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While whey and casein are packaged together in milk, when isolated these two proteins have different effects.
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Whey is higher in some amino acids like leucine and it has a much faster rate of digestion and absorption compared to casein.  Casein has a unique slow digestion because of slowed gastric emptying.  Thus the absorption of amino acids into the circulation is much slower compared to whey and other protein sources.  Based on these qualities it has been shown in younger subjects that whey has a greater stimulatory effect on muscle protein synthesis.  Older individuals have a blunted response to protein and thus it may be that the benefits of whey are even greater in the elderly.  Canadian researchers had elderly subjects perform a single leg resistance exercise protocol and then supplement with either whey or casein immediately after exercise.  Both drinks contain 20 grams of protein, but the whey drink had 2.6 grams leucine compared to 1.6 grams in the casein drink.  Muscle protein synthesis was calculated in the non-exercised leg and exercised leg for 4 hours after exercise.  Compared to casein, muscle protein synthesis was 65% greater in the non-exercised leg and 69% greater in the exercised leg.  Thus in elderly men whey promoted a considerably greater anabolic effect on muscle at rest and after resistance exercise.  

. Burd NA, Yang Y, Moore DR, Tang JE, Tarnopolsky MA, Phillips SM. Greater stimulation of myofibrillar protein synthesis with ingestion of whey protein isolate v. micellar casein at rest and after resistance exercise in elderly men.  Br J Nutr. 2012 Jan 31:1-5.