Soy isoflavones are phytochemical constituents of soybeans, a legume that has served as the basis for various foods in many Southeast Asian countries for thousands of years. The major isoflavones in soy are genistein and daidzein. Soyfoods also contain other valuable constituents, including protein, saponins, and phytosterols.
Soy isoflavone is regarded as a "natural" dietary supplement ingredient to replenish the aging body's declining estrogen levels and thus relieve menopausal symptoms, such as hot flashes, as well as decrease the risk of heart disease and osteoporosis, without promoting breast cancer. Soy isoflavones show great promise in reducing the uncomfortable hot flashes of menopause, and new research indicates that soy may also help protect against certain chronic diseases, including osteoporosis, heart disease, and some cancers.
Isoflavones are considered nonsteroidal estrogens: They are strikingly similar in chemical structure to the estrogen hormones women produce, but they are also different enough not to be full-fledged steroidal hormones. Many of soy's desirable health benefits are attributed to the isoflavones, the naturally occurring plant compounds found in soybeans in varying amounts. Studies show that isoflavones may play a part in lowering LDL (or "bad" cholesterol) in the blood, reducing hot flashes and bone loss in menopausal women, and inhibiting the growth of some cancer cells.
Soy isoflavones and other plant estrogens are effective hormone replacements after a hysterectomy. Other researchers have reported the isoflavones are also estrogenic enough to promote bone formation. In fact, the European drug ipriflavone, used to treat osteoporosis, is a synthetic isoflavone; daidzein is one of its metabolites.
The isoflavones act as antiestrogens--that is, in opposition to estrogen--and many researchers believe this is why they reduce the risk of breast and endometrial cancer. Estrogen signals cells to proliferate, which is why it can be carcinogenic. Genistein, which has only 1/1,000 the hormonal activity of estrogen, attaches to the breast cells' estrogen receptors and by doing so blocks the more potent female hormone from attaching. So in summary, when a woman has little natural estrogen production (post-hysterectomy or postmenopause), isoflavones can attach to open estrogen receptor sites on cells and produce a weak estrogen effect. When there is too much estrogen (during PMS, for example), isoflavones can compete with the natural estrogen for receptor sites, and because their effect is weak in comparison, they blunt the estrogen effect.
Isoflavones also reduce cancer risk by inhibiting the activity of tyrosine kinase, an enzyme that promotes cancer cell growth. Isoflavones are also powerful antioxidants. Like other antioxidants, they can reduce the long-term risk of cancer by preventing free radical damage to deoxyribonucleic acid (DNA), the complex molecule that contains genes. According to Rice-Evans' research, genistein is the most potent antioxidant among the isoflavones, followed by daidzein. Genistein may also increase the body's production of superoxide dismutase (SOD), a powerful antioxidant that quenches superoxide radicals.
In areas outside cancer research, the most immediate and observable effect of supplemental soy isoflavones is the reduction of menopausal hot flashes. The soy lessened menopausal symptoms, lowered blood pressure and resulted in a healthier blood lipoprotein profile. The supplements led to these benefits, without the side effects associated with conventional hormone replacement therapy. Soy isoflavones also appear to reduce cardiovascular disease risk via several distinct mechanisms. Researchers have reported that soy foods lower blood cholesterol levels.
In addition, soy isoflavones may maintain normal vascular function.