The specific role of isoflavones on estrogen metabolism in premenopausal women

This was a placebo-controlled randomized human intervention study in premenopausal, omnivorous women without a history of breast carcinoma. For the isoflavone group, 33 participants were analyzed; women were aged 25 to 55 years and were studied in the Tampa Bay area of Florida, USA.

Premenopausal women in the active arm received an oral soy protein supplement providing 40 mg of genistein per day (20 mg per dose) for 12 weeks, compared with placebo.

Overall, isoflavone supplementation favorably altered estrogen-related measures and menstrual characteristics. The clearest effect was an increase in total menstrual cycle length from baseline to the end of study in the isoflavone group compared with placebo, 3.52 1.35 versus -0.06 1.12, P=0.04. Follicular cycle length also increased in the active group but did not reach statistical significance, 1.46 0.63 versus 0.14 0.43, P=0.08. Hormone patterns were also shifted in a potentially favorable direction, with SHBG increased in 41.4% of women, free estradiol decreased in 53.85%, and estrone decreased in 55.56%.

The active arm was small, and the intervention lasted only 12 weeks, limiting confidence in durability and clinical relevance. The population was restricted to premenopausal omnivorous women without breast cancer, which narrows generalizability. Some hormone and cycle outcomes were not statistically significant, and dietary intake and body weight changed during the study, which could complicate interpretation.