Whey and casein are proteins both derived from milk. While each type of protein offers a different amino acid profile, they’re also digested at different rates in the body. Whey is digested fast, while casein takes a lot longer, and therefore, considered a slow-digesting protein. Research has shown that whey can spike protein synthesis (muscle building), but only for a short time. On the other hand, casein has been shown to inhibit protein degradation (muscle breakdown) over a long period of time. This has led many bodybuilders to combine the two for better results in muscle growth – a practice that recent research also supports.

In a 10-week study conducted at Baylor University, researchers split 36 males randomly into 3 groups, who were given a different supplement prior to weight training. Group 1 received a placebo; Group 2 consumed 40 g of whey and 8 g of casein; while Group 3 ingested 40 g of whey, 3 g of branch chain amino acids and 5 g of glutamine. The results showed that Group 2 had the greatest increase in fat-free mass.

J Strength Cond Res. 2006 Aug; 20(3):643-53