BACKGROUNDHormonal effects of soy and isoflavones have been investigated in numerous trials with equivocal findings. We aimed to systematically assess the effects of soy and isoflavones on circulating estrogen and other hormones in pre- and post-menopausal women.METHODSThe Cochrane Library, MEDLINE and EMBASE (plus reviews and experts) were searched to December 2007. Inclusion of randomized or residential crossover trials of soy or isoflavones for 4 or more weeks on estrogens, SHBG, FSH, LH, progesterone and thyroid hormones in women was assessed independently in duplicate. Six percent of papers assessed were included. Data concerning participants, interventions, outcomes, potential effect modifiers and trial quality characteristics were extracted independently in duplicate.RESULTSForty-seven studies (11 of pre-, 35 of post- and 1 of perimenopausal women) were included. In premenopausal women, meta-analysis suggested that soy or isoflavone consumption did not affect primary outcomes estradiol, estrone or SHBG concentrations, but significantly reduced secondary outcomes FSH and LH [by ∼20% using standardized mean difference (SMD), P = 0.01 and 0.05, respectively]. Menstrual cycle length was increased by 1.05 days (95% CI 0.13, 1.97, 10 studies). In post-menopausal women, there were no statistically significant effects on estradiol, estrone, SHBG, FSH or LH, although there was a small statistically non-significant increase in total estradiol with soy or isoflavones (∼14%, SMD, P = 0.07, 21 studies).CONCLUSIONSIsoflavone-rich soy products decrease FSH and LH in premenopausal women and may increase estradiol in post-menopausal women. The clinical implications of these modest hormonal changes remain to be determined.

Background: Soy consumption is known to reduce plasma total cholesterol and LDL cholesterol in hypercholesterolemic subjects, but the responsible soy components and the effects in normocholesterolemic subjects remain unclear. Objective: The effects of soy isoflavone consumption on plasma total cholesterol, HDL-cholesterol, LDL-cholesterol, triacylglycerol, apolipoprotein A-I, apolipoprotein B, and lipoprotein(a) concentrations and on LDL peak particle diameter were examined in normocholesterolemic, premenopausal women. Design: Thirteen healthy, normocholesterolemic, free-living, premenopausal female volunteers took part in this randomized, crossover-controlled trial. Each subject acted as her own control. Three soy isoflavone intakes (control: 10.0 ± 1.1; low: 64.7 ± 9.4; and high: 128.7 ± 15.7 mg/d), provided as soy protein isolate, were consumed for 3 menstrual cycles each. Total cholesterol, HDL cholesterol, LDL cholesterol, and triacylglycerol were measured over the menstrual cycle. Apolipoprotein A-I, apolipoprotein B, lipoprotein(a), and LDL peak particle diameter were evaluated in the midluteal phase. Results: Total cholesterol, HDL-cholesterol, and LDL-cholesterol concentrations changed significantly across menstrual cycle phases (P < 0.005). During specific phases of the cycle, the high-isoflavone diet lowered LDL cholesterol by 7.6-10.0% (P < 0.05), the ratio of total cholesterol to HDL cholesterol by 10.2% (P < 0.005), and the ratio of LDL to HDL cholesterol by 13.8% (P < 0.002). Conclusions: Isoflavones significantly improved the lipid profile across the menstrual cycle in normocholesterolemic, premenopausal women. Although of small magnitude, these effects could contribute to a lower risk of developing coronary heart disease in healthy people who consume soy over many years.

Phyto-oestrogens are oestrogenic compounds found in plants and consist of isoflavones, lignans and coumestans. Epidemiological studies provide evidence for a protective role of isoflavones, and to a lesser extent lignans, against the development of numerous chronic diseases, including several cancers, cardiovascular disease and osteoporosis. The structural similarity of phyto-oestrogens to endogenous oestrogens has prompted the hypothesis that phyto-oestrogens exert hormonal or anti-hormonal effects relevant to the risk of hormone-dependent disease and/or their suitability as a dietary alternative to hormone replacement therapy. The many human studies that have evaluated the effects of isoflavones and lignans on various endpoints relating to risk of various diseases have greatly increased knowledge of how these compounds behave. At the same time, additional questions have been generated. For example, the increasing interest in extracting isoflavones from the soybean for incorporation into dietary supplements has raised important concerns regarding safety and efficacy. Overall, it is clear that phyto-oestrogens are an area of active and advancing research with great potential to continue to affect human health.

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