Flavonoids are important dietary phytochemicals with broad human exposures and a variety of claimed biological effects. Although
traditional sources of flavonoids are fruits and vegetables, dietary supplements have become an important source of flavonoids.
The enthusiasm for all things about flavonoids expressed by scientists and public alike has sometimes overlooked an important point:
these compounds are usually not bioavailable or have very low bioavailability. The current review focused on factors that affect bioavailability
of flavonoids, which in turn impact their efficacy and toxicity. These factors include physicochemical and biological ones, with
the emphasis on the latter, including membrane permeation, enzymatic metabolism, and excretion by efflux transporters. Current evidence
indicates that naturally occurring flavonoid glycosides typically need to be de-glycosylated before they are absorbed, because intact
flavonoid glycosides are poorly absorbed. However, once absorbed, flavonoid aglycones undergo rapid and extensive first-pass metabolism
via conjugation and subsequent excretion of the conjugates, which makes them poorly bioavailable. Fortunately, the presence of
enteric and enterohepatic recycling allows the flavonoids to be accumulated in vivo over time, because the recycling increases contact
time between the flavonoids and target tissues. We believe that continued studies in this area will help scientists devise a better means to
improve oral bioavailability of flavonoids so we can one day fully enjoy the health benefits of ingesting flavonoids.