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Whey and casein are popular protein supplements. Although they are both derived from milk, they have different amino acid compositions and behave differently in the body. The biggest difference relates to digestion rate. Whey is rapidly digested and absorbed. It is associated with a faster increase in protein synthesis but it is transient. Casein on the other hand is slowly digested and tends to have a better effect on protein balance over long periods of time partly by decreasing protein breakdown. Another difference is the higher leucine content of whey which is about two times greater than casein. Leucine is the most important amino acid for augmenting protein synthesis. A recent study by German researchers directly compared whey versus casein in a group of healthy men who were studied after 2 weeks of bed rest. They received on two separate visits either 0.4 g/kg of whey or casein in the fasted state and measures of protein synthesis were made for 6 hours after ingestion. Whey resulted in a two-fold greater increase in plasma leucine than casein with values peaking about 90 min after intake. The net increase in protein synthesis over the 6 hours was about 15% greater in the whey trial compared to casein. Although casein was effective at stimulating protein synthesis, this study shows that whey has a modest advantage when studied in healthy men after 2 weeks of bed rest. The findings are relevant to athletes during their lay off periods when they are physically inactive, and show whey protein may be the best choice to maintain muscle mass during those periods of rest.

Antonione R, Caliandro E, Zorat F, Guarnieri G, Heer M, Biolo G. Whey protein ingestion enhances postprandial anabolism during short-term bed rest in young men. J Nutr. 2008 Nov;138(11):2212-6.