Effects of soy isoflavones and the menstrual cycle on plasma leptin concentrations

Numerous studies have shown interactions between leptin and reproductive hormones in animals and humans. In order to evaluate the effects of soy phytoestrogens and the menstrual cycle on plasma leptin concentrations, 14 premenopausal women were given three soy powders for 3.5 menstrual cycles each. Each powder contained a different level of isoflavones, which were given relative to body wt (Diet A: 2.01±0.03, Diet B: 1.01±0.04, Diet C: 0.15±0.01 mg total isoflavones/kg body wt). Fasting blood samples from the early follicular (EF), midfollicular (MF), periovulatory (PO) and midluteal (ML) phases of the menstrual cycle were analyzed for reproductive hormones and leptin. Plasma leptin concentrations changed significantly between phases (p<.0001). Leptin was lowest in the EF (10.3±3.5 ng/mL) and MF (10.3±3.4 ng/mL) phases, increasing significantly in the PO phase (12.1±4.7 ng/mL), and increasing further in the ML phase (12.8±3.3 ng/mL). Plasma leptin was significantly lower in Diet A (10.0±1.6 ng/mL) and Diet B (9.8±1.6 ng/mL) than in Diet C (11.5±1.6 ng/mL), in the MF phase only (p<.02). A similar, but nonsignificant effect was also seen in the PO phase (p=.056). The day to day variability in plasma leptin concentrations showed individual coefficients of variation (CV) between 13-40%, with an average CV of 23%. These results show large intra-subject variability and significant effects of both the menstrual cycle and soy isoflavones on plasma leptin concentrations.