Although there are many hormones that directly and indirectly affect protein turnover (e.g., insulin, cortisol, testosterone, growth hormone, and insulin-like growth factor), only testosterone and insulin will be discussed here. Testosterone is of interest because of the significant controversy surrounding its unethical use in sporting events and its potent effects on protein metabolism. For many years testosterone was assumed to possess stimulatory effects on net protein synthesis, based on observations of male/female differences in lean mass as well as the increases noted for those who supplemented with pharmacological doses. Consequently, proper investigations into the metabolism and efficacy of testosterone administration followed.57 124-127 Even without resistance exercise, testosterone administration can increase lean body mass,124 126 127 and a resistance exercise training program can magnify these effects.127 At the muscle level, testosterone acts by increasing protein synthesis and intracellular amino acid reutilization and not degradation.57 Given the fact that acute resistance exercise also increases plasma testosterone concentration,128 129 it will be important to conduct properly designed studies to compare the efficacy of an optimal nutritional intervention to that of exogenous testosterone supplementation. Another interesting finding with potential relevance to an athlete's enthusiasm for very high protein intakes is the apparent negative correlation between protein intake and plasma testosterone concentration.128
Another key hormone important in protein metabolism and a major factor in the efficacy of CHO-protein nutrition is insulin. Insulin has a net stimulatory effect upon muscle protein synthesis,130 primarily through a reduction in muscle proteolysis.102103107131-133 The effect of insulin on protein synthesis appears to depend on whether there is an abundance of amino acids.130 Several studies have failed to find a stimulation of insulin on muscle protein synthesis,102 103 132 134 which is likely due to the hypoaminoacidemia induced by insulin.130 When amino acids are provided simultaneously with insulin (to prevent hypoaminoacidemia), there appears to be a stimulation of protein synthesis.102 103 131-133 Other studies have found that hyperin-sulinemia stimulates both muscle FSR and amino acid transport.12 Finally, the effects of insulin on protein metabolism are different before and after resistance exercise.130 In the resting state, insulin induces a more positive protein balance by increasing synthesis and increasing amino acid transport; after exercise, there was no effect on synthesis, yet there was a significant reduction in degradation and a threefold increase in amino acid transport.130 These findings provide the theoretical basis for the provision of protein and CHO in the early post-exercise period in athletes performing resistance-type exercise, a finding recently confirmed.109
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