ALTERED TIME-COURSE OF URINARY DAIDZEIN AND GENISTEIN EXCRETION DURING CHRONIC SOYA DIET IN HEALTHY MALE-SUBJECTS

Soybean consumption is associated with reduced rates of prostate and o ther cancers, possibly due in part to the presence of isoflavones. The metabolism and disposition of these soya-derived phytoestrogens after chronic soya exposure were studied on a metabolic unit in six healthy males (21-35 yrs of age) who consumed an unrestricted hospital diet a nd a 12-oz portion of soymilk with each meal for one month. The daily isoflavone intake was about 100 mg of daidzein (mostly as daidzin) and about 100 of mg of genistein (mostly as genistin). At two-week interv als, excretion of isoflavones in urine was studied, during which time the subjects consumed a constant basal diet for three to four days, in gested the full daily 36-oz portion of soymilk within 30 minutes each day for one to two days, and collected urine continuously. The urinary recovery of ingested daidzin plus daidzein (46.9 +/- 15.2%, mean +/- SD) and genistin plus genistein (14.6 +/- 9.2%) did not change with pr olonged soya ingestion. The absorption half-lives (t(1/2)) for daidzei n and genistein and the appearance t(1/2) for equol (1 subject) were i nitially 1.5 +/- 0.4, 1.9 +/- 0.6 and 2.2 hours, respectively, and 2.5 +/- 1.1 (p = 0.06 compared with baseline), 1.4 +/- 0.9 (p = 0.03 comp ared with baseline), and 4.2 hours, respectively, during one month of soymilk ingestion. The excretion t(1/2) for daidzein, genistein, and e quol were initially 2.9 +/- 0.5, 3.8 +/- 0.7, and 5.2 hours, respectiv ely, and 3.9 +/- 1.2 (p = 0.03), 5.5 +/- 1.6 (p = 0.02), and 9.7 hours , respectively, during one month of soymilk ingestion. These results i ndicate that chronic soya exposure did not induce significant changes in the metabolic pathways of isoflavones but altered the time courses of daidzein and genistein excretion. Thus chronic exposure to soya mig ht prolong the tissue exposure to the presumed biologically active fre e and unconjugated forms of these isoflavones and thereby enhance thei r oncoprotective effects.