It has been hypothesized that L-arginine improves exercise performance by increasing nitric oxide synthesis and levels of insulin and growth hormone (GH). Metabolic and hormonal responses to chronic L-arginine supplementation may clarify the mechanisms underlying its putative physiologic effects on physical performance. Therefore, the aim of this study was to investigate the effects that 4 weeks of supplementation with L-arginine would have on metabolic and hormonal parameters at rest and in response to exercise. Fifteen healthy runners were divided into treatment (ARG; 6 g L-arginine) and placebo (PLA; 6 g cornstarch) groups. On the first visit, blood samples were collected for baseline, and the supplement or placebo was provided. After 4 weeks of supplementation (second visit), blood samples were collected at the following intervals: at rest, immediately after the first 5-km time-trial running test (5km-TT), immediately after the second 5km-TT, and after 20 minutes of recovery (+20). In addition to exercise performance (total running time), plasma nitrate, nitrite, nitrate plus nitrite, cyclic guanosine monophosphate, lactate, ammonia and serum insulin, GH, insulin-like growth factor 1, and cortisol concentrations were evaluated. There were significant increases in plasma nitrite, cyclic guanosine monophosphate, lactate, ammonia and serum GH, and cortisol at the first 5km-TT, immediately after the second 5km-TT, and +20 in both ARG and PLA. Nitrate plus nitrite and nitrate increased only at +20. No significant change was observed in serum insulin and insulin-like growth factor 1 in any sample period. Total running time did not differ significantly between the 2 tests, in either ARG or PLA. Thus, according to our results, 4 weeks of L-arginine supplementation did not cause beneficial changes in metabolic and hormonal parameters, beyond those achieved with exercise alone.
Keywords: +20; 5-km time-trial running test; 5km-TT; ADMA; ARG; Amino acids; Exercise; GH; HPLC; Healthy adult runners; Hormones; IGF-1; IP-1; IP-2; NO; NOx; Nitric oxide; Nutritional supplements; PLA; POST; PRE; TRT; after 20-minute recovery; after 4 weeks; after the second 5km-TT; asymmetric dimethylarginine; baseline measurements; cGMP; cyclic guanosine monophosphate; eNOS; endothelial nitric oxide synthase; first 5km-TT; growth hormone; high-performance liquid chromatography; insulin-like growth factor 1; l-arginine–supplemented group; nitrate plus nitrite; nitric oxide; placebo group; total running time.
© 2013.