Effect of dietary quercetin on oxidative DNA damage in healthy human subjects

The effect of dietary intake of flavonols (predominantly quercetin) on oxid ative DNA damage was studied in thirty-six healthy human subjects (sixteen men, twenty women). The study was a randomised crossover study, comprising two 14 d treatments of either a low-flavonol (LF) or high-flavonol (HF) die t with a 14 d wash-out period between treatments. Subjects were asked to av oid foods containing flavonols, flavones and flavanols during the LF dietar y treatment period and to consume one 150 g onion (Allium cepa) cake (conta ining 89.7 mg quercetin) and one 300 ml cup of black tea (containing 1.4 mg quercetin) daily during the HF dietary treatment. A 7 d food diary was kep t during each dietary period and blood samples were taken after each dietar y treatment. Products of oxidative damage to DNA bases were measured in DNA from leucocytes. The study had more than 95% power to detect a change of 2 0% in DNA damage products (P < 0.05). Plasma vitamin C and plasma quercetin concentrations were also measured. No significant differences in intake of macronutrients or assessed micronutrients, measured DNA base damage produc ts, or plasma vitamin C were found between the HF and LF dietary treatments . The plasma quercetin concentration was significantly higher after the HF dietary treatment period (228.5 (SEM 34.7) nmol/l) than after the LF dietar y treatment period (less than the limit of detection, i.e. <66.2 nmol/l). T hese findings do not support the hypothesis that dietary quercetin intake s ubstantially affects oxidative DNA damage in leucocytes.