Inhibition of carcinogenesis by dietary polyphenolic compounds

Plants consumed by humans contain thousands of phenolic compounds. The effe cts of dietary polyphenols are of great current interest due to their antio xidative and possible anticarcinogenic activities. A popular belief is that dietary polyphenols are anticarcinogens because they are antioxidants, but direct evidence for this supposition is lacking. This chapter reviews the inhibition of tumorigenesis by phenolic acids and derivatives, tea and cate chins, isoflavones and soy preparations, quercetin and other flavonoids, re sveratrol, and lignans as well as the mechanisms involved based on studies in vivo and in vitro. Polyphenols may inhibit carcinogenesis by affecting t he molecular events in the initiation, promotion, and progression stages. I soflavones and lignans may influence tumor formation by affecting estrogen- related activities. The bioavailability of the dietary polyphenols is discu ssed extensively, because the tissue levels of the effective compounds dete rmine the biological activity. Understanding the bioavailability and blood and tissue levels of polyphenols is also important in extrapolating results from studies in cell lines to animal models and humans. Epidemiological st udies concerning polyphenol consumption and human cancer risk suggest the p rotective effects of certain food items and polyphenols, but more studies a re needed for clear-cut conclusions. Perspectives on the application of die tary polyphenols for the prevention of human cancer and possible concerns o n the consumption of excessive amounts of polyphenols are discussed.