Tori Hudson, N.D.

Reference: Caan B, Natarajan L, Parker B, et al. Soy food consumption and breast cancer prognosis. Cancer Epidemiol Biomarkers Prev 2011;20(5):854-858.

Design: Data from the Women’s Healthy Eating and Living (WHEL) was used to examine the effect of soy intake on breast cancer prognosis in 3,088 breast cancer survivors. These women were early stage breast cancer patients who were followed for an average of 7.3 years. Soy isoflavone intakes were measured after the diagnosis with a food frequency questionnaire. The association between soy intake and breast cancer recurrence and/or death was then tracked.

Participants: 3,088 breast cancer survivors.

Primary outcome: Risk of breast cancer recurrence or death from breast cancer in breast cancer patients as it relates to soy intake.

Key findings: As soy isoflavone intake increased, the risk of death decreased. Women at the highest levels of soy isoflavone intake (> 16.3 mg isoflavones/day) had a 54% reduction in risk of death.

Practice Implications: This is the third epidemiologic study to report no adverse effects of soy foods on the prognosis of breast cancer. Soy foods, which contain isoflavones, show both antiestrogenic and estrogen-like properties. The confusion and controversy has been that many studies have shown that isoflavones may protect against an initial breast cancer. In very few laboratory studies certain soy isoflavone components have been able to enhance the proliferation of breast cancer cells in select doses, and have been able to both promote and inhibit mammary tumor growth in rats.

However, in 2009, some clarity began to emerge for breast cancer patients. In breast cancer survivors, one study in Asian women (the Shanghai Breast Cancer Survival Study) and the other in U.S. women (the Life after Cancer Epidemiology study), suggested that soy containing foods do not negatively affect breast cancer prognosis, to not counteract the effect of the breast cancer drug tamoxifen and may in fact provide potential benefits in decreasing risk of recurrence or death from breast cancer.  

The current study has explored this issue further, by examining data from a randomized controlled trial, the WHEL study. The results of this study, and the two previous in 2009, should give practitioners and women alike, great reassurance in the safety of soy consumption for women with who have/have had a diagnosis of breast cancer. We no longer need to advise against soy consumption for breast cancer survivors. This is great news given all the potential health benefits of soy for bone health, cardiovascular health and soy as a part of a whole foods and healthy diet.