High quality proteins contain all of the amino acids in their correct ratios for growth and metabolism. In this regard 20 to 25 grams of high quality protein contains around 8 to 10 grams of essential amino acids. Some of these essential amino acids are the branched chain amino acids, which are pivotal in stimulating muscle growth. Consuming high quality protein post-exercise is known to cause skeletal muscle hypertrophy, by stimulation of the mammalian target of rapamycin (mTOR) signal pathway. However, it is unclear as to the best source of protein for this purpose. Whey protein contains a high concentration of branched chain amino acids, and is rapidly absorbed, but the speed of absorption causes the plasma amino acid concentrations to return to baseline within 2 to 3 hours. In contrast, more slowly digested proteins such as casein produce a more prolonged rise in plasma amino acids that can remain elevated for around 6 hours. Soy protein is considered to have an intermediate rate of digestion between whey and casein.
Researchers have investigated the ability of high quality proteins to stimulate mTOR as a marker of skeletal muscle growth. For example in one study, researchers fed healthy human subjects either 19 grams as a protein blend or 18 grams of whey protein following intense leg resistance training1. The protein blend contained 50 % of its protein from sodium caseinate, 25 % from whey protein isolate and 25 % from soy protein isolate. Both proteins contained around 9 grams of essential amino acids. As has been shown previously, the whey protein resulted in a larger and earlier post-exercise rise in plasma branched chain amino acids. In contrast, the protein blend caused a slower and more gradual increase in plasma levels of branched chain amino acids, but these levels remained elevated for longer. The skeletal muscle fractional synthetic rate was similar for both the whey protein and the protein blend, but the protein blend sustained the fractional synthetic rate for longer. The mTOR signally increased to a similar amount in both groups.
These results support previous studies in that they show a muscle growth effect for post-exercise protein. In this regard whey protein is more rapidly absorbed that other forms of protein and this results in more rapid rises in plasma levels of amino acids. However, the actual stimulation of skeletal muscle growth may not be greater in whey protein as the rises in plasma amino acids are not sustained. Mixing several proteins with different characteristics provides a more consistent elevation of plasma amino acids which provides a similar stimulation of the fractional synthetic rate of skeletal muscle, but only the blend of proteins was able to elevate S6K1 phosphorylation in a prolonged way (S6K1 is a kinase thought to be involved in muscle building). Of course it could be argued that repeated ingestion of whey protein may offer a solution to its more rapid absorption and utilisation rates and this is common practice amongst strength athletes. Combination of whey protein with other food also delays its absorption and prolongs plasma levels of amino acids.