by 34% after CAS ingestion but not after WP ingestion. Postprandial protein synthesis was stimulated by 68% with the WP meal and to a lesser extent (+31%) with the CAS meal.
Under the conditions of this study, i.e., a single protein meal with no energy added, two dietary proteins were shown to have different metabolic fates and uses. After WP ingestion, the plasma appearance of dietary amino acids is fast, high, and transient.
This amino acid pattern is associated with an increased protein synthesis and oxidation and no change in protein breakdown. By contrast, the plasma appearance of dietary amino acids after a CAS meal is slower, lower, and prolonged with a different whole body metabolic response: protein synthesis slightly increases, oxidation is moderately stimulated, but protein breakdown is markedly inhibited."
