Classification and occurrence of dietary polyphenols
Dietary polyphenols are the most abundant antioxidants in human diets. With over 8,000 structural
variants, they are secondary metabolites of plants and denote many substances with aromatic ring(s)
bearing one or more hydroxyl moieties. They are subdivided into groups (Figure 1) by the number of
phenolic rings and of the structural elements that link these rings [11]: (1) The phenolic acids with the
subclasses derived from hydroxybenzoic acids such as gallic acid and from hydroxycinnamic acid,
containing caffeic, ferulic, and coumaric acid; (2) the large flavonoid subclass, which includes the
flavonols, flavones, isoflavones, flavanones, anthocyanidins, and flavanols; (3) the stilbenes; and (4)
the lignans and the polymeric lignins.