Soy Intake and Breast Cancer Risk

In my private practice experience, I find that many clients have been led to believe that soy-based foods may increase the risk of developing breast cancer.

As a result, they are wary about consuming these foods. Breast cancer is no joke.

According to Canadian Cancer Society Statistics (Ontario):

    1. In 2017, breast cancer was projected to be the most common cancer in Canadian women, with more incidents than lung and colorectal cancer combined.
    2. Breast cancer is the second leading cause of death from cancer in women, behind only lung cancer.

Given this data, it’s unsurprising that people may be readily looking for causes of breast cancer, and one place we all tend to look is food choices.

So, if someone has heard, from whatever the source, that the intake of soy-based foods could potentially play a role in increasing the risk of developing breast cancer, no one can blame them for their concern.

There’s only one problem though – all the strong human evidence investigating intake of soy and its effect on breast cancer risk shows that, if there is any effect at all, it’s protective.

So, where does the fear and misinformation stem from? Most often from animal or in vitro studies, which may be of limited utility.

When it comes to breast cancer risk and soy intake, the results from these types of studies tend not to be supported by the evidence in humans.

Today, I’m going to take a look at a number of high-quality human studies that support the notion of a protective, or at worst neutral, effect of soy intake on breast cancer risk.

Additionally, I will be looking at the topic of soy intake in those living with breast cancer – taking into consideration that there are different types of breast cancer and looking at whether the type of breast cancer a women is living with modifies the effect of soy intake on health outcomes.and treatment plans, and assessing if those two variables that affects how soy intake may contribute to risk of breast cancer recurrence or death.

Before we go any further I must also note that today’s post was sponsored by Archer Daniels Midland (ADM), but all opinions are my own.

Let’s get started!

Soy Intake and Breast Cancer Risk

The first and perhaps most important question that needs to be addressed is: are high levels of soy intake associated with an increased risk of breast cancer?

It’s important for the women out there who have been scared of soy intake to have an understanding of what the best available evidence has to say when it comes to assessing these outcomes.

When we take a close look at the evidence, the answer is pretty resounding:

2014 systematic review out of the Japanese Journal of Clinical Oncology

Conclusion: Soy possibly decreases the risk of breast cancer in Japanese women specifically.

(similar conclusions in Japanese populations in this 2013 study)

2014 meta-analysis out of the PLoS ONE journal

Conclusion: Soy intake may be protective against breast cancer in women from Asian countries, while no association between soy intake and breast cancer was found for women in Western countries.

2015 prospective cohort study from the Federation of American Societies for Experimental Biology

Conclusion: Soy intake is associated with reduced risk of breast cancer in a largely Caucasian and African-American population from North America.

So, what we see here from this sampling of results from the more notable human studies available, is that soy intake has, at worst, a neutral effect on risk in North American women and a potential positive effect on various populations of Asian women.

In other words, there is no population level evidence to suggest an increased risk of breast cancer is associated with high intake of soy-based foods.

So why might soy impact women living in Asia differently than other women?

Two possible explanations, as cited in the Food Science and Human Wellness Journal, include:

    1. Many women in Asian countries consume soy throughout their lifetime, whereas in North America, it may be a food that is introduced much later in life.
    2. There may be genetic differences that affect the way Asian women metabolize soy phytoestrogen

Breast Cancer Survivors

With that major concern addressed, the next big question we should ask is whether or not soy-based foods are suitable for women who have or have had breast cancer.

A 2009 cohort study in JAMA out of Shanghai China followed breast cancer survivors for several years and examined the potential relationship between their intake of soy-based foods and risk of breast cancer recurrence and/or death.

They found there to be an “evident” relationship between higher soy consumption and reduction in breast cancer recurrence and/or death.

This relationship existed independent of the type of breast cancer and whether or not they used tamoxifen (a common anti-cancer medication).

Again, one of the big things to keep in mind when interpreting these results is the geographical location and the fact that, per capita, women living in China are more likely to have consumed soy foods throughout their lifetime, and at a higher level than the average North American woman.

So, what happened when a similar study was conducted in the US?

A 2017 prospective study out of the Cancer journal looked at American breast cancer survivors and found similar but slightly different results.

The overall results suggest a protective effect of soy intake on breast cancer reoccurrence in survivors, but with some caveats.

For example, high levels of soy intake were protective in those who had taken tamoxifen, but not those who did not.

Soy intake was also only found to be protective in specific type of tumour ER+/PR+. The majority of breast cancer tumours are classified as ER+/PR+, which refers to the fact that the cancer grows in response to estrogen and progesterone.

Together, these are a pair of observational studies that provide valuable insights into soy intake in breast cancer survivors.

As you can see, there is no reported evidence of harm and, if anything, only evidence of a potential protective effect of soy intake in breast cancer recurrence.

Keep in mind that, as compared to the study in Shanghai cited above, the levels of soy intake in American women were quite a bit lower in this study, due to cultural differences.

This may or may not offer some explanation into the outcome differences.

Final Thoughts

Today’s article served a very important purpose.

Right now, as I experience it from the perspective of my private practice, there is a heightened concern over possible negative health effects of soy intake in women as it relates to breast cancer risk and survival.

A review of the available evidence suggests that there is no confirmed association between soy intake and breast cancer risk or recurrence.

In fact, in many contexts, there is evidence of a positive effect of soy intake on breast cancer related health outcomes, however more research is needed.

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