Breast cancer patients can be assured that enjoying soy food products won't hurt them and may even impart some benefit for reducing the risk of recurrence. But when discussing the pros and cons of soy, clinicians should be aware that there are differences in the quantity and quality of soy in popular food products.
Soy is known to contain both estrogenlike and anti-estrogenic properties, and previous research has yielded conflicting results. A 2003 study suggested that the soy protein genistein may cause ER-positive tumors to grow faster while a 2006 meta-analysis of research on soy intake and breast cancer noted only a modest reduction in risk (Cancer Res 61:5045-5050; J Natl Cancer Inst 98:459-471).
"Can we be satisfied with prospective cohort studies?," said Donald I. Abrams, MD, president of the Society for Integrative Oncology. "Although we crave randomized, double-blind, placebo-controlled trials to generate incontrovertible evidence, such a study would be obviously impossible. We cannot assign a population to never consume soy foods. Placebo tofu definitely sounds unpalatable...most thought leaders conclude that soy foods are probably fine [for women with breast cancer]. Isoflavone supplementation is discouraged as they may be more potent phytoestrogens. Less processed soy products (edamame, soy milk, tofu, miso) are preferred over heavily processed soy items such as soy cheese, soy hot dogs, or soy turkey."
Excerpted from an editorial in the Journal of the Society for Integrative Oncology online, February 2010.
More recent studies have bolstered the benefits of soy (see Fact box), including the latest research from Xiao Ou Shu, MD, PhD, and colleagues. They studied the association of soy food consumption after a breast cancer diagnosis using data on 5,033 patients from the Shanghai Breast Cancer Survival Study. Dr. Shu is a professor of medicine in the division of epidemiology at Nashville's Vanderbilt University Medical Center and Vanderbilt- Ingram Cancer Center. Co-authors are from the Shanghai Institute of Preventive Medicine. Dr. Shu reported having received a research development fund from the United Soybean Board in 2005, according to JAMA.
|•||Soy product and isoflavone intake and breast cancer risk defined by hormone receptor status, Cancer Sci online, September 29, 2009|
|•||Adolescent and adult soy food intake and breast cancer risk: Results from the Shanghai Women's Health Study, Am J Clin Nutr 89:1920-1926, 2009|
|•||Soy isoflavones have an antiestrogenic effect and alter mammary promoter hypermethylation in healthy premenopausal women, Nutr Cancer 61:238-244, 2009|
The women were sent a structured questionnaire. The authors collected clinical information and assessed habitual dietary intake at various times over a 36- month period. A food frequency questionnaire measured the consumption of soy foods (tofu, soy milk, fresh soy beans) and other foods such as meat, fish, and cruciferous vegetables. As of June 2009, the 36-month interview was completed for 4,354 of 4,934 eligible patients. The major endpoints for the study were death from any cause, cancer recurrence or metastasis, or death from breast cancer (JAMA 302:2437-2443, 2009).
In the multivariate analysis, Dr. Shu's group adjusted for known clinical predictors and lifestyle factors related to soy intake and survival, including age at diagnosis, tumor stage, method of treatment (chemotherapy, radiotherapy, etc), hormone-receptor status, and tamoxifen use.
After a median follow up of 3.9 years, the authors documented 444 total deaths and 534 recurrences or breast cancerrelated deaths in the study group. They found that soy protein or soy isoflavone intake after cancer diagnosis was inversely associated with mortality and recurrence.
When comparing the hazard ratios for the highest and lowest quartiles of soy protein intake, the authors calculated HRs of 0.71 for total mortality and 0.68 for recurrence. The corresponding HRs for mortality when soy isoflavone intake was considered were 0.79 for mortality and 0.77 for recurrence. The multivariate-adjusted, four-year mortality rate for women in the lowest quartile was 10.3% while the recurrence rate was 11.2%. For women in the highest quartile, the multivariate-adjusted, four-year mortality rate was 7.4% and the recurrence rate was 8%.