are both high quality proteins found in milk. Despite being the two predominant proteins in milk, they have unique
characteristics. Two distinguishing features that may give whey an edge in terms of promoting muscle protein synthesis are its rapid absorption and higher leucine content. Researchers from the Netherlands tried to sort these effects out in head to head comparison of proteins. They had 48 healthy older men consume 20 grams of either whey, casein, casein hydrolysate while blood markers and muscle rates of protein synthesis were measured for the next 6 hours. Leucine content was 2.5 grams in whey and 1.7 grams in both casein proteins. There was no difference in muscle protein synthesis response between casein and casein hydrolysate. However, ingestion of a single bolus of whey increased muscle protein synthesis nearly two-fold more than the casein proteins. Plasma leucine levels were highest after whey, intermediate after casein hydrolysate, and lowest after casein. There was a significant association between plasma leucine levels and muscle protein synthesis. These findings provide evidence that the greater increase in muscle protein synthesis to ingestion of whey protein is attributed to its rapid absorption and its higher leucine content.
Pennings B, Boirie Y, Senden JM, Gijsen AP, Kuipers H, van Loon LJ. Whey protein stimulates postprandial muscle protein accretion more effectively than do casein and casein hydrolysate in older men. Am J Clin Nutr. 2011 Mar 2.