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SECOND INTERNATIONAL SYMPOSIUM
ON THE ROLE OF SOY
IN PREVENTING AND TREATING CHRONIC DISEASESeptember 15-18, 1996
Brussells, Belgium
SCIENTIFIC PROGRAM
Satellite Symposium
Soybean Isoflavones :
Levels in Foods and PharmacokineticsPharmacokinetics of Genistein and Daidzein in Rats
Roger A. King, CSIRO Division of Human Nutrition, P.O. Box 10041, Gouger Street, Adelaide, South Australia 5000.In these studies we have assessed the influence of glycosidic conjugation of genistein on its pharmacokinetics and have compared the pharmacokinetics of genistein and daidzein in rats. Rats received 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. The soy extract also provided the equivalent of 19.4mg daidzein/kg body weight as glycones. Genistein and daidzein concentrations were measured in plasma, urine and feces at intervals up to 48 h after dosing. Plasma genistein concentration at two hours was 12.3+/-2.9 mol/L in the genistein group compared to 4.29+/-0.24 mol/L (p=0.02) in the soy extract group.
Plasma daidzein concentration at 2 h in the soy extract group was 8.25+/-0.64 mol/L, which was significantly higher than that for genistein (p=0.002). There was no significant difference in plasma genistein at 8 h and at later times between genistein treated and soy extract treated rats. The mean urinary excretion rate during the first two hours after dosing was markedly higher in the genistein group compared to the soy extract group (0.30+/-0.09 mol/h and 0.02+/-0.01 mol/h respectively, p=0.01), but the percentage of dose recovered in urine over 48 h was not different between groups (22.0+/-2.7% genistein treated; 19.4+/-1.2% soy extract treated). The recovery of daidzein in urine from the soy extract group was approximately 40% more than for genistein. There were no significant differences between groups in the recovery of genistein in feces (22.1+/-2.8% and 21.3+/-2.5% of dose respectively). Only 6.2+/-0.9% of the daidzein from the soy extract was recovered in the feces. The results suggest that the initial rate of absorption of genistein from its glycosidic forms is less than aglycone, probably due to the need for bacterial hydrolysis, and that the pharmacokinetics of daidzein and genistein differ, with daidzein being more bioavailable than genistein when administered as glycosides.Soy Symposium Home http://soyfoods.com/symposium/
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