Interviews Nutrition Research Science

Will soy consumption increase my risk of breast cancer?

This article was part of a group assignment created by Katherine Fisher, Ana Maafs and Thanit (Pao) Vinitchagoon for NUTR0497 Translating Nutrition Evidence into Multimedia for the Public, offered in Fall 2019. The consumer’s article aims to shed some light on the previous debate about soy consumption and risk of breast cancer in American vs. Asian populations.

Soy is one of the many foods that has been the victim of scientific debate. Different studies yielded contradictory results, the media got over-excited, and there was mass confusion about dietary advice regarding soy. Even more so among people with breast cancer. This article aims to unpack the science behind the health effects of soy on breast cancer and will share the latest recommendations for soy consumption based on current evidence.

Current Breast Cancer Trends in the US

According to the Breast Cancer Organization, the number of new breast cancer cases in the US had reduced in 2000, partially due to a decrease in the use of hormone replacement therapy. However, the American Cancer Society reports that since 2011 the number of new cases has increased again. This is probably due to improvement in diagnosis techniques and earlier detection of cases. In addition, women with breast cancer are living longer. Still, breast cancer is the second leading cause of cancer death in women, following lung cancer.

The Debate About Soy and Breast Cancer Risk

The conflicting evidence about soy and breast cancer is related to isoflavones, a chemical compound naturally found in soy, beans and legumes. Isoflavones, along with other similar phenolic compounds, are present in soy and have been associated to antioxidant, antimicrobial, and anti-inflammatory health benefits. Isoflavones stand out because they mimic our body’s estrogen activity. Estrogen is a group of sex hormones that promote the development and maintenance of female characteristics in the human body. Many cases of breast cancer are related to estrogen activity. The fact that soy isoflavones may have a similar effect led some researchers to recommend caution when consuming soy products. Cell and animal studies showed that isoflavones stimulated cancer growth.

However, evidence from the 1990s and 2000s suggested that in Asian populations, soy and isoflavones had a protective effect against cancer risk, but not in Western populations. During an interview with Dr. Zhang, a professor from Tufts University who studies food patterns in populations with cancer, she explained why results show different associations. These differences can be attributed to four elements: study population, study design, characteristics of soy consumption, and starting age of soy consumption. “The difference might be at least 10-fold on how much soy food is consumed in America compared to those who live in Asian countries,” she added.

Compilation of Study Results

A single study gives only a snapshot of a problem within a particular context. Combining several snapshots from diverse study settings can widen our understanding of the problem to make results more generalizable. Researchers from Shengjing Hospital of China Medical University pooled together five different studies – three from China and two from the US – to understand the effect of soy consumption on breast cancer outcomes. Dr. Chi and collaborators gathered data from more than 11,000 individuals living with breast cancer. They compared the group with the highest versus the lowest soy consumption. Overall, they found that women with higher soy consumption had 16% decreased risk of cancer death, and 26.5% decreased risk of cancer recurrence than those with low soy consumption.

So, is Soy a Sufficient Cause of Concern in the U.S.?

Apparently not. Considering the current evidence, the World Cancer Research Fund classified several risk factors according to their association with breast cancer. Soy is not there. It might be that evidence about individual foods is less defined and determining the effect of one specific dietary component is difficult. “The influence of one single food is small and it should be considered as part of overall dietary patterns,” Dr. Zhang mentioned. American women generally consume low amounts of soy to have a significant effect on breast cancer risk. Nonetheless, it could make a positive impact on overall health if soy – or other protective foods – is consumed in addition to or in replacement of other dietary components, such as red and processed meats.

But not all Soy is Created Equal

The most pressing concerns for consumers aware of potential breast cancer risk is not whether to eat soy and how much to eat, but rather how processed the soy they are consuming is and what role it plays in individual’s diet. Dr. Zhang mentions that the body’s response to soy from supplements and ultra-processed food may be different than from non-ultra-processed soy foods. She recommends reducing the amount of ultra-processed foods to have a healthier diet.

With the recent surge of soy products that have invaded the market, consumers of soy should be mindful about where they are getting their plant-based proteins. According to the Soyfoods Association of North America, fermented and unfermented soy can currently be consumed as a beverage, texturized, beans, meat alternative, tofu, edamame, condiment, and even spread. As if this variety is not enough, there is even a burger made with soy. And this burger is only one singular product among the dozens of meat alternatives that have been developed in the last few years. Evidence to date is mostly related to simple soy foods and beverages; therefore, more research on the specific sources of soy is needed.


Most recent evidence suggests that soy may have either beneficial or neutral effects on breast cancer outcomes. The differences in ethnicity, usual amount consumed, types of soy products and types of dietary patterns between populations make comparison difficult. Based on current research, it appears to be safe for women in the U.S. to consume soy products as part of a healthful diet and in periodic replacement of red and processed meats. Moreover, the consumption of soy, and food in general, should mainly be from non-ultra-processed foods, since high consumption of ultra-processed foods might have negative health effects.

To learn more about the subject, view part of Dr. Zhang’s interview by clicking here.

Katie Fisher is a second year FANPP student and hails from a background in agricultural economics. Her focus at Tufts Friedman School is to understand how food and nutrition policy affects consumers. Originally from Arizona, she loves to explore New England every chance she gets and can be found doing that or binge-watching Netflix most weekends.

Ana Maafs is a second year MS NICBC student. Originally from Mexico City, she worked as a nutritionist for a few years before coming to Boston. She is interested in behavior change at individual and community level. She enjoys having bubble tea from different places around Chinatown.

Thanit (Pao) Vinitchagoon is a U.S.-based Registered Dietitian who worked in a hospital in Bangkok, Thailand for a couple of years before joining the Friedman School as a Ph.D. student in Nutrition Interventions, Communications, and Behavior Change. His interest is in eating disorder prevention and the “non-diet” approach to nutrition and health. He enjoys eating out occasionally in different places since experimenting with new foods (for him) is very fun!

The Friedman Sprout is a monthly student run newspaper that aims to serve the student population at the Friedman School of Nutrition Science and Policy, prospective students, and alumni. Our mission is to report on newsworthy information that affects the Friedman community including nutrition research, food policy, internship and volunteer opportunities, as well as school events. Our editorial slant is that of sustainability in food and nutrition.

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