A little copy/pasta from the PES website:
When researchers at the world-renowned Mayo Clinic tested a combination of whey + casein protein, as found naturally in milk protein, they discovered that the two protein sources work together to build lean muscle.
100% whey protein in isolation only gets you so far... it can spike protein synthesis with an influx of amino acids, but quickly returns to baseline, leaving you where you started.
Enter casein, a slower-digesting protein that's typically consumed for its anti-catabolic properties as it promotes a steady release of amino acids over time. Casein actually results in more retention of ingested protein than whey!
In another published study, researchers found the combination of whey protein with casein protein resulted in a quick increase in protein synthesisalong with a sustained elevation in plasma leucine levels. Plasma leucine levels are a key dictator of muscle protein synthesis, and thus, a combination of whey protein and casein protein may serve to maximize muscle protein accumulation.
Whey protein hits the bloodstream at a fast rate, but it is casein protein that has been shown to create the strongest leucine balance. It only makes sense to consume them together.
Studies have repeatedly shown that milk may be the optimal post workout drink due to the unique delivery and interplay of milk protein's components: whey protein and casein protein, together.
In fact, the research shows that even if amino acid content of a protein is held constant, milk protein augments lean muscle accrual to a greater degree than fast-digesting hydrolyzed proteins due to the unique pattern of amino acid delivery that only milk protein can provide. Nature had it right all along: milk protein is king. Did I mention that Milk Protein Isolate is the first ingredient in our SELECT Protein?