Common features in the pathways of absorption and metabolism of flavonoids

Interpretation of the in vivo biological activity of flavonoids from in vitro data requires an understanding of their bioavailability, which includes absorption and metabolism. The bioavailability of flavonoids depends on the chemical structure and whether the molecule is conjugated. Although the apparent bioavailability of flavonoids appears to be highly variable between types of flavonoid, from the very poorly absorbed anthocyanins to the well-absorbed isoflavones, the pathways involved in the absorption and metabolism are common to all flavonoids. The flux through metabolic pathways is determined by: (1) specificity and activity of transporters; (2) specificity and activity of metabolizing enzymes; and, (3) flavonoid stability. Figure 2.1 summarizes the current state of the art on the pathways that are known to impact bioavailability of flavonoids. Each step is considered individually here.