Whey and casein are two of the most popular protein sources for athletes. Although they are both considered milk proteins, intense research over the last several years has shown they have very distinct qualities. The biggest difference relates to how fast they are digested.
Whey is rapidly digested and absorbed and therefore provides a fast way to supply amino acids to the body. Just like sugar provides quick energy, whey provides amino acids in a burst. Casein, on the other hand, is slowly digested and provides a more steady release of amino acids.
Whey and casein also differ in respect to their amino composition. Whey contains about two times more leucine than casein. Leucine is the key amino acid that triggers protein synthesis.
A recent study by German researchers directly compared whey versus casein in a group of healthy men. Instead of studying whey and casein after exercise, these researchers wanted to know how they behaved independent of exercise. So they studied the men after 2 weeks of bed rest to simulate an extended period of physical inactivity. They received on two separate visits either 0.4g/kg of whey or casein protein (about 32g for a 175 pound person) in the fasted state. Protein synthesis measurements 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 minutes after intake. The net increase in protein synthesis over the 6 hours was 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 that whey protein may be the best choice to maintain muscle mass during 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.