Effects of soy foods on ovarian function in premenopausal women

It has been proposed that the high intake of soy foods among Asians may par tly explain their lower rates of breast cancer, perhaps by lowering endogen ous oestrogen levels, although this has been inadequately studied. Twenty h ealthy cycling premenopausal women (ten Asians and ten non-Asians) particip ated in a 7-month soy intervention study which was designed to investigate the effect of supplementation on ovarian function. Asian soy foods (tofu, s oymilk, green soybean peas) in the amount of approximately 32 mg of isoflav ones per day were added to the women's diets for three menstrual cycles. Th e women's baseline (two cycles) serum hormone levels were compared to level s during soy intervention (three cycles) and levels after intervention (two cycles). During the entire study period, subjects provided almost daily ov ernight urine samples and blood specimens during specified days of their me nstrual cycles. The day of urinary luteinizing hormone (LH) peak was used a s a marker for the day of ovulation. Knowledge of day of ovulation allowed comparison of hormone measurements at baseline to those obtained during int ervention and recovery cycles with standardization of day of cycle. Soy int ervention was associated with a statistically significant reduction in seru m luteal oestradiol level (-9.3%, P < 0.05), but there were no significant changes in follicular phase oestradiol, follicular or luteal phase progeste rone, sex hormone-binding globulin or menstrual cycle length. This signific ant reduction in luteal phase oestradiol was, however, observed only among Asian (-17.4%) but not among non-Asian (-1.2%) participants; urinary excret ion of isoflavones was higher among Asians than non-Asians (29.2 vs 17.1 mu mol day(-1), P = 0.16) during the intervention period. Thus, supplementati on using traditional soy foods reduced serum oestradiol levels among Asian participants in this study. Differences in the type of soy products (i.e, t raditional soy foods versus soy protein products), amount of isoflavones, a nd race/ethnicity of participants may have contributed to the divergent res ults. Larger soy intervention studies designed specifically to include part icipants of different race/ethnicities and using both traditional soy foods and soy protein products providing comparable doses of isoflavones are nee ded to definitively determine the effect of soy on ovarian function. (C) 20 00 Cancer Research Campaign.