Women who have estrogen-sensitive breast cancer have often been advised to stay away from soy foods and soy isoflavone supplements. This recommendation was based on studies in test tubes and in animals that showed the isoflavone genistein may stimulate the growth of estrogen-receptor breast cancer cells. Since it was not known if these results applied to humans, it made sense for women who have estrogen-receptor positive breast cancer to follow this recommendation until more information was available. That information is now available and the new recommendation is that soy consumption at moderate intake should actually be encouraged.
Considerable evidence from laboratory and human studies indicates soy exerts significant anticancer effects, particularly in hormone-sensitive cancers such as breast and prostate cancer. Specifically, as an anticancer agent, soy isoflavonoids:
- Act as antioxidants.
- Reduce estrogen levels, particularly free estrogen. (Lower levels of estrogen have been associated with a decreased risk of breast cancer.)
- Prevent the formation of new blood vessels, thus preventing tumors from obtaining a blood supply necessary for continued growth.
- Prevent tumor cells from dividing and growing by inhibiting enzymes involved in cell replication.
Population studies have consistently found that soy consumption may help reduce a woman’s risk of developing breast cancer. Clinical and experimental studies further support the benefits of soy.
In the last few years, several studies with a total of nearly 10,000 women have found that ingestion of soy foods do not increase the risk of recurrence of breast cancer. In fact, a meta-analysis of these studies found was that increasing soy consumption was associated with lowering the risk of recurrence and/or all-cause mortality even in women currently taking tamoxifen. Specifically, ingestion of >10 mg/day of isoflavone was associated with a statistically significant 25% decrease in recurrence of breast cancer.1
In a recent study, soy intake was assessed in 616 women with a history of breast cancer and these women were followed for an average of 52 months.2 Results showed a reduction of mortality from breast cancer of 36% in those consuming 17-29 mg/day of total isoflavones versus those consuming <7.56 mg/day. Consuming more than 29 mg/day did not offer any significantly greater protection unless the woman was estrogen receptor positive (ER+). These women showed a 41% reduced mortality to breast cancer at the higher intake level.
Soy intake was from tofu, processed soy products, soy milk, and whole soybeans. Results indicated that total soy protein intake was also of benefit. Overall, 5-year survival rates were 62.5% for those consuming less than 2 g/day of soy protein versus 98.6% for those consuming more than 13 g/day.
All together, these results from this study further support the positive effects of moderate intake of soy foods in women with a history of breast cancer, especially those with ER+.
1. Nechuta SJ, Caan BJ, Chen WY, 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):123-132.
2. Zhang YF, Kang HB, Li BL, Zhang RM. Positive effects of soy isoflavone food on survival of breast cancer patients in China. Asian Pac J Cancer Prev. 2012;13(2):479-482.