Glucuronides Are the Main Isoflavone Metabolites in Women

Three experiments were conducted to characterize the metabolism of isoflavones from soy milk in women: two meals in 2 wk separated by a 1-wk washout period (Experiment 1), one meal feeding (Experiment 2) and six consecutive days of feeding (Experiment 3). Urine and plasma samples were extracted directly or predigested before extraction with H-2 beta-glucuronidase/sulfatase or B-3 beta-glucuronidase so that isoflavone glucuronide and sulfate conjugates could be determined by difference. Among the three experiments, no significant differences were found in the proportion of glucuronide, sulfate and aglycone isoflavones recovered from plasma samples taken 3 h after isoflavone dosing or in 24-h urine samples taken after isoflavone dosing. In the 6-d feeding study, samples taken on d 5 and 6 did not differ significantly in isoflavone content or proportion of the metabolites studied. The percentages of daidzein and genistein glucuronides were 73 +/- 4 and 71 +/- 5% of total daidzein and genistein excreted in urine, and 62 +/- 4 and 53 +/- 6% of total daidzein and genistein present in plasma, respectively. Percentages of aglycone daidzein and genistein were 4 +/- 1 and 5 +/- 1% of total daidzein and genistein in urine, and 18 +/- 2 and 26 +/- 7% of total daidzein and genistein present in plasma, respectively. These studies showed that about one fifth of circulating isoflavones are aglycones. Concentrations of isoflavones chosen for in vitro studies should take this into account. Because the glucuronide isoflavones predominate in vivo, these metabolites should not be overlooked as possible contributors to observed effects of isoflavones.