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Casein and Whey Retain Their Unique Effects When Consumed Together
The concept that whey and casein, when consumed separately, represent fast and slow proteins is well established.  Whey increases muscle protein synthesis rapidly but the effect is short-lived, whereas casein delivers a lower but more sustained anabolic effect.  What has not been tested is whether these unique qualities of whey and casein remain when they are consumed together. 
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In order address this issue, researchers developed a rather elegant and sophisticated method whereby they infused lactating cows with a stable (not radioactive) isotope so that the milk the cows produced naturally contained the tracer in the whey and casein.  In other words the whey and casein contained a tracer that came directly from the cow as opposed to synthetically adding it to milk already produced.  They then isolated whey and casein and made a liquid formula containing half casein and half whey.  Thus when subjects consumed the liquid concoction, the researchers could study the metabolism of both whey and casein separately even though they were consumed together.  Young healthy men and women then were fed 58 grams of protein containing equal amounts of the labeled whey and casein protein fractions so that they could be tracked separately.  The study showed that whey and casein retained their fast and slow digestion profiles when consumed together.  The more gradual digestion of casein resulted in more sustained elevations in circulating amino acids.  Whereas early on both whey and casein promoted similar increases in muscle protein synthesis, at four hours after intake, the amino acids from casein were being retained in muscle at double the rate as those from whey.  This study shows that different protein sources can be mixed together and they still retain their distinct attributes in respect to speed of digestion and effects on protein synthesis.

   . Soop M, Nehra V, Henderson GC, Boirie Y, Ford GC, Nair KS.  Co-ingestion of whey protein and casein in a mixed meal - demonstration of a more sustained anabolic effect of casein. Am J Physiol Endocrinol Metab (May 8, 2012).