Flavonoids undergo substantial hepatic metabolism and the metabolites might significantly contribute to the effects of these dietary constituents. The metabolites of flavonoids in liver can be summarized as follows: 1) For flavones, the hydroxylation appears to occur at the C-4'-, C-3', C-6 and C-8- position when there is a single or no hydroxy group on the B-ring. The methoxyl groups positioned at the C-7, C-6 or C-4'- position of flavones are demethylated. The glucuronidation occurs at the 6, 7, 4' or 3'- hydroxyl moiety. Flavone glycosides and aglycones appear to undergo similar metabolic pathways. 2) For flavonols, the hydroxylation appears at the 3' and 4'- position and flavonols with a 4'-methoxy group are easily O-demethylated to their corresponding hydroxylated analogs. The glucuronidation takes place at the 7, 3, 3' and 4'- hydroxyl moiety. 3) For isoflavones, the microsomal hydroxylation is observed at the C-3'-, C-6 and C-8- position when there is a single or no hydroxy group on the B-ring. The demethylation takes place at the C-6- or C-4'- position when there is one methoxy group on C-4' or C-6-position, respectively. The glucuronidation occurs at the 4, or 5- hydroxyl moiety. 4) For chalcones, the C-3'-, and C-8- positions undergo hydroxylation. The C-8- position is a very active site for metabolism of chalcones.
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