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Mark Messina, PhD, of Soy Connection/United Soybean Board, writes about misconceptions associated with soy consumption and breast cancer.
While U.S. dietary recommendations have remained fairly consistent since they were first issued in 1980, consumer perceptions about the healthfulness of foods fluctuate due to a host of influencers, from peer opinions to scientific studies. These perceptions shape food company ingredient choices and packaging decisions. The result can be a confusing marketplace for shoppers who may not know where to turn to learn the facts.
Soy is a food and ingredient that is frequently misunderstood. One of the most common misunderstandings is its effect on breast cancer. The truth is that soy does not cause cancer and may actually offer benefits for women with breast cancer.1-7 The good news for women who enjoy soyfoods, from veggie burgers and energy bars to soymilk and tofu, is that soyfoods are safe to eat1 and are rich in a variety of vitamins and minerals, including folate and potassium; many are also high in fiber.8 Moreover, soy protein and soybean oil (a.k.a. vegetable oil) may reduce the risk of coronary heart disease.9
Despite the misperception around soy and breast cancer, according to United Soybean Board 2019 research,10 about three-quarters of consumers (74%) with an opinion on the healthfulness of soy rate soyfoods as “very” or “somewhat healthy.” Further, 42% of consumers under 35 reported eating soyfoods once a week or more in 2019. It’s evident that consumers feel positive about soyfoods-so, what’s driving the disconnect when it comes to soy and breast cancer?
Initial concerns about this relationship date to the late 1990s, when studies in mice showed isoflavones stimulate the growth of existing estrogen-sensitive breast tumors.11-12 Isoflavones, which are commonly classified as phytoestrogens or plant estrogens, occur naturally in soybeans.13
These results were unexpected. For nearly a decade prior to these studies, the National Cancer Institute was investigating the potential role of soyfoods and isoflavones in preventing breast cancer. Earlier research showed that isoflavones could function as antiestrogens, which suggested that they might help prevent the development of estrogen receptor–positive breast cancer.14 This theory was strengthened by the observations that breast cancer rates were historically low in countries where soyfoods are a common part of diets.15
While the research in mice appeared to counter those findings, looking at what happens to mice who are fed soy isoflavones doesn’t necessarily tell us how these compounds affect women. Still, for many years, these studies were nearly all that were available.
That changed dramatically in 2009 with the publication of findings from the Shanghai Breast Cancer Survival Study.16 This study, which involved over 5,000 women with breast cancer, found that women who consumed higher amounts of soy after they were diagnosed with breast cancer had a lower likelihood of recurrence and improved survival. Three years later, researchers conducted an analysis of this study plus two similar U.S. studies, involving nearly 10,000 participants in total. It showed that among women who had been diagnosed with breast cancer, those who consumed the most soy had a 25% lower chance of having a recurrence of their disease.17-18
Consistent with these studies are the positions of the American Cancer Society19 and the American Institute for Cancer Research,20-21 which is that women who have had breast cancer can safely eat soyfoods. Further, the World Cancer Research Fund International21 concluded that eating soy may be one of several lifestyle steps that improve survival after a breast cancer diagnosis.
While this research is encouraging, observational studies that link dietary habits with disease outcomes can’t provide definitive data because they can’t establish cause-and-effect relationships. For that, we need intervention studies where study participants are fed a specific food or diet.
So far, intervention studies have not been conducted to determine the effect of soy consumption on reoccurrence or survival in women with breast cancer. But there have been many studies of the effects of soyfood or isoflavone consumption on markers or indicators of breast cancer risk. These markers include breast tissue density and breast cell proliferation. Denser breast tissue and faster growth of breast cells are both linked to greater risk of breast cancer.22
Studies that examined the impact of soy on these markers involved heathy women, women at increased risk of breast cancer, and women with breast cancer. These studies consistently showed that neither soy nor isoflavones adversely affected these markers. After reviewing these data, the European Food Safety Authority, which is analogous to the FDA, recently concluded that isoflavones don’t adversely affect breast tissue.23
The scientific understanding of the safety of soyfoods and their possible benefits for women with breast cancer has greatly evolved since researchers first began investigating this area of research. The bottom line: the evidence overwhelmingly shows that consuming soy does not increase risk of developing breast cancer risk, and that women with a history of breast cancer can safely eat soyfoods.
About the author:
Dr. Mark Messina is an internationally recognized expert on the health effects of soy. He has appeared on CNN and is regularly quoted by the media in publications, including The New York Times, Newsweek, and USA Today. Dr. Messina has presented to both consumer and professional audiences in 44 countries on topics including soy and cancer risk, heart health, menopause, and men’s health.
Dr. Messina is the co-author of The Simple Soybean and Your Health. His research has appeared in numerous professional journals, including The American Journal of Clinical Nutrition, The Journal of Nutrition, The Lancet, and the Journal of the National Cancer Institute.
He is a former program director in the Diet and Cancer Branch of the National Cancer Institute, National Institutes of Health, where he initiated a multimillion-dollar research program investigating the role of soy in cancer prevention.
Dr. Messina co-owns Nutrition Matters Inc., a nutrition consulting company He is an adjunct associate professor in the Department of Nutrition, School of Public Health, at Loma Linda University, and is the executive director of the Soy Nutrition Institute.
2. Lampe JW. “Emerging research on equol and cancer.” Journal of Nutrition, vol. 140, no. 7 (July 2010): 1369S-1372S
3. Nagata C. “Factors to consider in the association between soy isoflavone intake and breast cancer risk.” Journal of Epidemiology, vol. 20, no. 2 (2010): 83-89
4.Chen M et al. “Association between soy isoflavone intake and breast cancer risk for pre- and post-menopausal women: a meta-analysis of epidemiological studies.” PLoS One, vol. 9, no. 2 (February 20, 2014): e89288
5. Nechuta SJ 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.” American Journal of Clinical Nutrition, vol. 96, no. 1 (July 2012): 123-132
6. Zhang YF et al. “Positive effects of soy isoflavone food on survival of breast cancer patients in China.” Asian Pacific Journal of Cancer Prevention, vol. 13, no. 2 (2012): 479-482
7. Chi F et al. “Post-diagnosis soy food intake and breast cancer survival: A meta-analysis of cohort studies.” Asian Pacific Journal of Cancer Prevention, vol. 14, no. 4 (2013): 2407-2412
8. USDA National Nutrient Database for Standard Reference Release. 16108, Soybeans, mature seeds, raw. Available at https://ndb.nal.usda.gov/ndb/foods/show/16108.
9. U.S. Food and Drug Administration. “Soybean Oil and Reduced Risk of Coronary Heart Disease.” Available at https://www.fda.gov/media/106649/download. July 31, 2017. Health claim: “Supportive but not conclusive scientific evidence suggests that eating about 1½ tablespoons (20.5 grams) daily of soybean oil, which contains unsaturated fat, may reduce the risk of coronary heart disease. To achieve this possible benefit, soybean oil is to replace saturated fat and not increase the total number of calories you eat in a day. One serving of this product contains 14 grams of soybean oil.”
10. 2019 United Soybean Board Food Industry Insights annual survey.
11.Helferich WG et al. “Phytoestrogens and breast cancer: a complex story.” Inflammopharmacology, vol. 16, no. 5 (October 2008): 219-226
12. Setchell KD et al. “Soy isoflavone phase II metabolism differs between rodents and humans: implications for the effect on breast cancer risk.” American Journal of Clinical Nutrition, vol. 94, no. 5 (November 2011): 1284-129
13. Murphy PA et al. “Solvent extraction selection in the determination of isoflavones in soy foods.” Journal of Chromatography. B, Analytical Technologies in the Biomedical and Life Sciences, vol. 777, no. 1-2 (September 25, 2002): 129-138
14. Folman Y et al. “The interaction in the immature mouse of potent estrogens with coumestrol, genistein and other utero-vaginotrophic compounds of low potency.” The Journal of Endocrinology, vol. 34, no. 2 (February 1966): 215-225
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19. Rock CL et al. “Nutrition and physical activity guidelines for cancer survivors.” American Cancer Society. CA: A Cancer Journal for Clinicians, vol. 62, no. 4 (July/August 2012): 242-274 Available at https://onlinelibrary.wiley.com/doi/full/10.3322/caac.21142.
20. American Institute for Cancer Research. “Soy and Cancer Survivorship.” Available at http://www.aicr.org/patients-survivors/healthy-or-harmful/soy.html.
21. World Cancer Research Fund/American Institute for Cancer Research. “Diet, Nutrition, Physical Activity and Breast Cancer Survivors.” Updated 2018. Continuous update project. Available at http://www.aicr.org/continuous-update-project/breast-cancer.html. Accessed July 14, 2018.
22. Rice MS et al. “Mammographic density and breast cancer risk: a mediation analysis.” Breast Cancer Research: BCR, vol. 18, no. 1 (September 21, 2016): 94
23. Huser S et al. “Effects of isoflavones on breast tissue and the thyroid hormone system in humans: a comprehensive safety evaluation.” Archives of Toxicology, vol. 92, no. 9 (2018): 2703-2748. Available at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6132702/