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Hormonal Effects of Soya in Pre-Menopausal Women Increased menstrual cycles length, decreased estrogens and increased sex hormone binding globulins (SHBG) have all been associated with reduced risk of breast cancer in pre-menopausal women. No Statistically significant alterations of menstrual cycle length have been reported in any of the intervention studies that controlled for menstrual cycle phase, although cycle length increased non-significantly in six out of seven of the studies by an average of 1/1d (1-6). This tendency towards increased cycle length is consistent with a recent cross-sectional study that reported an association between Soya intake and menstrual cycle length in Singapore Chinese Women. Effect on estrogen concentrations have been inconsistent in the intervention studies performed in pre-menopausal women, although a cross-sectional study showed an inverse association between Soya intake and the blood estradiol concentrations in Japanese women. It is of interest that in the by Wu et al., a significant decrease in estradiol was seen only in Asian and not in non-Asian subjects. In addition, Xu et al. reported a significant lowering of urinary estrogens by Soya isoflavones, despite the fact that significant effects on blood estrogen concentrations were not observed. In addition to the main circulating estrogens, specific estrogens metabolites are genotoxic and therefore are though to reflect less formation of genotoxic metabolites. Several studies evaluated the effects of Soya consumption on blood concentrations of SHBG, progesterone and gonadotropins. No studies reported statistically significant effects on SHBG although all of the studies showed non-significant decreases. Two studies showed statistically significant reductions in the concentrations of lutenizing hormone and follicle stimulating hormone. These data suggest that Soya may exert small effects on circulating hormones in pre-menopausal women, resulting in modestly reduced menstrual cycle length and estrogen levels, both associated with reduced risk of breast cancer. Of greater significance may be the shift in estrogen metabolism away from genotoxic metabolites. Hormonal effects of Soya in Post-Menopausal Women Consumption of isolated Soya protein or Soya foods exerts few effects on endogenous hormones in post-menopausal women. Studies have reported decreased estrone sulfate and increased SHBG. Soya consumption does not alter circulating estradiol, lutenizing hormone of follicle stimulating hormone. Similar to their findings in pre-menopausal women Xu et al. found reduced urinary total estrogen in post-menopausal women. Two studies reported increases in the vaginal cell maturation index, although thre reported no effect on this index and one reported no effect on endometrial histology. Consumption of Soya or red clover extracts exerts no effects on follicle stimulating hormone, lutenizing hormone, estradiol, SHBG or vaginal or endometrial parameters. Isoflavones consumption whether consumed as an extract or in Soya protein, has been shown to reduce the frequency and severity of menopausal symptoms such as hot flushes, but only about 10-20% more than the placebo. Several studies have shown no effects. Thus, Soya consumption exerts a few effects on hormones in postmenopausal women. The main effects are a reduction in urinary estrogens and genotoxic estrogen metabolites, and a small reduction of menopausal symptoms, with no adverse effects on the endometrium. Hormonal Effects of Soya in Men There has been some speculation that an androgen-lowering effect might be a mechanism by which Soya consumption could protect against prostrate cancer. At the same time, there has been some concern that Soya phyto-estrogens might exert harmful effects on sperms quality. Nagata et al reported significant inverse correlations between Soya foods consumption and serum concentrations of estradiol in Japanese men. Borderline significant associations were seen between Soya and estrone, total testosterone and free testosterone. Three dietary intervention studies have reported the effects of Soya or Soya phyto-estrogen consumtion on reproductive hormones in men. Habito et al. reported that 4 wks. Of tofu consumtion did not alter blood concentrations of estradiol, testosterone, di-hydro-testosterone and androstandiol glucuronide, although the mean testosterone: estradiol ratio was 10% lower, SHBG was 9% higher and the free androgen index was 7% lower after tofu consumption. This slight reduction in androgen activity was not confirmed by Nagata et al. who reported that 2 mo of soya milk consumption did not alter blood concentrations of estradiol, total and free testosterone or SHBG, although estrone concentrations tended to decrease. Mitchell et al. reported no changes in serum concentrations of estradiol, testosterone, follicle stimulating hormone, or lutenizing hormone in men consuming an isoflavones tablet. Only one study directly evaluated semen and testicular endpoints in men consuming soya. Mitchell et al. reported that consumption of a tablet containing 40mg/d of soya isoflavones for 2 months had no effect on testicular or ejaculate volume or sperm concentration, count or motility. These three studies performed in adult men suggest that soya consumption may decrease androgenic activity slightly in men without adverse effect on sperm quality. Conclusions
The reported studies suggest that soya consumption
exerts small effects on hormones in both men and women. Although these
effects are generally in a beneficial direction, their clinical
significance is yet to be established. Future studies should focus on
elucidating the responsible components and the optimal forms and does as
well as the factors that influence particular sub groups to respond to
soya. These factors may include ethnicity as well as individual phyto-estrogen
metabolism. Finally, of great importance is establishing the clinical
relevance of these small effects.
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