Can L-arginine Influence the Acute Hormonal, Metabolic, and Physiological Responses at Rest and Prior to Exercise?

Abstract

L-arginine is a conditionally essential amino acid and has recently been purported as ergogenic for both strength and aerobic athletes; however, the value of oral L-arginine supplementation in physically active participants is controversial. The purpose of this dissertation was threefold: first to examine the hormonal and metabolic responses of low vs. high relative doses of oral L-arginine at rest; second to determine the GH response when L-arginine was ingested prior to a bout of whole-body resistance exercise in strength trained men; and third to evaluate the hormonal and metabolic responses when L-arginine was consumed prior to a bout of submaximal aerobic exercise in trained cyclists. Study 1: fourteen physically active men (age: 25 ± 5 y; body mass: 78.0 ± 8.5 kg; height: 179.4 ± 4.7 cm) volunteered to be in a randomized, double-blind, repeated measures design. Each subject was provided three treatment conditions (placebo: flour, low dose: 0.075 g•kg-1 or high dose: 0.150 g•kg-1 body mass of L-arginine). L-arginine plasma concentrations significantly increased to a similar concentration at any time point in both the low and high dose conditions, while there was no change over time in the placebo condition. There was no significant difference between conditions for plasma growth hormone (GH), nitrate+nitrite (NOx), insulin-like growth factor-1 (IGF-1), or insulin. Study 2: fourteen strength trained men (age: 25 ± 4 y; body mass: 81.4 ± 9.0 kg; height: 179.4 ± 6.9 cm) participated in a randomized crossover design. L-arginine (0.075 g•kg-1 body mass) or a placebo was ingested 60 minutes prior to performing an acute bout of resistance exercise (3 sets of 8 exercises, 10 repetitions at ~75% 1RM). There were no differences between conditions for GH, GH-releasing hormone (GHRH), ghrelin, or IGF-1 at any time point. GH-inhibiting hormone (GHIH) was significantly lower in the L-arginine condition. However; integrated area under the curve (iAUC) for GH was significantly blunted in the L-arginine condition. Study 3: fifteen aerobically trained men (age: 28 ± 5 y; body mass: 77.4 ± 9.5 kg; height: 180.9 ± 7.9 cm; training experience: 5.9 ± 3.4 y, VO2 max: 59.6 ± 5.9 ml•kg-1•min-1) participated in a randomized crossover design. L-arginine (0.075 g•kg-1 body mass) or a placebo was ingested prior to performing an acute bout of submaximal aerobic exercise. There were no difference between conditions for GH, non esterified fatty acids (NEFA), lactate, glucose, VO2, VCO2, RER, and NOx. However, there was a small but significant elevation in plasma glycerol at the 45 minute time point after L-arginine consumption. In summary, L-arginine consumed orally at rest was effective at increasing plasma L-arginine. L-arginine prior to resistance exercise attenuated plasma GH compared to resistance exercise alone. Lastly, L-arginine before aerobic exercise did not enhance GH, glucose, lactate, NOx, or any cardio-respiratory parameters; however there was a small but significant increase in glycerol during exercise. In conclusion, L-arginine is effective at increasing L-arginine plasma concentrations; however the hormonal and metabolic responses are small and further research is required to examine the ergogenic potential.