Bioavailability and tissue distribution of dietary phytoestrogens in rats

Abstract

The fate of isoflavones in rats after oral and intravenous injection (IV) of soy was investigated. A liquid chromatography-mass spectrometry method to determine isoflavones in rat plasma was developed with a short run time of 16 minutes. Serum samples from a multigeneration reproductive study conducted to examine the effects of diets containing alcohol-washed soy protein supplemented with increasing amounts of Novasoy(TM), in male and female Sprague Dawley (SD) rats, were analyzed for isoflavones. Gender and age differences were observed and warranted additional investigations into the effects of gender, source of isoflavones and the role of glycosidation of isoflavones on the biovailability and pharmacokinetics of soy isoflavones. The bioavailability of isoflavones from Novasoy(TM), a mixture of synthetic glucosides, and a mixture of synthetic aglyconees was examined in female and male (3-month-old) SD rats, and male Fisher 344 (20-month-old) rats. For the synthetic aglycone and synthetic glucoside sources, bioavailability was independent of gender. However, for Novasoy(TM), a gender difference was observed for daidzein, with higher plasma concentrations found in male rats. Bioavailability values for daidzein, genistein and glycitein were significantly (p < 0.05) higher (up to sevenfold) from Novasoy(TM) and the synthetic glucosides compared with the synthetic aglyconees. Therefore, the source of isoflavones had significant effects on isoflavone bioavailability. Similar studies using aged (20-month-old) male rats, revealed significantly (p < 0.05) higher bioavailability values for glycitein from Novasoy(TM) (27 +/- 13%) and the synthetic glucosides (21 +/- 10%) compared with the aglycone mixture (8 +/- 3%). Significantly ( p < 0.05) higher values for bioavailability of daidzein were observed in aged rats dosed with Novasoy(TM) an the glucoside form compared with the aglycone mixture. Thus, the source of isoflavones had significant effects on daidzein and glycitein bioavailability in aged male rats. These findings will be of importance in risk-benefit analyses of soy isoflavones by regulatory agencies.