Soy and Breast Cancer

I have a lovely patient who suffers terribly with menopausal hot flushes.  It’s been years and they show no signs of stopping.  The problem is, she also has a condition that increases her risk of breast cancer.  Hormone replacement with estrogen, while it would help her hot flushes, would be dangerous for her.  Some time ago her oncologist told her she also should avoid soy because it has estrogen effects and may increase her breast cancer risk as well.

You probably know I hate to see anyone suffer.  I hate it even more when the reason for the suffering is based on faulty or outdated logic.  I know newer research has shown that soy foods and soy isoflavone supplements do not increase the risk of breast cancer, but I didn’t have the research to back that claim up.  Off to the research database!

First, some background info.  The reason so many doctors and scientists assumed soy was dangerous for breast cancer patients is because soy contains substances, called isoflavones, that are structured like estrogens.  There’s evidence they can bind to estrogen receptors in cells.

It was assumed that, since most breast cancer tumors are responsive to estrogen, that any estrogen activity would stimulate the cells to grow.  In fact, highly successful treatments for breast cancer like Tamoxifen and Arimidex act by blocking ALL estrogen activity.  As you can imagine, these medications cause a lot of side effects like hot flushes, vaginal dryness and other symptoms that mimic menopause.

You know what happens when we assume, right?  More recently, scientists have decided to question that assumption and look to see if soy intake (both soy foods and soy supplements) actually does increase the risk of breast cancer.

What did they find?  LOTS of studies are out there, but I just want to mention a few.  There was a review article published in late 2013 that looked at 131 different studies on soy foods and soy and red clover isoflavones.  There was evidence that eating soy foods was protective against breast cancer.  Even stronger evidence is that breast cancer patients taking Tamoxifen had no increased risk of recurrence when they used soy.

Another study published in February of 2014 analyzed 35 studies looking at associations between breast cancer risk and soy intake.  The study concluded that in Asia, soy intake reduced the risk of breast cancer in women both before and after menopause.  However, there was no change in breast cancer risk demonstrated in women in Western countries with soy intake.  There was certainly no evidence of an INCREASED risk of breast cancer in women using soy.

A very large study published in 2013 asked over 3800 women about their dietary patterns, including soy intake, when they enrolled in the study.  Over 14 years the authors tracked several variables in study participants, particularly breast cancer diagnosis, breast cancer mortality and all-cause mortality.  There was no difference in breast cancer risk or mortality (from breast cancer or other causes) in women with the highest soy intake vs. those with the lowest soy intake.

A review article published in Germany in 2016 also concluded that soy did not increase the risk of breast cancer and increases survival after breast cancer diagnosis.

It is pretty clear that soy intake does not increase the risk of breast cancer and may actually be protective in some populations.  Why is this?  There is a theory that soy isoflavones, while mimicking estrogens in structure, do not actually behave like estrogens.  Therefore, when they bind to estrogen receptors in cells, they block the actions of the person’s own estrogen molecules.  This theory could explain why soy isoflavones do not increase breast cancer recurrence in patients taking Tamoxifen for estrogen-sensitive breast cancer.  The soy actually behaves a bit LIKE Tamoxifen without the side effects.

If you have considered trying soy isoflavones to reduce hot flashes or using soy as a good source of complete dietary protein, there is plenty of good evidence that it won’t hurt you.  Even if you have a higher-than-normal risk of breast cancer, or have actually developed breast cancer, there’s no evidence that soy is harmful.  As always, you should always discuss supplementation with your doctor to make sure any supplements won’t interfere with your treatment plan.

QUESTION:  Do you have menopausal hot flashes?  Have you considered trying soy?  Have you been told it could be harmful?

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