Lack of effect of isoflavonic phytoestrogen intake on leptin concentrations in premenopausal and postmenopausal women

Objective: To assess the effect of soy isoflavone ingestion on plasma leptin concentrations in premenopausal and postmenopausal women. Design: Randomized, crossover studies, with blinding of participants and laboratory personnel. Setting: Procedures involving free-living individuals were carried out at the University of Minnesota General Clinical Research Center. Patient(s): Fourteen regularly cycling premenopausal women, and 18 postmenopausal women. Intervention(s): Each premenopausal participant consumed, on a daily basis, each of three soy protein powders containing different levels of isoflavones for three menstrual cycles plus 9 days, with plasma samples collected every other day the last 6 weeks of each diet period. Similarly, each postmenopausal participant consumed each of the three powders for 93 days, with plasma samples collected daily on days 64 to 66 and 92 to 94 of each diet period. The powders, dosed on a per-kilogram body weight basis, provided mean isoflavone intakes of 8, 65, and 130 mg/day, for the control, low-isoflavone, and high-isoflavone diet periods, respectively. Main Outcome Measure(s): Plasma leptin concentrations. Result(s): Isoflavone intake had essentially no effect on leptin concentrations in either premenopausal or postmenopausal participants. Concentrations in the premenopausal women were higher during the periovulatory and midluteal phases as compared to the early follicular and midfollicular phases. Conclusion(s): Despite the well-documented effect of estrogens to enhance leptin production, even high levels of isoflavone consumption do not alter leptin concentrations in women. Further studies are needed to more precisely delineate the nature of estrogenic and/or antiestrogenic effects of isoflavones in humans.