The Study
Effects of Quercetin and Citrulline on Nitric Oxide Metabolites and Antioxidant Biomarkers in Trained Cyclists
This study gave different supplements to cyclists and measured changes in their blood, but it didn't prove that any supplement actually caused the changes — it just saw some small differences that might have happened by chance. It's like noticing your friend feels better after eating an apple, but you can't say the apple made them better without testing more people and being sure.
Analysis score
Maximum 90 for a randomized controlled trial.
Where the score came from
Scientists gave cyclists daily pills of quercetin, citrulline, both, or sugar pills for a month to see if they improved oxygen flow or reduced body stress.
Where does this study sit?
Reviews of RCTs (Meta-analyses)
Max 100Randomized Trials
Max 90Reviews of Cohort Studies
Max 85Cohort Studies
Max 72Reviews of Case-Control Studies
Max 63Case-Control Studies
Max 58Cross-Sectional & Case Series
Max 50Expert Opinion
Max 575 / 100
Quality score
Participants are randomly assigned to treatment or control groups, minimizing bias. The gold standard for testing whether an intervention causes an effect.
Key takeaways
Summary
Based on the study abstract and findings.
- 1The drop in FRAP from quercetin is unexpected and may mean it’s not helping as expected—possibly even reducing natural defenses.
- 2The lack of nitric oxide boost means these supplements didn’t improve blood flow as theory predicted.
- 3Cyclists who took quercetin had lower antioxidant levels (FRAP) than those who took sugar pills.
- 4No group had higher nitric oxide, SOD, or overall antioxidant levels than the sugar pill group.
Score breakdown, methodology, conflicts of interest, evidence analysis & raw study data
Publication
Journal
Nutrients
Year
2025
Authors
Jennifer A. Kurtz, Rafaela G. Feresin, Jake Grazer, Jeff Otis, Kathryn E. Wilson, J. A. Doyle, K. Zwetsloot
Related Content
Claims (6)
When citrulline is metabolized, it leads to increased nitric oxide production in blood vessel lining cells, which causes blood vessels to widen and improves oxygen delivery to skeletal muscle.
In trained cyclists, taking quercetin, citrulline, or both together does not change the activity of superoxide dismutase or the body's overall antioxidant capacity compared to taking a placebo.
In trained cyclists, taking 3 grams of citrulline twice daily for 28 days does not raise levels of nitric oxide metabolites in the blood compared to a placebo.
In trained cyclists, taking quercetin, citrulline, or both for 28 days does not change levels of nitric oxide metabolites, superoxide dismutase activity, or total antioxidant capacity compared to a placebo, even though nitric oxide metabolites increased slightly over time in some individuals.
Trained cyclists show higher levels of plasma nitric oxide metabolites after 28 days of any intervention, including placebo, suggesting that factors like training, seasonal changes, or when measurements are taken influence these biomarkers more than the intervention itself.
In trained cyclists, taking 500 mg of quercetin twice daily for 28 days is linked to a measurable decrease in systemic antioxidant capacity as measured by FRAP, compared to a placebo.
Not medical advice. For informational purposes only. Always consult a qualified healthcare professional before making health decisions.