Phytoestrogens

Phytoestrogens are plant compounds that have effects similar to those of estrogen, the major female sex hormone, in the body. They can be divided into two main groups: isoflavones, which are found in soybeans, red clover, kudzu root, among others; and lignans, substances that are found in flaxseed, whole grains, and some fruits and vegetables. Soy is a unique dietary source of the much-studied isoflavones called genistein and daidzein.

Phytoestrogens are thought to compete with natural estrogens for estrogen receptors on cells. By binding to the receptors, phytoestrogens prevent estrogen from stimulating certain tissues and theoretically lower the risk of developing cancers that are spurred by estrogen. Isoflavones are much more potent than lignans when it comes to interacting with estrogen receptors in the body.

Although they compete for the same binding sites on cells, phytoestrogens do not act exactly the same way natural estrogens do. Chemically, for example, isoflavones are only about one-thousandth as potent as natural estrogens. But in some of the body's tissues, phytoestrogens mimic the action of estrogen and may alleviate the symptoms of menopause in older women; in other tissues, they block the action of estrogen and thereby lower the risk of developing some kinds of cancer, especially of the breast.

Phytoestrogen-rich soy supplements and isoflavone supplements in pill form are promoted as alternatives for estrogen replacement therapy (ERT) in menopausal women.

Population studies have found a link between soy-rich diets and a low risk of breast and endometrial cancer in women and prostate cancer in men. The research that has attracted the most attention has compared Asian women to American women and found that Asian women who eat traditional diets that contain a lot of soy have a much lower risk of breast cancer and rarely experience hot flashes and other menopausal symptoms. These protective effects are attributed to the phytoestrogens found in soy. Several controlled studies have found that giving either soy-supplemented diets or isoflavone supplements extracted from red clover results in a decrease in the frequency of hot flashes experienced by menopausal women.

Laboratory and animal studies show that phytoestrogens can inhibit tumor growth. Studies with cultured human breast cancer cells, leukemia cells, and prostate cancer cells have demonstrated the ability of phytoestrogens to slow tumor growth.

Research has also shown that phytoestrogens can inhibit the oxidation of the harmful LDL (low-density lipoprotein) cholesterol, a step believed to trigger the formation of fatty plaque in arteries. Soy experts believe that phytoestrogens are partly responsible for the cholesterol-lowering effects seen with soy protein. Research has also found that isoflavone supplements extracted from red clover increased the ability of arteries to stretch and allow blood to flow freely-a function called arterial compliance-in postmenopausal women.

There is also some research to suggest that phytoestrogens may help prevent osteoporosis. For example, population studies indicate that phytoestrogens may protect against bone loss in older women, including those whose diets provide less than the recommended amounts of calcium. Asian women, for instance, consume very little milk and milk products; they also tend to be thin and small-boned-factors that theoretically would increase their risk of osteoporosis. Yet their incidence of this disease is somewhat less than that of American women, even though Americans generally consume more calcium than Asians. The soy-rich Asian diet is credited with the reduced risk of osteoporosis.

Phytoestrogens are found naturally in soy, legumes, flaxseed, whole grains, fruits and vegetables, and red clover. Soy protein and whole soy foods like tofu, roasted soybeans, and soy milk are especially good sources.

Isoflavone supplements extracted from either soy or red clover are available in pill and capsule forms. A number of phytoestrogen supplements sold in health food stores contain a mixture of isoflavones and extracts of wild yam, dong quai, and kudzu root-herbal sources of phytoestrogens-along with various vitamins and minerals. Soy protein is available in powdered form, which can be mixed with milk or juice.

The recommended intakes of isoflavones vary considerably, but many experts point to the average Asian intake of about 25 to 50 mg a day as a safe and possibly effective dose. However, some high-potency phytoestrogen supplements provide higher doses of isoflavones of 500 mg or more.

Some experts are concerned that taking high doses of phytoestrogens could backfire, actually increasing the risk for some kinds of cancer. One group that could possibly be at risk are postmenopausal women with undiagnosed estrogen-dependent breast cancer. Most experts believe that a daily dosage of 50 mg of phytoestrogens is safe for most people, including men. But, it is not known if higher doses taken on a regular basis could cause problems.