In trained cyclists, 28 days of supplementation with quercetin, citrulline, or their combination has no significant effect on plasma nitric oxide metabolites, superoxide dismutase activity, or overall antioxidant capacity relative to placebo, despite a non-group-specific increase in nitric oxide metabolites over time.

In trained cyclists, the body already maintains high levels of nitric oxide and antioxidant defenses from regular training. Adding quercetin or citrulline does not increase these further because the enzymes and pathways that make nitric oxide and antioxidants are already working at full capacity. Any small rise in nitric oxide over time comes from training, not supplements. The body also balances oxidants and antioxidants tightly, so extra antioxidants from supplements don’t change the overall levels because the system adjusts to keep everything stable.