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This is not intended to be a comprehensive review of the literature as several in-depth reviews have appeared in the last few years which summarize the results of human studies in relation to polyphenols(1-9). Furthermore, although isoflavones are polyphenols, we have not made recommendations for this class.
What are polyphenols?
Significant research effort is now focused on minor dietary constituents, vitamins and trace elements, phytochemicals (carotenoids, flavonoids, indoles, isothiocyanates, etc.), zoochemicals (conjugated linoleic and n-3 fatty acids, etc.), fungochemicals and bacteriochemicals (formed during food fermentations and by the gut microflora).
The best studied polyphenols (also called flavonoids, and belonging to the bigger class of phytochemicals or phytonutrients) are (epi)catechins (including green tea polyphenols) and procyanidins (class: flavanols), quercetin (class: flavonols), and the isoflavones (genistein and daidzein, found in soya). Other, less studied classes, at least for health effects in humans, are anthocyanins and hydroxycinnamic acids.
Biological activities
All polyphenols share several structural features, particularly phenolic rings, and are amongst the most powerful of all the dietary antioxidants in vitro. This property led to extensive use of the word 'antioxidant', and overall polyphenol content of a food source is commonly evaluated by measuring the antioxidant activity e.g. by TEAC, ORAC, TRAP, etc(10). In the past, direct conclusions from in vitro antioxidant activity of a food were extrapolated to health benefits in vivo in humans. However, it is now clear that this was far too simplistic. Polyphenol action in vivo is through indirect antioxidant protection by activating endogenous defence systems, modulating cellular signalling processes involving nuclear factor-kappa B (NF-[kappa]B), activator protein-1(AP-1) DNA binding, glutathione biosynthesis, phosphoinositide 3 (PI3)-kinase/protein kinase B (Akt), mitogen-activated protein kinase (MAPK), translocation into the nucleus of nuclear factor erythroid 2 related factor 2 (Nrf2), and inhibition of oxidative or pro-inflammatory enzymes such as cyclo-oxygenase, lipoxygenase, NADPH oxidase, and some metalloproteins. For example, the inhibition of one specific enzyme, COX-2, affects inflammation, and consequently the development of a larger number of chronic diseases(11). Epicatechin, a polyphenol found in cocoa, green tea and many fruits, affects nitric oxide formation via NADPH oxidase(12,13), hence modifying vascular biomarkers such...