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Myth Busting - Soy foods & breast cancer

Where did this myth come from? You may have heard soy foods contain isoflavones, which are very similar to oestrogen in their action in the body. And, in around 70% of breast cancers, oestrogens stimulate growth of the cancer cells. So it is tempting to link eating more isoflavones in soy with increased growth of breast cancer cells. At first glance the case against soy foods may seem strong, however on closer inspection the evidence certainly doesn’t stack up.

As women age and particularly after menopause oestrogen levels within breast tissue can increase, which can increase the risk of breast cancer. Research now indicates that when natural oestrogens levels are higher, isoflavones from soy foods may actually slow down the growth of cancer cells by ‘getting in the way’ of the natural oestrogens which have a much stronger potential to stimulate cancer cells. Not convinced? This greater understanding of how isoflavones work has been supported by population studies which consistently show that soy foods do not increase the risk of breast cancer; in fact women who eat more isoflavones from soy food like tofu or soy drinks are 30% less likely to have the breast cancer return and are more likely to live longer than those people who don’t eat soy based foods.

Background Bites: A study investigated the links between pre breast cancer diagnosis soy intake and mortality. This study included 3842 women in the Multiethnic Cohort (MEC) study of African Americans, Native Hawaiians, Japanese Americans, Latinos, and Caucasians aged 50 years and older. Consistent with the current body of evidence the authors found that pre-diagnostic soy intake was unrelated to mortality in postmenopausal women.

Conroy S et al.

The Effects of Soy Consumption before Diagnosis on BreastCancer Survival:

The Multiethnic Cohort Study Nutrition and Cancer. 2013;65 (4):527-537

The potential influence of soy isoflavones on breast cancer prognosis as well as their interaction with the hormonal therapy tamoxifen have led to concern about soy food consumption among breast cancer patients.1 These concerns have stemmed from the results of in-vitro and animal studies, but the relevance of these results in women consuming soy foods is not established.2 Importantly, recent evidence indicates that there is no increased risk of breast cancer recurrence or increased mortality with increased soy intake in breast cancer patients.3-10 The largest study to date on the influence of soy food and breast cancer outcomes tracked over 9,500 American and Chinese breast cancer survivors for seven years, including women taking tamoxifen. The authors concluded that eating soy food is safe for breast cancer survivors and contrary to previous concerns, women who consumed the highest intakes of soy foods had a 29% reduced risk of breast cancer specific mortality and a 36% reduced risk of recurrence compared with those with the lowest intake of soy foods.3

  1. NSW Cancer Council. Position statement. Soy, phyto-oestrogens and cancer prevention. 2006
  2. Setchell K et al. Am J Clin Nutr 2011;94:1284-94
  3. Nechuta S et al. Am J Clin Nutr 2012;96:123–32
  4. Cann B et al. Cancer Epidemiol Biomarkers Prev. 2011;20(5); 854–8
  5. Zhang Y et al.  Asian Pacific J Cancer Prev. 2012;13:479-482
  6. Korde L, Wu A, Fears T. Cancer Epidemiol Biomarkers Prev. 2009;18:1050-1059
  7. Lee S, et al. AJCN. 2009;89:1920–6.
  8. Messina M, Wood C. Nutr Journal. 2008;7:17
  9. Shu X, et al. JAMA, 2009; 302(22)
  10. Wu AH, Yu MC, Tseng C-C, Pike MC. British Journal of Cancer 2008;98:9–14

Conroy S et. The Effects of Soy Consumption before Diagnosis on Breast Cancer Survival: The Multiethnic Cohort Study. Nutrition and Cancer. 2013;65(4): 527-537.

Nechuta S et al. Soy food intake after diagnosis of breast cancer and survival: an in-depth analysis of combined evidence from cohort studies of US and Chinese women. Am J Clin Nutr. 2012;96(1):123-32.
Information from Grains & Legumes Nutrition CouncilTM, GLNC or #glnc.