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Dietary Polyphenols as Antioxidants and Anticancer Agents: More Questions than Answers

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Author(s): Miao-Lin Hu

Journal: Chang Gung Medical Journal
ISSN 2072-0939

Volume: 34;
Issue: 05;
Start page: 449;
Date: 2011;
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Keywords: dietary polyphenol | antioxidant | pro-oxidant | anticancer | signal transduction

ABSTRACT
High intake of fruit and vegetables is believed to be beneficial to human health. Fruit, vegetables and some beverages,such as tea and coffee, are particularly rich in dietary polyphenols. Various studies have suggested (but not proven) thatdietary polyphenols may protect against cardiovasucalar diseases, neurodegenerative diseases and some forms of cancer.Dietary polyphenols may exert their anticancer effects throughseveral possible mechanisms, such as removal of carcinogenicagents, modulation of cancer cell signaling and antioxidantenzymatic activities, and induction of apoptosis as well as cellcycle arrest. Some of these effects may be related, at least partly, to their antioxidant activities. In recent years, a new conceptof the antioxidant effects of dietary polyphenols has emerged,i.e., direct scavenging activity toward reactive species andindirect antioxidant activity; the latter activity is thought toarise primarily via the activation of nuclear factor-erythroid-2-related factor 2 which stimulates the activities of antioxidant enzymes such as glutathione peroxidase (GPx), glutathioneS-transferase, catalase, NAD(P)H: quinone oxidoreductase-1 (NQO1), and/or phase IIenzymes. The direct antioxidant activity of dietary polyphenols in vivo is probably limitedbecause of their low concentrations in vivo, except in the gastrointestinal tract where theyare present in high concentrations. Paradoxically, the pro-oxidant effect of dietary polyphenols may contribute to the activation of antioxidant enzymes and protective proteins in cultured cells and animal models because of the adaptation of cells and tissues to mild/moderate oxidative stress. Despite a plethora of in vitro studies on dietary polyphenols, manyquestions remain to be answered, such as: (1) How relevant are the direct and indirectantioxidant activities of dietary polyphenols in vivo? (2) How important are these activitiesin the anticancer effects of dietary polyphenols? (3) Do the pro-oxidant effects of dietarypolyphenols observed in vitro have any relevance in vivo, especially in the potential anticancer effect of dietary polyphenols? Apparently, more carefully-designed in vivo studiesare needed to answer these questions.

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