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Soy and Breast Cancer

I’ve been working in the cancer nutrition field for just shy of two decades, and I’ve been asked about soy foods and breast health for nearly that entire time!

There are now several very large, human studies in which thousands of women have been followed for many years. These studies consistently show that compared with women who do not eat soy, women who regularly consume soy foods have less risk of breast cancer. More than one of these studies also suggests that even after a woman has had breast cancer, soy is safe.

Some of these studies show that breast cancer survivors who consume soy foods have a lower risk of breast cancer recurrence than survivors who avoid soy. These studies have been conducted in both Asian and US populations. This is important because soy has long been a part of many Asian cuisines, but it is a relatively new introduction to the American diet [i][ii]. 

To be fair, these studies are observational. This means researchers collect diet information from women, then follow them for many years to see who gets what diseases. Researchers look at a particular disease, for example breast cancer, and try to determine if this is related to whether or not a woman eats soy, or some other food. In an observational study, it is always possible that the true connection with better breast health is not soy, but something else that is related to eating soy [iii][iv].

For example, suppose that women who eat soy foods also tend to eat less fried food. And maybe they also happen to eat more vegetables and they tend to be thinner, and to exercise more. Any one of these other things—avoiding fried foods, the vegetables, the lower body weight, or more exercise—could be the reason why soy-eating women have a lower risk of breast cancer. So maybe it’s not the soy itself decreasing breast cancer risk, but something else.

This means that large, observational studies can’t conclusively prove that soy protects against breast cancer, but they are reassuring in affirming that soy does not increase breast cancer risk. And they do point toward a protective effect of soy on breast health, regardless of other lifestyle and diet choices, for both women in general, and breast cancer survivors.


Fermented vs. Unfermented Soy

Some people believe that fermented soy is healthier than unfermented. For example, most types of tofu are unfermented, but tempeh is fermented. Miso is a fermented soy food, but soymilk is not. To understand this difference, consider milk and yogurt. Milk is not fermented. Yogurt, which turns into yogurt when certain bacteria are introduced into milk, is fermented.

The research on soy and breast health has not, for the most part, separated out the difference between fermented and unfermented soy. These studies have simply tallied up the total amount of all soy foods eaten. Typically this would include both fermented and unfermented soy.

I personally believe that variety is always better, and I try to include both fermented and unfermented soy in my diet. In the same way that you can absorb different nutrients from raw vs. cooked vegetables, you get different nutrients from fermented vs. unfermented soy. And research does suggest that a greater variety in the way people consume their plant foods—whether it’s soy, other beans, vegetables, fruit, nuts, or seeds—is the healthiest pattern of eating.



I believe one reason so many women have concerns about soy arises from the term “phytoestrogens.”

Some soy nutrients—the isoflavones—have chemical structures that look a bit like the estrogen found in a woman’s body. However, it’s important to note that phytoestrogens are not the same thing as female estrogens, and soy foods do not contain estrogen.

I also believe it’s unfortunate that people think of soy in relation to its so-called “estrogenic” effects. This gives the impression that the only actions of soy foods in the body are related to soy’s impact on estrogen or estrogen receptors, but this simply isn’t true.

Health experts who study soy note that it has many beneficial effects in the body, including antioxidant activity and the ability to alter cell behavior down to the molecular level. Soy foods seem to enhance a cell’s ability to withstand damage. Soy nutrients also can encourage damaged cells to die, in a process called apoptosis, before they go on to become cancerous.

Soy nutrients turn up the body’s production of something called sex hormone binding globulin (SHBG). The more SHBG in the body, the lower the levels of circulating, “free” estrogen. This is important, because it is only free estrogen that can bind with cells—normal or cancerous—in breast tissue and encourage growth. More SHBG means less free estrogen in circulation.

Soy can turn on the production of cell proteins that dampen cell growth; these proteins are linked with lower cancer risk. Soy can turn down the production of proteins that are linked with higher risk of cancer­—those that spur growth. Soy appears to have anti-inflammatory activities. The list of soy’s potential beneficial effects is impressive indeed. And none of these actions I’ve described have anything to do with estrogen [iv][v].

Unfortunately, because everyone (including health care providers) uses the term “phytoestrogens” to describe soy nutrients, it’s hard to recognize these other beneficial activities of soy. And it’s very easy to get hung up on the word “estrogen.”

When it comes to breast cancer, estrogen is something women and their doctors fear. But just remember that phytoestrogens are not the same thing as the estrogens produced by the body. Also keep in mind that soy has many anti-cancer effects that have nothing to do with estrogen at all! [vii][viii].


A Final Note

Soy foods are a healthy option, while soy supplements may not be a good choice. All of the research supporting the potential benefits of soy for breast health have looked at soy foods, not supplements. So if you’re a woman concerned about breast health, stick to healthy, whole, soy foods, such as tofu, tempeh, soymilk, and edamame. Don’t rely on powders and pills.

You can feel confident in whatever choice you make about soy foods: Eat these foods if you enjoy them, or skip them altogether if soy food isn’t your cup of tea!

[i] Nechuta SJ, Caan BJ, Chen WY, Lu W, Chen Z, Kwan ML, Flatt SW, Zheng Y, Zheng W, Pierce JP, Shu XO. 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):123-32.
[ii] 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):479-82.
[iii] Caan BJ, Natarajan L, Parker B, Gold EB, Thomson C, Newman V, Rock CL, Pu M, Al-Delaimy W, Pierce JP. Soy food consumption and breast cancer prognosis. Cancer Epidemiol Biomarkers Prev. 2011;20(5):854-8.
[iv] Dong JY, Qin LQ. Soy isoflavones consumption and risk of breast cancer incidence or recurrence: a meta-analysis of prospective studies. Breast Cancer Res Treat. 2011;125(2):315-23.
[v] Kang X, Zhang Q, Wang S, Huang X, Jin S. Effect of soy isoflavones on breast cancer recurrence and death for patients receiving adjuvant endocrine therapy. CMAJ. 2010;182(17):1857-62.
[vi] Butler LM, Wu AH, Wang R, Koh WP, Yuan JM, Yu MC. A vegetable-fruit-soy dietary pattern protects against breast cancer among postmenopausal Singapore Chinese women. Am J Clin Nutr. 2010;91(4):1013-9.
[vii] Shu XO, Zheng Y, Cai H, Gu K, Chen Z, Zheng W, Lu W. Soy food intake and breast cancer survival. JAMA. 2009;302(22):2437-43.
[viii] Guha N, Kwan ML, Quesenberry CP Jr, Weltzien EK, Castillo AL, Caan BJ. Soy isoflavones and risk of cancer recurrence in a cohort of breast cancer survivors: the Life After Cancer Epidemiology study. Breast Cancer Res Treat. 2009;118(2):395-405.

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