In vitro bioavailability and cellular bioactivity studies of flavonoids and flavonoid-rich plant extracts: questions, considerations and future perspectives

In vitro techniques are essential in elucidating biochemical mechanisms and for screening a wide range of possible bioactive candidates. The number of papers published reporting in vitro bioavailability and bioactivity of flavonoids and flavonoid-rich plant extracts is numerous and still increasing. However, even with the present knowledge on the bioavailability and metabolism of flavonoids after oral ingestion, certain inaccuracies still persist in the literature, such as the use of plant extracts to study bioactivity towards vascular cells. There is therefore a need to revisit, even question, these approaches in terms of their biological relevance. In this review, the bioavailability of flavonoid glycosides, the use of cell models for intestinal absorption and the use of flavonoid aglycones and flavonoid-rich plant extracts in in vitro bioactivity studies will be discussed. Here, we focus on the limitations of current in vitro systems and revisit the validity of some in vitro approaches, and not on the detailed mechanism of flavonoid absorption and bioactivity. Based on the results in the review, there is an apparent need for stricter guidelines on publishing data on in vitro data relating to the bioavailability and bioactivity of flavonoids and flavonoid-rich plant extracts.