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Soy consumption may reduce mortality among women with breast cancer
Higher dietary intake of isoflavones — the compound found primarily in soy — appeared associated with reduced all-cause mortality among women diagnosed with breast cancer, according to results of an epidemiological analysis.
“At the population level, we see an association between isoflavone consumption and reduced risk [for] death in certain groups of women with breast cancer,” Fang Fang Zhang, MD, PhD, nutrition and cancer epidemiologist at Friedman School of Nutrition Science and Policy at Tufts University, said in a press release. “Our results suggest, in specific circumstances, there may be a potential benefit to eating more soy foods as part of an overall healthy diet and lifestyle.”
Soy foods contain antiestrogenic- and estrogen-like properties. Therefore, controversy remains about whether women diagnosed with breast cancer should be advised on their soy intake.
Zhang and colleagues analyzed data from 6,235 women diagnosed with breast cancer who participated in the NIH–funded Breast Cancer Family Registry.
Women completed food frequency questionnaires that assessed their dietary intake. Researchers then categorized the women into four groups based upon the estimated amount of isoflavone consumed.
After median follow-up of 9.4 years, 1224 deaths had occurred.
Women in the highest quartile of dietary isoflavone intake exhibited a 21% reduced risk for all-cause mortality compared with those in the lowest quartile ( 1.5 mg daily vs.<0.3mg daily, HR = 0.79; 95% CI, 0.64-0.97). The association between higher isoflavone intake and reduced mortality risk appeared limited to women with hormone receptor–negative disease (HR = 0.49; 95% CI, 0.29-0.83) and women who did not receive hormone therapy for breast cancer (HR = 0.68; 95% CI, 0.51-0.91).
HemOnc Today spoke with Zhang about the results, as well as whether these findings put to rest concerns about soy consumption among women with breast cancer.
Question: Can you describe the overall health benefits of soy?
Answer: Suggestive health benefits of soy include improved lipid profiles, lower blood pressure, and contributions to bone health. For its role in cancer, some studies suggest that soy inhibits cell proliferation, has antioxidant properties and can reduce the production of hormone estrogen in the human body. The health benefits of soy are still under investigation.
Q: Can you explain the concern with soy and the potential association with breast cancer?
A: Soy contains isoflavone, which is a class of phytoestrogen — a plant-based estrogen that is structurally similar to the hormone estrogen that circulates in the human body. High levels of circulating hormone estrogen are well-known risk factors for breast cancer. Because isoflavone has similar structures of the hormone estrogen, it can bind to the hormone receptors of the cell and have estrogen-like effects.
Q: How did the idea for this study come about?
A: It is controversial whether women should consume more soy or avoid soy consumption to prevent the occurrence or recurrence of breast cancer. Isoflavone may interfere with cancer therapies for women treated with hormone therapies, such as tamoxifen. This is what we set out to investigate.
Q: How did you conduct your study?
A: Our prospective cohort study included more than 6,000 women diagnosed with breast cancer. The cohort derived from an existing registry known as the Breast Cancer Family Registry, sponsored by the NIH. Women answered questions about their typical diet, specifically how frequently they consume various foods and in what portion sizes. Researchers analyzed this information to estimate how much isoflavone these women typically consume, and followed participants over time to assess survival outcomes. Of note, we did not include isoflavone intake from dietary supplements.
Q: What did you find?
A: First, we found an overall 21% reduction in total mortality among the women in our study who consumed the highest quartile of isoflavone intake. Second, we found the largest reduction in mortality among women with ER– and PR–negative breast cancers or among those who did not receive hormone therapy. We did not observe a significant reduction in mortality among women with ER– and PR–positive breast cancer or among those who received hormonal therapy.
Q: Does the amount of soy consumed have a significant effect on breast cancer occurrence/recurrence?
A: The amount of soy a person consumes varies depending upon where he or she lives. The average consumption of isoflavone from the diet is 3 mg per day or less among women residing in North America and Europe. This level is substantially lower than the amount of soy consumed by women who live in China or Japan. For example, the average consumption of isoflavone in the Chinese diet is between 25 mg and 50 mg per day. However, despite this significant difference, evidence supports the survival benefits of a higher intake of soy among women who live in China, as well as those who live in the United States and Canada.
Q: Do the findings put to rest any concern about an association between soy and breast cancer?
A: Large-scale studies in the United States and China have consistently shown no harm of soy consumption in women with breast cancer. Thus, patients should not be advised to stop soy food consumption if they have already regularly consumed soy in their diet. Women with breast cancer can consume a reasonable amount of soy foods as part of a healthy and balanced diet. However, I would caution the use of soy supplements. Our study did not assess this; therefore, we do not know if supplements provide a benefit. Patients should consult with their physicians before using dietary supplements that contain soy.
Q: What is next for research?
A: We need a better understanding of whether soy consumption influences a woman’s response to cancer treatment and, if so, through which biological mechanisms soy shapes those responses. Such knowledge can shed light on opportunities to develop chemopreventive agents for patients with breast cancer. On a broader view, although many survivors are highly motivated to seek information about dietary changes to improve their long-term health, most are still burdened by suboptimal dietary intake. Our research has shown that cancer survivors in the United States have poor adherence to federal dietary guidelines, and their intake patterns are worse than those of the general population. This gap between a desire for improved health and poor dietary habits highlights the strong need for identifying and addressing barriers to dietary guideline adherence among cancer survivors.
Q: Is there anything else that you would like to mention?
A: Oncology care providers play an important role in reinforcing the importance of a healthy diet, and they can refer patients to registered dietitians or other reputable sources to improve survivors’ dietary intake. Patients with cancer are encouraged to discuss nutritional issues with their oncology care providers and consider nutrition an important component of a healthy lifestyle. – by Jennifer Southall
Reference:
Zhang FF, et al. Cancer. 2017;doi:10.1002/cncr.30615.
For more information:
Fang Fang Zhang, MD, PhD, can be reached at Tufts University, Friedman School of Nutrition Science and Policy, 150 Harrison Ave., Boston, MA 02111; email: fang_fang.zhang@tufts.edu.
Disclosure: Zhang reports no relevant financial disclosures.