Effect of soy consumption on breast cancerThere's uncertainty regarding the effects of soy isoflavones in post-menopausal women. Studies have shown that soy isoflavones may provide post-menopausal women with the same benefits of hormone replacement therapy (HRT): reduction of hot flashes, vaginal dryness and protection against heart disease and bone loss (osteoporosis). 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.
Does soy increase risk of breast cancer?Whether soy can help protect against breast cancer in post-menopausal women is unclear at this time. It remains controversial whether women diagnosed with breast cancer should be advised to eat more or less soy foods. 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 study at the Wake Forest University School of Medicine, entitled "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 . A more recent study investigated the association of dietary intake of isoflavone with all-cause mortality in 6235 women with breast cancer . During a follow-up of about 9 years 1224 deaths were reported. The mortality for women who had the highest versus lowest quartile of dietary isoflavone intake was 21% lower.
References Breast and uterine effects of soy isoflavones and conjugated equine estrogens in postmenopausal female monkeys. J Clin Endocrinol Metab. 2004 Jul;89(7):3462-8.
 Dietary isoflavone intake and all-cause mortality in breast cancer survivors: The Breast Cancer Family Registry. Cancer. 2017 Mar 6. doi: 10.1002/cncr.30615.