Select Protein is a mixture of both whey protein isolate and casein protein. Select Protein is a Three Phase Protein Phase 1 is the pre digested leucine peptides which helps spike muscle protein synthesis. Phase 2 is the whey protein to give the body more essential amino acids to aid in new muscle proteins. Both the pre digested leucine peptides and the whey digest relatively quickly. Phase 3 is the slower digesting casein protein, which helps protein synthesis continue around the clock!