ALTERED KINETICS AND EXTENT OF URINARY DAIDZEIN AND GENISTEIN EXCRETION IN WOMEN DURING CHRONIC SOYA EXPOSURE

Soybean consumption may be protective for breast cancel; possibly due in part to the presence of the isoflavones daidzein and genistein, whi ch are weakly estrogenic. The metabolism and disposition of these phyt oestrogens during chronic soya exposure were studied on a metabolic un it. Six healthy 22- to 29-year-old women consumed an unrestricted hosp ital diet for most of the study and ingested 12 oz of soymilk with eac h meal for one month. At two-week intervals, excretion of isoflavones in urine was studied, during which time the subjects consumed a consta nt basal diet for three to four days, ingested the full daily 36-oz po rtion of soymilk within 30 minutes each day for one to two days, and c ollected urine continuously. Urinary recovery of genistein [initially 23.9 +/- 173% (SD) of ingested genistin + genistein], daidzein (initia lly 66.2 +/- 23.5% of ingested daidzin + daidzein), and equol (initial ly 28% of the ingested precursors daidzin + daidzein in 1 subject and < 1% in 5 subjects) decreased progressively over four weeks of daily s oya ingestion by 42% for genistein (p < 0.05) and 31% for daidzein (p < 0.01) but increased by 3- to 100-fold for equol (4 subjects, p < 0.0 5). Total amounts excreted and peak levels were similarly affected. Th e absorption half-lives (t(1/2)) for genistein and daidzein were initi ally 2.7 +/- 0.8 and 1.6 +/- 0.5 hours, respectively, and during four weeks of soymilk ingestion decreased to 2.0 +/- 0.6 (p = 0.04) and 1.4 +/- 0.2 hours (p = 0.06), respectively, suggesting more rapid absorpt ion. The appearance t(1/2) of equol can be estimated for only one subj ect initially (2.9 hrs), but during four weeks of soya ingestion it co uld be estimated for three more subjects (4.7 +/- 2.3 hrs). The excret ion t(1/2) values for genistein and daidzein were initially 67 +/- 0.8 and 4.4 +/- 0.7 hours, respectively, and during four weeks of soymilk ingestion decreased to 4.2 +/- 1.2 (p = 0.005) and 3.2 +/- 1.1 hours (p = 0.005), respectively, suggesting more rapid excretion. For equol, the excretion t(1/2) was initially 9.1 hours (1 subject), and after t wo and four weeks of soymilk ingestion it was 13.4 +/- 9.7 and 5.5 +/- 1.6 hours (4 subjects, p = 0.046, 2 wks vs. 4 wks), respectively. The se results indicate that metabolism and disposition of ingested isofla vones are altered during chronic soya ingestion in women, perhaps from increased metabolic degradation to formation of non isoflavone metabo lites. Increased production of the longer- and stronger-acting estroge nic equol in some women during chronic soymilk ingestion may alter the estrogenic potency of dietary soya isoflavones.