Acute effects of l-arginine intake on heart rate variability after a submaximal exercise test in healthy men: randomized clinical trial

l-arginine (L-ARG) is a semi-essential amino acid and a precursor for nitric oxide (NO) synthesis via endothelial nitric oxide synthase (eNOS). While NO has been implicated in autonomic modulation and vascular function, the effects of acute L-ARG intake on heart rate variability (HRV) after exercise remain unclear. This study aimed to assess the acute effects of L-ARG supplementation on HRV and cardiovascular recovery following submaximal aerobic exercise in healthy men. In a triple-blind, randomized, placebo-controlled, crossover trial, 37 physically active males (18-30 years) underwent two treadmill exercise protocols: one following ingestion of 3 g of L-ARG and the other with placebo. HRV and cardiovascular parameters (heart rate, systolic and diastolic blood pressure) were measured at baseline and across 20 min of post-exercise recovery. Time- and frequency-domain HRV indices were analyzed using validated algorithms. Two-way repeated measures ANOVA and post-hoc tests were applied (p < 0.05). Significant time effects were observed for HRV and cardiovascular variables across both protocols (p < 0.001), indicating physiological recovery. However, no statistically significant differences were found between the L-ARG and placebo conditions for any HRV or hemodynamic outcome. A trend toward faster vagal reactivation (rMSSD) was observed with L-ARG, but effect sizes were small and not clinically relevant. Acute supplementation with 3 g of l-arginine did not significantly influence HRV or cardiovascular recovery following submaximal aerobic exercise in healthy young men. These findings suggest limited autonomic effects of L-ARG in populations with high baseline HRV.