A phase I double-blind clinical trial was conducted to evaluate the effects of a high oral dose of soy isoflavones administered daily for 84 days to healthy postmenopausal women. Principal outcome measures included DNA damage, apoptosis, and changes indicative of estrogenic stimulation.
After eligibility and equol-producer status were determined, stratified randomization was used to assign women to the isoflavone (active) or placebo group. Of the 30 women who completed the study, 18 were in the active group. DNA damage was assessed via COMET and apurinic/apyrimidinic site assays in lymphocytes. Apoptosis was evaluated via terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling and activated caspase-3 assays in lymphocytes. Estrogenic/antiestrogenic effects were assessed using a self-report questionnaire and by assaying for estrogen, follicle-stimulating hormone, luteinizing hormone, and sex hormone-binding globulin in blood.
In treated postmenopausal women, there was no indication that high doses of soy isoflavones caused DNA strand breakage, increased apurinic/apyrimidinic sites, or increased apoptosis in peripheral lymphocytes. There were no significant changes in mean values for estrogenic effects or other laboratory measurements. Very few adverse events occurred, and the only drug-related adverse events were mild or grade 1 in severity.
Unconjugated soy isoflavones appear to be safe and well tolerated in healthy postmenopausal women at doses of 900 mg/day.