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.
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.
Commentary: 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, a phytoestrogen, 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 but in a very few laboratory studies certain isoflavone components of soy 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), suggest that soy containing foods do not negatively affect breast cancer prognosis, do 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.
Caan B, Natarajan L, Parker B, et al. Soy food consumption and breast cancer prognosis. Cancer Epidemiol Biomarkers Prev 2011;20(5):854-858.