Adequate proteins are critical for athletes because they need the amino acids to serve as building blocks for the formation of new proteins. Not all dietary proteins are equal in their biological effects. An emerging concept in protein quality is that digestion rate has a major impact on protein effects in the body. This has led to the classification of proteins as either slow or fast. Some proteins are digested slowly, whereas others are digested quickly and result in quite different effects. Prior work has shown that fast-digesting proteins like whey rapidly increase protein synthesis but the effect is short-lived. Casein on the other hand is a slow-digesting protein that inhibits protein breakdown and therefore can sustain long periods of an anabolic environment for muscle growth. Therefore it makes sense to combine both whey and casein to take advantage of their favorable effects on protein synthesis and protein breakdown. One recent study did just that. This study showed that a group of men who received a protein supplement consisting of both whey and casein had greater increases in lean body mass after 10 weeks of training than a placebo group or a group that just received whey protein. The study provides evidence for the concept of mixing proteins to maximize growth potential.

Kerksick CM, Rasmussen CJ, Lancaster SL, Magu B, Smith P, Melton C, Greenwood M, Almada AL, Earnest CP, Kreider RB. The effects of protein and amino acid supplementation on performance and training adaptations during ten weeks of resistance training. J Strength Cond Res. 2006 Aug;20(3):643-53.