Essential Amino Acids for Muscle Protein Accretion

The stimulation of muscle protein synthesis is reliant on the adequate availability of amino acid (AA) precursors. The increase in circulating AA concentrations in the blood serves as a signal to initiate synthetic processes. A sufficient increase in blood AA is required to overcome a concentration gradient that normally favors the intracellular space. We have demonstrated that the rate of muscle protein synthesis is directly related to the rate of appearance of AA in the intracellular space. Muscle proteins comprise various essential and nonessential AA; however, only ingestion of the essential amino acids (EAA) is required to stimulate muscle protein synthesis. While EAA alone stimulate muscle protein anabolism, the effect is magnified when combined with exercise. Furthermore, the timing of ingestion in relation to exercise is important in achieving a maximal anabolic response. The benefits of free-form EAA and intact protein on exercise-induced muscle anabolism are discussed. Finally, the effects of aging must be considered in the optimal EAA formulation.