Absorption, bioavailability, and metabolism of flavonoids

To unravel mechanisms of action of dietary flavonoids in their potential role in disease prevention, it is crucial to know the factors that determine their release from foods, their extent of absorption, and their fate in the organism. Research on absorption, metabolism, and bioavailability of flavonoids will answer these questions. The subclass, flavonols, with quercetin as the major dietary flavonol, was the first to be studied, and information on other subclasses of flavonoids is emerging. Most flavonoids, except for the subclass of catechins, are present in plants bound to sugars as beta-glycosides. This structural feature determines whether the flavonoid can be absorbed from the small intestine or has to go to the colon before absorption can occur. Generally, but exceptions have been described, glucosides are the only glycosides that can be absorbed from the small intestine. Absorption from the small intestine is more efficient than from the colon and will lead to higher plasma values. After absorption from the small intestine, flavonoids are conjugated with glucuronic acid or sulfate or O-methylation may occur. The conjugation reactions, which occur in the small intestine upon absorption, are very efficient. As a result, no free flavonoid aglycones can be found in plasma or urine, except for catechins. Plasma concentrations due to a normal diet will be less than 1 muM. Flavonoids that cannot be absorbed from the small intestine, and absorbed flavonoids secreted with bile, will be degraded in the colon by microorganisms, which will break down the flavonoid ring structure. The resulting phenolic acids have partly been characterised. These phenolic acids can be absorbed and have been measured in plasma and urine. Future research will need to address tissue distribution, cellular uptake, and cellular metabolism.