Effects of Quercetin and Citrulline on Nitric Oxide Metabolites and Antioxidant Biomarkers in Trained Cyclists

Abstract

Background: Quercetin (QCT) and citrulline (CIT) have been independently associated with improved antioxidant capacity and nitric oxide (NO) production, potentially enhancing cardiovascular function and exercise performance. This study aimed to evaluate the combined and independent effects of QCT and CIT supplementation on NO metabolites and antioxidant biomarkers in 50 trained cyclists undergoing a 20 km cycling time trial (TT).

Methods: In a randomized, double-blind, placebo-controlled design, forty-two male and eight female trained cyclists were assigned to QCT + CIT, QCT, CIT, or placebo (PL) groups. Supplements were consumed twice daily for 28 days. Biochemical assessments included NO metabolites (nitrate/nitrite), ferric reducing antioxidant power (FRAP), superoxide dismutase (SOD) activity, and antioxidant capacity, measured pre- and post-TT.

Results: NO metabolites were significantly elevated post-supplementation (p = 0.03); however, no significant interaction effects were observed for NO metabolites, FRAP, SOD, or antioxidant capacity across the groups (p > 0.05). Post-hoc analyses revealed that QCT significantly reduced FRAP concentrations compared to PL (p = 0.01), while no significant changes in SOD or antioxidant capacity were found across any groups.

Conclusions: These findings suggest that combined and independent QCT and CIT supplementation did not significantly improve these biomarkers, suggesting that baseline training adaptations, supplementation timing, and individual variability may influence the efficacy of these compounds in enhancing exercise performance and oxidative stress markers. The ergogenic efficacy of QCT + CIT on antioxidant-related markers remains inconclusive.

Keywords: aerobic capacity; dietary supplements; endurance exercise; oxidative stress; redox signaling.

Figure 1

Plasma concentration of nitric oxide (NO) metabolites pre- to post-supplementation. Data are displayed as mean + SD. n = 47: QCT + CIT (n = 11), QCT (n = 13), CIT (n = 11), PL (n = 12).

Figure 2

Ferric reducing antioxidant power (FRAP) concentration pre- to post-supplementation. Data are displayed as mean + SD. n = 40: QCT + CIT (n = 9), QCT (n = 11), CIT (n = 9), PL (n = 11).

Figure 3

Superoxide dismutase (SOD) concentration pre-to-post supplementation. Data are displayed as mean + SD. n = 20: QCT + CIT (n = 6), QCT (n = 4), CIT (n = 6), PL (n = 4).

Figure 4

Antioxidant capacity concentration (AOC) pre-to-post supplementation. Data are displayed as mean + SD. n = 20: QCT + CIT (n = 6), QCT (n = 4), CIT (n = 6), PL (n = 4).