Flavonoids are of great interest for their antioxidant and health-promoting activities. Ascorbate (vitamin C) has antioxidant activities but also sometimes displays pro-oxidant activities <i>in vitro</i> and reportedly <i>in vivo</i>. This research investigated to what extent flavonoids moderate oxidative stress from vitamin C <i>in vivo</i>.<p>Dietary experiments were conducted in two phases using adult male Wistar rats. First, all animals were maintained for two weeks on a control flavonoid-free diet with the dietary requirement (27 IU) of vitamin E/kg diet. In the subsequent four weeks, the animals were treated in four groups (8 rats/group), being fed the following diets: flavonoid-free control (C), ascorbate-supplemented (7.55 mmol/kg diet) (A), flavonoid-supplemented (2.67 mmol/kg diet) (F) and flavonoids (2.67 mmol/kg diet) plus ascorbate (7.55 mmol/kg diet)-supplemented (T). Measurements were done on in vivo biomarkers of oxidative stress, tissue antioxidants and on tissue in vitro susceptibility to oxidative stress.<p>In the combined feeding of ascorbate plus flavonoids, endogenous thiobarbituric acid reactive substances (TBARS) increased in liver by 114%. No effects of dietary ascorbate or flavonoids were seen on endogenous TBARS in brain or heart, or on plasma thiols or erythrocyte fragility.<p><i>In vitro</i>, the susceptibility to TBARS formation of liver homogenate (incubated for 60 min at 37ºC in air) showed a significant 60% increase in ascorbate-fed animals compared to control, but no increase in animals fed ascorbate plus flavonoids, suggesting that the additional feeding of flavonoids helped to prevent the increase produced by ascorbate-feeding. Incubation of liver mitochondria with 300 µM ascorbate in vitro produced a large (2-7 fold) increase in TBARS, but there was no difference among mitochondria from the different feeding groups.<p>The ability of flavonoid-feeding in protecting against oxidative stress from ascorbate in vivo could not be demonstrated in this study, even showing pro-oxidant effects of flavonoids in combination with ascorbate in liver. However, in vitro tests in liver suggest a protective effect of flavonoid-feeding against susceptibility to oxidative stress from ascorbate. Further investigations are needed in order to resolve the differences observed in vitro and in vivo and to determine the endogenous effects of specific flavonoids under ascorbate-induced oxidative stress.

Flavonoids are of great interest for their antioxidant and health-promoting activities. Ascorbate (vitamin C) has antioxidant activities but also sometimes displays pro-oxidant activities <i>in vitro</i> and reportedly <i>in vivo</i>. This research investigated to what extent flavonoids moderate oxidative stress from vitamin C <i>in vivo</i>.<p>Dietary experiments were conducted in two phases using adult male Wistar rats. First, all animals were maintained for two weeks on a control flavonoid-free diet with the dietary requirement (27 IU) of vitamin E/kg diet. In the subsequent four weeks, the animals were treated in four groups (8 rats/group), being fed the following diets: flavonoid-free control (C), ascorbate-supplemented (7.55 mmol/kg diet) (A), flavonoid-supplemented (2.67 mmol/kg diet) (F) and flavonoids (2.67 mmol/kg diet) plus ascorbate (7.55 mmol/kg diet)-supplemented (T). Measurements were done on in vivo biomarkers of oxidative stress, tissue antioxidants and on tissue in vitro susceptibility to oxidative stress.<p>In the combined feeding of ascorbate plus flavonoids, endogenous thiobarbituric acid reactive substances (TBARS) increased in liver by 114%. No effects of dietary ascorbate or flavonoids were seen on endogenous TBARS in brain or heart, or on plasma thiols or erythrocyte fragility.<p><i>In vitro</i>, the susceptibility to TBARS formation of liver homogenate (incubated for 60 min at 37ºC in air) showed a significant 60% increase in ascorbate-fed animals compared to control, but no increase in animals fed ascorbate plus flavonoids, suggesting that the additional feeding of flavonoids helped to prevent the increase produced by ascorbate-feeding. Incubation of liver mitochondria with 300 µM ascorbate in vitro produced a large (2-7 fold) increase in TBARS, but there was no difference among mitochondria from the different feeding groups.<p>The ability of flavonoid-feeding in protecting against oxidative stress from ascorbate in vivo could not be demonstrated in this study, even showing pro-oxidant effects of flavonoids in combination with ascorbate in liver. However, in vitro tests in liver suggest a protective effect of flavonoid-feeding against susceptibility to oxidative stress from ascorbate. Further investigations are needed in order to resolve the differences observed in vitro and in vivo and to determine the endogenous effects of specific flavonoids under ascorbate-induced oxidative stress.