ABSORPTION AND EXCRETION OF THE SOY ISOFLAVONE GENISTEIN IN RATS

Rodent models have been used to study the anticarcinogenic properties of the soy isoflavones, particularly genistein, but there is little in formation regarding the pharmacokinetics of the absorption and excreti on of genistein. In this study, rats were given a single oral dose of genistein (20 mg/kg body weight) or an equivalent dose of its glycone forms, as an isoflavone-rich soy extract. Concentrations of genistein were measured in plasma, urine and feces at intervals up to 48 h after dosing. plasma genistein concentration at 2 h after dosing was 11.0 /- 2.3 mu mol/L in genistein-treated rats compared with 4.93 +/- 0.22 mu mol/L (P = 0.025) in soy extract-treated rats, but there were no si gnificant differences at 8 h and later times. The mean urinary excreti on rate during the first 2 h after dosing was more than 10 times highe r in the genistein group compared with the soy extract group (0.27 +/- 0.08 mu mol/h and 0.020 +/- 0.011 mu mol/h, respectively, P = 0.017) but the percentage of dose recovered in urine over 48 h was not differ ent between groups (19.9 +/- 2.4% genistein treated; 17.5 +/- 1.1% soy extract treated). There were no significant differences between group s in the recovery of genistein in feces (21.9 +/- 2.8% and 21.1 +/- 2. 5% of dose, respectively). Only 6.1 +/- 0.9% of the daidzein from the soy extract was recovered in the feces. The results suggest that the e xtent of absorption of genistein is similar for the glycone and aglyco ne forms. Although higher initial plasma concentrations may be achieve d with the aglycone, similar long-term concentrations exist for both f orms of isoflavone.