Background and Objective: Nitric oxide (NO) precursor supplements have garnered significant interest among athletes and are commonly incorporated into the formulation of many pre-workout supplements. It is believed that these supplements may enhance athletic performance by increasing nitric oxide levels through both enzymatic and non-enzymatic pathways. Therefore, the synergistic effect of combining the supplements that elevate nitric oxide levels may have a significant impact on exercise performance. This study aimed to investigate the acute effect of a combination of NO precursors, comprising beetroot juice, L-arginine, and L-citrulline, on serum nitric oxide levels and exercise performance in trained men.
Methods: In a double-blind, placebo-controlled, crossover trial, 13 trained male athletes were recruited. Participants were randomly assigned to receive either the supplement (beetroot juice [240 mgr nitrate ≈ 4 mmol], 3 g L-arginine, and 3 g L-citrulline) or placebo before an exhaustive exercise test on separate sessions. Blood samples were collected at three time points from antecubital vain immediately before supplementation, before the exercise, and after the exhaustive exercise. Serum levels of nitric oxide and lactate in addition to some measures of exercise performance were assessed. Data were analyzed using repeated measures analysis of variance (ANOVA) and paired t-tests at a significance level of p < 0.05.
Results: data indicated that serum Nitric oxide metabolites(NOx) levels increased significantly following supplementation with the combination of NO precursors compared to the placebo (from 44.25 µmol/L to 163.25 µmol/L upon supplementation; from 47.83 µmol/L to 57.50 µmol/L upon placebo intake; p = 0.001) and remained significantly elevated after the exhaustive exercise (from 163.25 µmol/L to 172.08 µmol/L upon supplementation; from 57.50 µmol/L to 70.83 µmol/L upon placebo intake; p = 0.001). Serum lactate levels also increased following the exhaustive exercise (from 18.76 mg/dl to 83.76 mg/dl with supplementation; from 13.29 mg/dl to 80.84 mg/dl with placebo); however, supplementation had no significant effect on lactate response to exercise (p > 0.05). Average heart rate and rate of perceived exertion increased by exercise protocol (p = 0.001). Regarding performance measures, average heart rate (145.89 vs. 151.60) and ratings of perceived exertion (15.38 vs. 16.07) were significantly lower with the supplement condition compared to the placebo (p = 0.001). However, no significant difference was observed in the time to exhaustion (p > 0.05).
Conclusion: Acute supplementation with a combination of NO precursors significantly increased serum nitric oxide metabolites concentration and slightly reduced the subjective perception of fatigue during exhaustive exercise. Nevertheless, it had no significant effect on time to exhaustion during the exhaustive exercise test. Therefore, based on the current data, an acute increase in serum nitric oxide alone seems insufficient to significantly delay exhaustion in trained men.