Exercise, protein metabolism, and muscle growth

Exercise has a profound effect on muscle growth, which can occur only if mu scle protein synthesis exceeds muscle protein breakdown; there must be a po sitive muscle protein balance. Resistance exercise improves muscle protein balance, but, in the absence of food intake, the balance remains negative ( i.e., catabolic). The response of muscle protein metabolism to a resistance exercise bout lasts for 24-48 hours; thus, the interaction between protein metabolism and any meals consumed in this period will determine the impact of the diet on muscle hypertrophy. Amino acid availability is an important regulator of muscle protein metabolism. The interaction of postexercise me tabolic processes and increased amino acid availability maximizes the stimu lation of muscle protein synthesis and results in even greater muscle anabo lism than when dietary amino acids are not present. Hormones, especially in sulin and testosterone, have important roles as regulators of muscle protei n synthesis and muscle hypertrophy. Following exercise, insulin has only a permissive role on muscle protein synthesis, but it appears to inhibit the increase in muscle protein breakdown. Ingestion of only small amounts of am ino acids, combined with carbohydrates, can transiently increase muscle pro tein anabolism, but it has yet to be determined if these transient response s translate into an appreciable increase in muscle mass over a prolonged tr aining period.