Flavonoid Occurrence, Bioavailability, Metabolism, and Protective Effects in Humans: Focus on Flavan-3-ols and Flavonols

Flavonoids are profusely abundant in the human diet and might be responsible for the advantages ascribed to dietary regimens rich in plant foods. Notwithstanding the evidence from epidemiological and cohort studies, an unambiguous and exhaustive pattern has not yet emerged regarding the mechanisms underlying their food-related health properties. It is now increasingly evident that the sole direct action of substances available in planta is not sufficient to fully explain the benefits of polyphenol-rich dietary habits. To elucidate this conundrum, a better comprehension of the interplay between human and microbial metabolism and dietary polyphenols is needed. This chapter provides a detailed overview of the current knowledge on bioavailability and in vivo biotransformation of two widespread classes of flavonoids: flavonols and flavan-3-ols. According to data collected, these substances are not always endowed with the highest bioactivities in vivo, and a deeper knowledge of their metabolites is needed to properly assess their overall contribution to human health and to single out substances to be monitored for the exhaustive profiling of flavonoid bioavailability.