Last week I received a copy of a very interesting 2009 study which examines the role of soy, tamoxifen and estrogen receptors in breast cancer survival.

The study was published in the esteemed Journal of American Medical Association (JAMA) on December 9, 2009.  If you’d like to read the entire article, here it is:  Soy Food Intake & Breast Cancer Survival 2009 study.

The objective of the study, called the Shanghai Breast Cancer Survival Study was to evaluate the association of the intake of soy foods after a breast cancer diagnosis.  It was quite a large study – over 5,000 female breast cancer survivors aged 20-70 years with diagnoses between March 2002-April 2006 were followed up through June 2009.  It was one of the largest population-based studies of breast cancer survival when it was published.  I’d encourage you to read it, it’s interesting.

Many are Confused About Whether Soy is Safe or Not

I’m writing about this today, some 3 years after publication, because there still seems to be quite a lot of confusion about the role of soy’s phytoestrogens (plant-based estrogens) among breast cancer survivors and those actively battling breast cancer.  We are often told by our doctors and their nurses to be wary of too much soy. Their advice is well-meaning, but flawed – that because the phytoestrogens in soy can supposedly act as weak estrogens, those who had estrogen receptor positive tumors (meaning estrogen appeared to fuel the growth of the tumors) should exercise caution and not eat too much soy.

This study makes it clear that this is not how soy works. Here’s a direct quote:

“In our comprehensive evaluation of soy food consumption and breast cancer outcomes using data from a large, population-based cohort study, we found that soy food intake was inversely associated with mortality and recurrence. The inverse association did not appear to vary by menopausal status and was evident for women with ER-positive and ER- negative cancers and early and late-stage cancers.”

For those not accustomed to the language used in scientific studies, “inversely associated with” means that the more soy foods that were eaten, the less mortality and recurrence was exhibited in the study participants.

Soy Phytoestrogens vs. Our Estrogen

The Study also found that soy isoflavones compete with the body’s estrogen in the binding of estrogen receptors, they increase the synthesis of sex hormone-binding globulin (thus lowering the bioavailability of sex hormones like estrogen), they reduce estrogen synthesis and increase the clearance of steroid hormones from circulation.  It is thought that these anti-estrogenic effects may be one of the underlying mechanisms through which the consumption of soy foods is associated with better breast cancer outcomes.

Soy Phytoestrogens vs. Tamoxifen

Additionally, the study found that soy food intake was associated with improved survival, regardless of tamoxifen use.  Interestingly, the study concluded that for women who took tamoxifen and had low soy intake, the tamoxifen helped their overall survival rates.  For those who ate high levels of soy foods, tamoxifen was not related to further improvement of survival rates.  More importantly, women who had the highest level of soy food intake and who did not take tamoxifen had a lower risk of mortality and recurrence rate than women who did take tamoxifen and who had the lowest level of soy food intake.  This suggests that high soy food intake and tamoxifen use may have a comparable effect on breast cancer survival.

I know which one I’d rather take!

How much is enough?

The study indicated that 11 grams per day of soy protein was sufficient to confer the benefits they observed. For an idea of how much that is:

1 cup of soy milk = 6-7 grams soy protein

1 cup soy yogurt = 6 grams soy protein

1/4 cup roasted soy nuts = 11 grams soy protein

4 oz organic tofu = 13 grams soy protein

1/2 cup edamame = 11 grams soy protein

Please choose organic soy whenever you are eating it because in some parts of the world, it is a genetically modified crop.

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