The two primary proteins in milk are casein and whey. Several highly publicized studies have shown that their metabolism and uptake characteristics vary in the human body. Whey is absorbed rapidly, peaking in the blood after about 90 minutes. Casein, on the other hand, is a slow-acting protein. It undergoes a type of clotting in the stomach that results in a more gradual release of amino acids into the blood.
Studies comparing absorption characteristics of whey and casein show that casein promotes a slow trickle of amino acids into the blood that lasts as long as seven hours. Amino acids in the blood blunt muscle-protein breakdown. The rapid absorption of whey leads to greater oxidation of absorbed amino acids in the liver and a higher rate of muscle protein synthesis (because of more immediate amino acid availability) but fewer anticatabolic effects than casein provides.
One important point about those celebrated milk protein studies is that they were done using nonexercising subjects during resting conditions. More recent research that studied exercising populations’both animal and human’show that supplying essential amino acids both before and after exercise potently promotes muscle protein synthesis, leading to gains in muscle size and strength.
But what about casein and whey? Since they get absorbed at different rates, would one prove superior to the other in promoting muscle gains after weight training? That was the precise focus of a recent study.1 Researchers gave one of three drinks to healthy volunteers one hour after they completed a leg extension workout. The three drinks consisted of:
1) 20 grams of casein
2) 20 grams of whey
3) A placebo, or inactive drink
To track the metabolic fate of the proteins, the authors measured the routes of two essential amino acids, leucine and phenylalanine, in the blood. As expected, both proteins caused a positive protein balance. Leucine levels peaked faster after the whey drink, but levels of phenylalanine didn’t. Since amino acids compete for absorption carriers in the body, leucine may have proved superior in that regard. At the start of the study both proteins showed similar uptakes for phenylalanine, but casein showed a higher level in the blood as time continued, which helps confirm casein’s slow-acting attributes.
On the other hand, at the conclusion of the study both proteins caused similar blood levels of phenylalanine. That led the authors to observe that from an anabolic perspective there were no significant differences between whey and casein intake following weight training. They also note that the only metabolic fate for phenylalanine is to be taken up by muscle for use in protein synthesis, while leucine, a branched-chain amino, can be metabolized in muscle for energy. The phenylalanine levels are a more accurate indicator than leucine of muscle anabolic response. The rapid absorption of whey also leads to more rapid leucine oxidation; the authors calculated negligible leucine oxidation from casein but a 57 percent oxidation from whey.
Of the two amino acids examined in the study, only phenylalanine is used solely in muscle-protein synthesis. Since the fate of phenylalanine was similar for both whey and casein, the authors conclude that both proteins are equally effective in promoting increased muscle protein synthesis after weight training.
1 Tipton, K.D., et al. (2004). Ingestion of casein and whey proteins results in muscle anabolism after resistance exercise. Med Sci Sports Exer. 36:2073-81.