Anti-Inflammatory, Antithrombotic and Antioxidant Efficacy and Synergy of a High-Dose Vitamin C Supplement Enriched with a Low Dose of Bioflavonoids; In Vitro Assessment and In Vivo Evaluation Through a Clinical Study in Healthy Subjects.

BACKGROUND/OBJECTIVES: Vitamin C is frequently used in several dietary supplements due to its proposed health-promoting properties, while phenolic compounds and especially flavonoids have been suggested to provide synergistic antioxidant and cardiovascular benefits. However, the specific interactions between these compounds and their individual contributions to biological activity remain underexplored. This study aimed to evaluate the antioxidant potential and anti-inflammatory and antiplatelet biological effects of a high-dose (1 g) vitamin C-low-dose (50 mg) bioflavonoid (VCF)-based supplement using both in vitro and in vivo approaches in human platelets. METHODS: Total phenolic content was quantified and antioxidant capacity was assessed using DPPH, FRAP, and ABTS assays and compared to individual phenolic standard compounds, including (simple phenolics like gallic acid, flavonoids like quercetin and catechin, and polyphenols like curcumin and tannin), and a standard supplement containing only high-dose vitamin C (VC). ATR-FTIR spectroscopy was used to assess molecular interactions between vitamin C and flavonoids. In vitro anti-inflammatory and antiplatelet activities of all supplements and standards were assessed by quantifying their IC(50) values against ADP, PAF, and thrombin-induced platelet aggregation. The in vivo evaluation of the efficacy and synergy of VCF supplement versus VC was achieved by a two-arm clinical study in healthy volunteers by quantifying their platelet reactivity, which was measured via EC(50) values on the aforementioned platelet agonists (PAF, ADP, and Thrombin) before (t = 0) and after receiving either solely VC or VCF supplementation for four weeks. RESULTS: From all phenolic standards, the flavonoids and especially a mixture of flavonoids (catechin + quercetin) showed higher in vitro antioxidant capacity and anti-inflammatory and antiplatelet efficacy, followed by polyphenols and then simple phenolics. The VCF supplement showed the most potent antioxidant capacity, but also the strongest anti-inflammatory and antiplatelet activities too, in comparison to the VC and the mixture of flavonoids, suggesting higher synergy and thus bio-efficacy as a result of the co-presence of flavonoids and vitamin C in this supplement. Platelet reactivity decreased over time for PAF and thrombin in both arms of the trial, but no significant differences were observed between treatment groups, suggesting that the number of flavonoids used was not sufficient to translate the in vitro findings to the in vivo setting. CONCLUSIONS: VC-containing supplements provide antioxidant, anti-inflammatory, and antiplatelet benefits, while the incorporation of flavonoids may provide synergistic health benefits, but more in vivo assessment is needed to fully evaluate the dose efficacy.