Optimal muscle strength and function lie at the heart of almost all athletic endeavors. However, within that broad context, considerable differences exist in the goals of training that are dependent on the particular sport. For example, in weight lifting, muscle size and strength are most important. In distance running, on the other hand, not only is endurance the major goal, but also increased muscle size is a detriment because of the extra body weight. Goals may differ even within a general sport category. Thus, weightlifting may be done for power lifting, which is judged by the amount of weight lifted; for body building competition, in which appearance is most important; or for conditioning for another sport, in which case the goal depends on the particular sport. Consequently, it must be recognized that the response of muscle protein to all possible types of training, as well as to the protein supplements in these various circumstances, may be diverse. Unfortunately, sufficient data do not exist to distinguish the diverse responses in different types of exercise. Furthermore, the responses of trained versus untrained subjects may differ, but current data are insufficient for resolving this issue also.
It is impossible to form a consensus position regarding the benefit of protein/amino acid supplements in exercise training. Apparently credible review articles have been written extolling the benefits of specific supplements,1 whereas others have dismissed the use of all protein/amino acid supplements as being "worthless."2 Determining whether supplements provide any benefit has probably been hampered by the failure to select appropriate endpoints for evaluation of a positive effect. However, studies focused at a more basic level have failed to agree on the response of protein metabolism in exercise, and thus the metabolic basis for protein. Therefore, it has been shown that whereas nitrogen balance may become negative on a fixed protein intake at the onset of an endurancetraining program,3 urea production is not stimulated during exercise, regardless of the level of protein intake.4
An often ignored but important complication of dietary studies is the level of energy intake. Thus, during training the effectiveness of a given amount of protein intake to spare endogenous protein will be affected by the level of energy intakes.5 Because of these and other complications, studies at the whole body level have not yielded a clear picture of the need for, or response to, dietary protein/ammo acid supplements. Consequently, this issue needs to be examined at the tissue level.
Both muscle protein breakdown and synthesis are increased in response to exercise in untrained subjects.6 Thus, muscle protein turnover is increased by exercise, and there may be a relation between muscle protein turnover and strength. Amino acid intake further stimulates muscle protein turnover after exercise. The effect of amino acids after exercise is greater than the stimulatory effect of amino acids on muscle protein synthesis when given at rest.7 These data suggest that not only may the exact composition and amount of a supplement be important, but also the timing of the intake of the supplement in relation to the performance of exercise factors must be considered in designing future studies to evaluate the efficiency of supplements.