Protein is constructed from amino acids, and whey protein is very high in the branch-chain amino acids leucine, isoleucine and valine. The three branch-chain amino acids are all essential, constituting 40 percent of the minimum daily requirement of indispensable dietary amino acids. Leucine, in particular, stimulates insulin secretion. There also is evidence that branch-chain amino acids don't increase work capacity during exercise. Whether or not branch-chain amino acids can improve immediate performance, they do seem important for building muscle.
Muscle protein is particularly high in branch-chain amino acids.
The availability of amino acids immediately after exercise seems to be of the greatest importance for muscle development. One scientific review of the literature suggests that recommended daily allowances of protein should be increased 50 to 100 percent for people who exercise regularly, and that optimal intakes may be even higher. Even if physical work capacity during exercise is not much improved by branch-chain amino acids , those same amino acids have produced 15 percent lower ratings of mental fatigue. Fewer branch-chain amino acids enter the brain during exercise through the large neutral amino acid carrier. Therefore, the carrier transports larger amounts of the amino acid tryptophan, leading to more brain serotonin synthesis and, hence, more mental fatigue.