The Quercetin Paradox, Evaluation of its Therapeutic Efficacy as a Nutraceutical in Clinical Trials: Evidence and Speculation.

Introduction: Quercetin, a dietary flavonoid typically consumed as a glycoside, is available in supplements in its aglycone form. This aglycone form is the most studied in preclinical and clinical research because of its antioxidant, anti-inflammatory, and metabolic properties. Several clinical trials have demonstrated the benefits of quercetin for enzymatic, metabolic and cardiovascular health, approximately 60% of these studies fail to verify the actual concentration administered, generating uncertainty on their findings. Objective: This study aimed to evaluate the concentration, stability, and cost-concentration of commercial quercetin nutraceutical formulations. Methodology: The concentrations of five commercial brands were quantified by high-performance liquid chromatography. Stability was assessed over 6 months at both room temperature and 40°C. Results: Only one brand contained the amount labeled; three contained approximately 80%, and one contained merely 14.8%. This study is the first to evaluate the long and thermal stability of quercetin in nutraceutical products, demonstrating that the active ingredient remains stable for at least 6 months. Brand C was identified as the most suitable option. Conclusion: There was significant variability in the quercetin concentration of commercial supplements. These findings underscore the need for stricter regulatory oversight to ensure the quality, therapeutic efficacy, and safety of nutraceutical products.