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Your Health – The Power of Soy

Incorporating phytoestrogens into your diet can help lower your risk of breast cancer.

Many of us live in fear of developing breast cancer. It often seems to strike without warning and appears fairly nonselective. Breast cancer is the most common cancer diagnosed in women, accounting for an estimated 16% of all cancer deaths. It remains the second leading cause of death due to cancer after lung cancer among women. For women ages 20 to 59, breast cancer is the leading cause of cancer mortality.1

But you can take some preventive measures that could save your life. Dietary changes can minimize your risk and regain some level of control against the development of breast cancer. Recent studies have shown that incorporating phytoestrogen-rich foods into your diet may reduce your future risk of developing breast cancer.2,3


Phytoestrogens are compounds found in plants with activity similar to estrogen in the body and their chemical make-up resembles your own body’s estrogen.Two major categories of phytoestrogens are isoflavones and lignans. Soy is the major source of isoflavones, while flaxseed is the major source of lignans.

The action of phytoestrogens in the body is complex. In some body tissues they act like a weak estrogen. But in other body tissues, like the breast, they appear to have antiestrogenic properties. Phytoestrogen’s affinity for the estrogen receptor (how strong they bind to the receptor) is approximately 35% that of estradiol, the estrogen produced from your ovaries.

Because of this resemblance to your body’s estrogen, phytoestrogens can mimic the action of your natural estrogen on some of your body’s tissues and organs. They can decrease hot flashes, and may have an effect on other symptoms of menopause, including mood disturbances, sleep difficulties, fatigue, and vaginal dryness.

Phytoestrogens seem to protect against the development of breast cancer. Japanese women eating a traditional diet rich in soy products have a low incidence of estrogen dependent cancers, such as breast cancer, compared to Western women. This incidence increases once Asian women westernize their diet. Breast, colon, prostate, endometrial, and ovarian cancers and coronary heart disease all have lower incidences in Asia and Eastern Europe than in western countries. Japan consistently has the lowest risk of hormone-dependent cancers. Studies in humans show that women who consume high quantities of soy products have reduced rates of breast cancer.4


The mechanism behind this protective effect is not yet fully understood. It may relate to the fact that phytoestrogens are a weak estrogen and, therefore, compete with the body’s own natural estrogen so that the breast is not exposed to as much estrogen in the long run. This effect would likely be important in pregnancy, the premenopausal years, and in adolescence,when estrogen levels are higher. Thus, these plant estrogens in some way block the action of the body’s own estrogen on the breast, possibly by occupying the same receptor in the breast that your own natural estrogen usually occupies.

This theory suggests that the protective effect of phytoestrogens on breast cancer risk needs to be started early in life, possibly in the womb. This is important to remember, since many women start consuming soy foods and flaxseed during menopause in order to decrease their menopausal symptoms.However, it appears that to prevent breast cancer through this hormone blocking mechanism, adding soy to the diet should be started much earlier.The anticancer mechanisms of phytoestrogens are also likely attributable to metabolic properties that do not involve estrogen receptors. A proposed mechanism for this nonhormonal protective effect of phytoestrogens in the diet is that they block an enzyme, tyrosine kinase, that is important in cancer cell growth. Blockage of this enzyme inhibits the growth of cancer cells by decreasing the action of certain cellular growth factors.


Data on the effects of a phytoestrogenrich diet during pregnancy and a decreased risk of breast cancer later in life in the babies exposed to phytoestrogens in utero are growing. This information,in both animals and humans, indicates that there may be a protective effect of a phytoestrogen-rich diet in pregnancy in preventing breast cancer in babies later in their life. Phytoestrogens freely pass from the mother’s bloodstream,through the placenta, and into the blood circulation of the fetus. The levels of phytoestrogens in the mother’s blood stream are very similar to those in the fetal circulation. Consequently, scientists have postulated that consuming phytoestrogens during pregnancy may have beneficial effects on the fetus that could decrease the future risk of hormonedependent cancers in these babies by modulating the amount of estrogen in their in-utero life.


I do not recommend obtaining phytoestrogens from supplements such as pills or powders. Phytoestrogen supplements are highly processed, and the effects of this processing on how well the body absorbs them are not clear. It may be that there are yet unrecognized cofactors in soy foods and flaxseed that could be responsible for the anticancer properties and health benefits of phytoestrogens that are processed away in supplements. This is analogous to the fact that anti-oxidants from vitamin supplements do not appear as effective in their anti-cancer properties as eating the natural fruits and vegetables that are rich in these substances.

It is also possible that the very high concentrations of phytoestrogens potentially available from supplements could be harmful. There are some supplements in which phytoestrogens are so concentrated that it would be like eating a pound of tofu at a sitting. Too much of a good thing can be dangerous.

How much soy is the right amount? No one knows the answer to this question.The average Asian woman’s diet contains about 3 to 4 ounces of soy foods such as tofu a day, yielding about 50 to 70 milligrams of isoflavones. Thus, if you include soy or flaxseed in one of your meals every day, you will get roughly the same amount.

Some soy products such as vegetarian or soy burgers are made with soy-protein concentrate, which contains almost no isoflavones. These soy products do not have the phytoestrogen benefits of unprocessed soy products such as tofu and miso. However, if you see soy-protein isolate on the label, the isoflavones are largely retained. Tofutti spreads, such as soy cream cheese, are often made with soy protein isolate. So check the label carefully if you are eating some of the more “ready made” soy products.


  1. Colditz GA. Epidemiology of breast cancer. Findings from the nurses’health study. Cancer. 1993;15(71):1480-1489.
  2. Ingram D, Sanders K, Kolybaba,Lopez D. Case-control study of phytoestrogens and breast cancer. Lancet.1997;350:990-994.
  3. Barnes S, Grubbs C, Setchell K,Carlson J. Soybeans inhibit mammary tumors in models of breast cancer. In: Parzia M, ed. Mutagens and Carcinogens in the Diet. New York: Wiley-Liss;1990:239-253.
  4. Adlercreutz H, Honjo H, Higashi A,et al. Urinary excretion of lignans and isoflavonoid phytoestrogens in Japanese men and women consuming a traditional Japanese diet. Am J ClinNutr. 1991;54(6):1093-1100.
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