Are phytoestrogens a "natural alternative" to estrogen replacement therapy?

Abstract

Phytoestrogens, compounds derived from plants and converted into weak estrogenic substances in the gastrointestinal tract, are heavily promoted as the “natural alternative” to estrogen replacement therapy for postmenopausal women or women who have undergone a hysterectomy. The use of soy products rich in these phytoestrogens is now widespread in the United States. Isoflavones are the best-known phytoestrogens, but only a few studies have been done of their effects. The results of these studies have not been promising. Their supposed benefits are based on the observation that the incidence of certain disorders or diseases affected by estrogen status—perimenopausal syndrome, cardiovascular disease, and osteoporosis—is lower in populations with diets high in phytoestrogens. There is no laboratory or clinical evidence to support claims of benefit or even to demonstrate hormonal effects of phytoestrogens. A recent commentary on phytoestrogens concluded: “Until such time as there are data from controlled studies on the dangers as well as the benefits of plant extracts, the drawing of firm conclusions about the long-term benefits of increased phyto-oestrogen consumption is unwise.”1(p164) For most of the world's population, the principal source of phytoestrogen is soy in the diet. Since October 1999, the Food and Drug Administration has allowed food companies to claim that soy protein reduces cholesterol concentrations and the risk of heart disease. Although consuming soy products seems to reduce cholesterol levels, there may be an increased cancer risk associated with their isoflavone components; this risk may be heightened when isoflavones alone are offered in the form of dietary supplements. The highest levels of naturally occurring isoflavones are found in soy beverages, cooked soybeans, and tempeh (textured fermented soy). One gram of soybeans contains about 1 mg of isoflavones. Some nutritional experts are willing to assume that a daily intake of 50 mg of isoflavones is safe. The typical daily consumption in Japan is 25 to 50 mg of isoflavones. Dietary supplements, however, can contain more than 85 mg of isoflavones in a single unit, and some manufacturers advise taking 2 a day. Soy protein concentrate powders have as much as 160 mg of isoflavones per serving. According to some evidence, large quantities of isoflavones may increase the risk of breast cancer.