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 information regarding the pharmacokinetics of the absorption and excretion 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 mumol/L in genistein-treated rats compared with 4.93 +/- 0.22 mumol/L (P = 0.025) in soy extract-treated rats, but there were no significant differences at 8 h and later times. The mean urinary excretion rate during the first 2 h after dosing was more than 10 times higher in the genistein group compared with the soy extract group (0.27 +/- 0.08 mumol/h and 0.020 +/- 0.011 mumol/h, respectively, P = 0.017) but the percentage of dose recovered in urine over 48 h was not different between groups (19.9 +/- 2.4% genistein treated; 17.5 +/- 1.1% soy extract treated). There were no significant differences between groups 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 extent of absorption of genistein is similar for the glycone and aglycone forms. Although higher initial plasma concentrations may be achieved with the aglycone, similar long-term concentrations exist for both forms of isoflavone.