Regulation of Infl ammation, Redox, and Glucocorticoid Signaling by Dietary Polyphenols
Polyphenols are secondary metabolites of plants with over 8000 structural variants and comprise of a wide range of substances having aromatic ring(s) bearing one or more hydroxyl moieties (Table 6.1). Chemically, polyphenols may vary from a simple molecule, such as phenolic acid, to highly polymerized compounds, such as tannins . The most widely distributed group of plant phenolics are fl avonoids having a general structure as that of diphenylpropanes, consisting of two aromatic rings linked through three carbon atoms that usually form an oxygenated heterocycle . Figure 6.1 shows the basic structure and the system used for the carbon numbering of a fl avonoid nucleus and the structures of some important polyphenols and their structural relationship with steroids. The fl avonoids subclasses include
fl avonols, fl avones, fl avanols, isofl avones, anthocyanins, and fl avanones (Table 6.2). When glycosylated, fl avonoids become more water-soluble and lose their reactivity toward free radicals. Considering the large number of polyphenolic compounds present in dietary sources, this chapter will focus on the better known and most studied polyphenols such as resveratrol, curcumin, and catechins. Minor sections on the absorption, metabolism, and bioavailability of polyphenols have also been considered as these aspects of polyphenols are still to be fully deciphered and proper understanding of these phenomena will provide new insights for designing new strategies to develop polyphenol-based therapies for chronic infl ammation.