Vitamin E-sparing and vitamin E-independent antioxidative effects of the flavonol quercetin in growing pigs

The aim was to investigate in vivo the antioxidative effects of the flavonol quercetin with special emphasis on interactions with vitamin E (αTOC) in growing pigs. It was assumed that any effects of quercetin on the antioxidant status would be more pronounced at low αTOC intake and that feeding diets with the addition of fish oil would induce an oxidative stress under these conditions. After a 5 week depletion period, in which all pigs (n = 26) were fed a low αTOC (7 mg/kg) diet, animals were assigned to one of 4 dietary groups (n = 6 or 7) according to a 2 × 2 factorial completely randomized design for four additional weeks. Factors consisted of (1) dietary supplementation of quercetin (0 or 10 mg quercetin/kg body weight/d) and (2) supplementation of fish oil (0 or 50 g/kg diet). The αTOC and flavonol concentrations in plasma and tissue as well as plasma thiobarbituric acid reactive substances (TBARS) and 8-iso-prostaglandin-F2α (8-iso-PGF2α) in plasma were measured. At low vitamin E intake, supplementation of quercetin tended to elevate plasma (0.42 vs. 0.54 μg/mL; P=0.063) and liver (0.16 vs. 0.24 μg/g; P=0.06) αTOC concentrations with no influence on the concentrations of markers of lipid peroxidation (TBARS, 8-iso-PGF2α), indicating a αTOC-sparing effect of quercetin. An additional αTOC-independent antioxidative effect of quercetin was not obvious under these conditions. However, further challenging the antioxidative mechanisms by the addition of fish oil at low αTOC intake resulted in a further reduction (P<0.05) of αTOC concentration in plasma (0.65 vs. 0.31 μg/mL) which could not be prevented by quercetin at least at the dose fed. However, the concomittant increase in lipid peroxidation indicated by elevated (P<0.05) concentrations of TBARS and 8-iso-PGF2α was ameliorated (P<0.05) by quercetin, suggesting a αTOC-independent antioxidative effect. Taken together, depending on the αTOC status, quercetin revealed a αTOC-sparing as well as αTOC-independent antioxidative effect at low αTOC intake.