Biotransformation and Gut Microbiota-Mediated Bioactivity of Flavonols

Consumption of fruits and vegetables has been associated with a reduced risk of multiple diseases, such as metabolic disorders. Flavonols are the most ubiquitous flavonoids in fruits and vegetables. However, dietary flavonols exhibit a general low oral bioavailability for their extensive biotransformation mediated by phase II enzymes in enterocytes and liver as well as by microbiota in the gut lumen. In this context, flavonols have brought attention to a paradox between low bioavailability and health-promoting effects. Flavonols are often transformed prior to absorption, which could change their biological activity. Compared to their parent compounds, the corresponding metabolites of flavonols in vivo might exhibit similar or higher intrinsic bioactivities, or perhaps a decreased efficacious effectiveness. Indeed, a growing body of evidence from biological function studies of metabolites supports the positive and significant contribution of in vivo metabolic processes, particularly conversion mediated by gut microbiota, to the health-promoting benefits of flavonols. As such, further understanding of the metabolic fate of flavonols and biological activities of their metabolites as well as the possible impact of microbiota-mediated conversion on the bioactivity is of great significance to guide a rational diet with flavonol-rich fruits and vegetables and/or flavonol-containing functional foods.