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288 A. SCALBERT ET AL. compared to other antioxidants, research on their health effects started more recently. This late interest for polyphenols is largely explained by the complexity of their chemical structures. Nu- merous studies on animal models have shown that, when added to the diet, they limit the development of cancers, cardiovas- cular diseases, neurodegenerative diseases, diabetes, and osteo- porosis (see4 for a review and references cited in the follow- ing sections). The purpose of this article is to critically review first the antioxidant effects of polyphenols and their relevance for health, and secondly the experimental evidence supporting a protective role of polyphenols against the main degenerative diseases effecting Westernized populations. Emphasis is placed on clinical and epidemiological studies. Evidence based on ani- mal experiments is also presented and discussed with regards to the doses of polyphenols administered which may widely differ from those experienced by humans with their diet. The mecha- nisms of action as suggested by animal and in vitro studies are also briefly summarized. Lastly, the possible risks associated to high polyphenol consumption are discussed. ANTIOXIDANT PROPERTIES OF POLYPHENOLS Degenerative diseases, such as cancers, cardiovascular dis- eases, osteoporosis, and degenerative diseases, are associated with aging. Oxidative damage to cell components, DNA, pro- teins, and lipids accumulates with age and contributes to the de- generation of the somatic cells and to the pathogenesis of these diseases. Antioxidants present in food can help limit this damage by acting directly on reactive oxygen species or by stimulating endogenous defence systems. The phenolic groups in polyphe- nols can accept an electron to form relatively stable phenoxyl radicals, thereby disrupting chain oxidation reactions in cellular components.5 The antioxidant potency of polyphenols has been evaluated in vitro by measuring their ability to trap free radicals and reduce other chemicals. Their potency is compared to that of a reference substance, usually Trolox (a water-soluble derivative of vitamin E), gallic acid, or catechin. In all cases, the reaction studied is the reduction of an oxidant by polyphenols. The most commonly used oxidants are listed below: 1. ABTS◦+ formed from ABTS (2,24-azonobis(3-ethylbenzot- hiazoline-6-sulphonic acid) in the presence of metmyoglobin and hydrogen peroxide.6 The reaction is monitored by col- orimetric determination of the colored ABTS◦+ radical. This method is often referred to as the TEAC method (Trolox Equivalent Antioxidant Capacity). 2. Radicals formed by heating AAPH (2,2′-azobis (2-amidino- propane) dihydrochloride) or ABAP (2,2′-azobis (2-aminod- opropane) hydrochloride). The radical concentration is usu- ally monitored by following the degradation of phycoerythrin by fluorimetry. This technique is known as the ORAC (Oxy- gen Radical Absorbance Capacity)7 or TRAP method (Total Radical-trapping Antioxidant Parameter).8 3. Ferric ions: This approach is known as the FRAP method (Ferric-Reducing Ability of Plasma).9 4. Radicals formed by autoxidation of linoleic acid.10 Figure 1 Chemical structures of a few common dietary polyphenols.PDF Image | Dietary Polyphenols and the Prevention of Diseases
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