Mammographic breast density as a biomarker of effects of isoflavones on the female breast

Isoflavones are polyphenolic compounds naturally present in foods of plant origin that bear a striking structural similarity to the mammalian estrogens. Soybeans, and products made from soybeans, are by far the richest identified sources of isoflavones (Table 1), and it has been hypothesized that soy consumption in Asian populations may contribute to the lower incidence of breast cancer and other hormone-dependent conditions in Eastern versus Western populations [1]. Isoflavones are capable of binding to estrogen receptors α and β [2], and they are weak estrogens. In common with other weak estrogens, and similar to selective estrogen receptor modulators such as tamoxifen, isoflavones can act as both estrogen agonists and estrogen antagonists, the latter suggesting potential antiproliferative effects of isoflavones on breast tissue. In Western diets, the lignans, another class of phytoestrogens, are quantitatively more important than isoflavones, but the evidence for lignans possessing estrogenic and/or anti-estrogenic activities is less well established, and fewer data are available on their potential beneficial health effects in humans.There are few prospective studies on the effects of isoflavones on breast cancer risk. In a recent large-scale, population-based case-control study among Chinese women, intake of soy foods during adolescence, a period when breast tissue is particularly sensitive to environmental stimuli, was associated with a significant reduction in breast cancer risk in later life [3]. In one of the largest studies to date, however, neither tofu nor miso consumption had a significant effect on risk of breast cancer among almost 35,000 women living in Hiroshima and Nagasaki, Japan [4]. No strong confirmation of an effect of soy on reducing risk of breast cancer is thus apparent from existing studies, but no studies to date have identified an increased risk of breast cancer associated with elevated intakes of isoflavones.There are many trials in the