The 73 postmenopausal women in the study, none of whom was taking hormone replacement therapy, averaged 61 years of age. The women were randomly assigned to 1 of 3 low-fat, low-cholesterol diets incorporating a daily portion of either 40 g of test protein as milk casein (control), or 56 mg or 90 mg of soy isoflavones. The test protein was incorporated into baked products, beverages and soups as part of a standard menu. Blood samples were collected at the beginning of the study and after 3 and 6 months in order to measure serum concentrations of several hormones. Though small hormonal fluctuations occurred in all three groups over time, the researchers found no significant changes in serum estrogen, cortisol, dehydroepiandrosterone sulfate, or follicle stimulating hormone in any of the subject groups. Circulating thyroid hormone levels were modestly increased in women taking the highest dose of soy isoflavones for more than 6 months. Other effects of soy isoflavones observed in the study, such as increased bone mineral density and higher HDL concentrations in women who took the highest dose for 6 months, suggest that soy may possess estrogen effects that are better reflected in these end-points rather than in serum hormone concentrations.
Persky, Victoria W et al. Effect of soy protein on endogenous hormones in postmenopausal women. Am J Clin Nutr 2002;75:145-53.
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American Journal of Clinical Nutrition