Isoflavones are polyphenolic compounds that are capable of exerting estrogen-like effects. For this reason, they are classified as phytoestrogens—compounds with estrogenic activity derived from plants (1). Legumes, particularly soybeans, are the richest sources of isoflavones in the human diet. In soybeans, isoflavones are present as glycosides (bound to a sugar molecule). Fermentation or digestion of soybeans or soy products results in the release of the sugar molecule from the isoflavone glycoside, leaving an isoflavone aglycone. Soy isoflavone glycosides are called genistin, daidzin, and glycitin, while the aglycones are called genistein, daidzein, and glycitein, respectively (chemical structures of isoflavone aglycones). Unless otherwise indicated, quantities of isoflavones specified in this article refer to aglycones—not glycosides.
* Isoflavones are a class of phytoestrogens—plant-derived compounds with estrogenic activity. Soybeans and soy products are the richest sources of isoflavones in the human diet.
* The results of recent randomized controlled trials suggest that substituting 50 g/d of soy protein for animal protein results in only a modest 3% reduction of LDL cholesterol. Isolated soy isoflavone supplements do not appear to have favorable effects on serum lipid profiles.
* Although hip fracture rates are generally lower among Asian populations consuming soy foods than among Western populations, it is not yet clear whether increasing soy isoflavone consumption in Western populations will help prevent osteoporosis.
* Overall, the results of numerous observational studies do not support the idea that high soy isoflavone intakes in adults are protective against breast cancer. Limited research suggests that higher intakes of soy foods early in life may decrease the risk of breast cancer in adulthood.
* Although scientists are interested in the potential for soy isoflavones to prevent or inhibit the progression of prostate cancer, evidence from observational studies that soy isoflavones are protective against prostate cancer is limited and inconsistent.
* Overall, there is little evidence that increasing soy isoflavone intake from food or supplements substantially improves menopausal hot flushes.
* Although diets rich in soy or soy-containing products appear safe and potentially beneficial, the long-term safety of high doses of soy isoflavone supplements is not yet known.
* At present, there is no convincing evidence that infants fed soy-based formula are at greater risk for adverse effects than infants fed cow’s milk-based formula.
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