Effect of soy consumption on breast cancer

Researchers have only recently started to investigate the possible effects of soy to breast cancer. More and more women who are adding soy to their diets to help prevent breast cancer but researchers are unsure of the exact effects.

How can soy protect against cancer?

Some animal studies and small human clinical trials have shown that soy foods may offer some protection against breast cancer. Researchers believe that isoflavones may help protect against breast cancer because isoflavones compete with natural estrogen in the body to bind to special estrogen receptors on cells. It is known that estrogen stimulates breast cell growth. Because isoflavones may block estrogen from reaching estrogen receptors, pre-menopausal women who include soy in their diet may decrease their risk of breast cancer.

Risk of soy for post-menopausal women

There's more uncertainty regarding the effects of soy isoflavones in post-menopausal women. Studies have shown that soy isoflavones may provide post-menopausal women with many of the benefits as hormone replacement therapy (HRT): reduction of hot flashes, vaginal dryness and protection against heart disease and bone loss (osteoporosis). One study with mice showed that genistein (an isoflavone found in soy) had adverse effects on the development of cancer. This is an animal study and it's unclear if the same effects will be seen in humans.
Whether soy can help protect against breast cancer in post-menopausal women is unclear at this time. Some researchers believe that soy consumption may actually increase the risk for breast cancer in post-menopausal women because the chemical structure of isoflavones is very similar to estrogen. Although experts say the effects are still unknown , no studies have clearly shown that that consumption of soy by humans increase breast cancer risk. Asian women, who consume more soy than Western woman, have a breast cancer rate one-fifth that of Western women. Also unknown are the effects of phytoestrogens on women who already have breast cancer or those at risk because of genetic factors.A recent study "Breast and Uterine Effects of Soy Isoflavones and Conjugated Equine Estrogens in Postmenopausal Female Monkeys", published in the The Journal of Clinical Endocrinology and Metabolism, suggested that high dietary levels of soy isoflavones do not stimulate breast or uterine proliferation in postmenopausal monkeys and may contribute to an estrogen profile associated with reduced breast cancer risk.