Plant Polyphenols as a Double-Edged Sword in Health Promotion: Lessons from the Experimental Models Using Simple Phenolic Acids

Abstract

erized molecules such as tannins are reportedly identi-fied. Phenolic acids include caffeic acid, vanillic acid, and coumaric acid, that account for approximately one-third of the total dietary intake of polyphenols. Flavonoids, account for the remaining two-thirds of polyphenol intake, can be further subdivided into 13 classes, including flavones, flavanones, flavonols, isoflavones, catechins (flavanols), anthocyanins and proanthocyanidins, with more than 5,000 identified compounds. Epidemiological studies have supported the hypothesis that high dietary intake of polyphenols is associated with decreased risk of a range of diseases including cardiovascular disease, cancer and neurodegenerative diseases (Fraga 2007; Martin and Appel 2010). Since the French paradox was described two decades ago (Renaud and Lorgeril 1992), several Abstract Polyphenols in foods possess potential to exhibit various beneficial effects in health pro-motion. Although polyphenols are well known to have antioxidant properties, a wealth of data suggests that most of the relevant mechanisms of disease prevention by polyphenols are not mainly related to their antioxidant properties. The protective function of phenolic compounds, which can be metabolically converted into electrophiles, might be ascribed to the induction of the cytoprotective responses against oxidative stress or toxic chemi-cals. Based on compelling evidence regarding the beneficial potential of polyphenols, various polyphenol-rich dietary supplements are being developed for public use. It is recently becoming apparent that commonly used dietary compounds can exert deleteri-ous effects at pharmacological (supraphysiological) doses. Moreover, some human inter-vention trials showed not only failure to protect by polyphenols but accelerated develop-ment of chronic diseases in susceptible subjects. Thus, the present monograph tries to provide experimental evidence supporting the idea that optimal polyphenol supplemen-tation can confer beneficial effects but high doses elicit adverse effects in an electrophilic reaction-dependent manner, thereby establishing a double-edge sword in health promo-tion. Current knowledge of polyphenols as phase II inducers is also discussed, with con-sideration of the metabolic activation, binding targets and factors influencing the bio-logical activities of polyphenols. In addition, this monograph attempts to provide a brief perspective on the beneficial and harmful effects of food polyphenols.