Metabolism of Dietary Estrogens

The major isoflavones, genistein and daidzein, commonly exist as inactive glycosides [11]. They are derived from precursors, biochanin A and formononetin, which are converted to genistein and daidzein, respectively, after breakdown by intestinal glucosidases [15]. Daidzein is further metabolized to the mammalian isoflavone metabolites, equol and O-demethylangiolensin (O-Dma) (Fig. 1) [15, 16]. Absorbed phytoestrogen metabolites undergo enterohepatic circulation and may be excreted in the bile as the 7-O-^-glucuronide conjugate [17-19], de-conjugated by intestinal flora, reabsorbed, reconjugated by the liver and excreted in the urine [15, 20-22]. Not all humans produce equol from daidzein, presumably due to differences in colonic bacterial populations among individuals. Since the conversion of daidzein to equol occurs by the action of gut microflora, it has been proposed that the type of bacteria found in the colon of equol excreters is different from those of non-excreters. Moreover, it has been reported that an individual is consistently either an equol excreter or non-ex-creter, suggesting that the individual variability may be intrinsic or perhaps even genetically determined [23]. In a large study conducted by Lampe et al. [24], 30 men and 30 women consumed a soy protein beverage containing 22 mg genistein and 8 mg daidzein for 4 days as a supplement to their habitual diets. Approximately one-third of the participants excreted equol after 3 days of consuming the soy supplement. There appears to be no difference in equol excre-



1a. Formononetin och3

1a. Formononetin oh o

1 b. Daidzein oh

1c. Equol oh o oh

1e. Genistein

1d. Biochanin A

1e. Genistein

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