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Whey and casein are the two principle proteins in milk. Both are very high quality proteins containing all the essential amino acids. But when taken individually, they have been shown to promote different effects.
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 Based on studies done with no exercise, whey has been described as a fast protein whereas casein is a slow protein. In the context of exercise, the few studies that have compared whey to casein head to head after resistance exercise have shown equivocal results. Using an improved methodology to label milk proteins and measure muscle protein synthesis, Danish researchers set out to directly compare the effects of whey and casein after a single bout of resistance exercise. Subjects performed a bout of resistance exercise and immediately after they consumed no food (control), or 20 grams of either whey protein or casein caseinate. Compared to controls, both whey and casein increased muscle protein synthesis. When examined over a 6 hour period after exercise, both whey and casein showed similar increases in muscle protein synthesis. However, during the first half of this 6 hr recovery window whey was 25% higher than casein, but during the second half casein was 32% higher than whey. This is consistent with the fast acting effects of whey and the slower but longer lasting effects of casein. The findings suggest that whey promotes a rapid and large increase in muscle protein synthesis but it's short-lived, compared to a more moderate and sustained effect of casein. Over 6 hours they tend to equal out. It is tempting to speculate that combining protein sources, perhaps a 50:50 whey to casein ratio (milk is 20% whey and 80% casein) would promote an ideal response.

  .    Reitelseder S, Agergaard J, Doessing S, Helmark IC, Lund P, Kristensen NB, Frystyk J, Flyvbjerg A, Schjerling P, van Hall G, Kjaer M, Holm L. Whey and casein labeled with L-[1-13C]leucine and muscle protein synthesis: effect of resistance exercise and protein ingestion. Am J Physiol Endocrinol Metab. 2011 Jan;300(1):E231-42.