Dietary substances and their glucuronides: structures, occurrence and biological activity

Covering up to 2025.Plant-derived polyphenols of various chemical classes are widely distributed in dietary substances, e.g. fruits, nuts, vegetables and teas. Such phenolic derivatives are natural antioxidants and have been linked with numerous health benefits, notably anti-cancer and anti-inflammatory properties. Additionally, they may behave as mild estrogens, as in the case of genistein. However, there has often been no clear correlation between in vitro properties, as measured in cell lines for instance, and in vivo performance. Moreover, it is not always clear what the true active species might be, as most phenols are readily subject to phase II metabolism, generating predominantly glucuronides and sulfates. In this highlight, we seek to address the question of whether dietary substance metabolites, especially glucuronides, which have been more widely studied, do indeed possess distinct activities in their own right compared to their parent substances. In most cases this will refer to enzyme inhibition and/or interaction with cell lines. General observations concerning glucuronidation are provided, accompanied by practical comments concerning the synthesis of glucuronides, which are not always available or marketed in useful quantities. The main structural classes of natural polyphenols are introduced, with comments including synthetic details and biological properties for important members of each class.