The Truth About Soy And Breast Cancer
Author: Dr. Stephen Chaney
With so much misleading information about Soy, I am sharing this great informational piece by Dr. Stephen Chaney...
You've probably heard the warnings:
"Soy may increase the risk of breast cancer!" "Women with breast cancer shouldn't use soy!"
The first warning was never true. Numerous clinical studies have shown that consumption of
soy protein is associated with a lower risk of developing
breast cancer.
Furthermore, the science behind the second warning has never been very strong.
The concerns that soy might stimulate the growth of breast cancer cells
was based primarily on cell culture experiments and one experiment in
mice - even though a second experiment in mice came to the exact
opposite conclusion.
Was The Hypothesis That Soy Could Increase The Risk Of Breast Cancer Recurrence Plausible?
The possibility that
soy isoflavones could stimulate the growth of
estrogen- responsive breast cancer
was biochemically plausible because soy isoflavones bind to the
estrogen receptor and have a very weak stimulatory effect (much weaker
than estrogen itself).
Even that evidence was not definitive because soy isoflavones also turn
on several tumor suppressor pathways in breast cells and help strengthen
the immune system - so they could just as easily inhibit the growth of
breast cancer cells.
However, because the concerns were plausible and had not been
definitively disproved, most experts, including me, recommended for
several years that women with estrogen- responsive breast cancer might
want to avoid soy protein.
Has The Hypothesis Been Rigorously Tested?
In fact, the definitive clinical studies have been performed, and it turns out for women who are breast cancer survivors,
consumption of soy foods does not increase either the risk of breast cancer recurrence or of dying from breast cancer.
The first of these studies was reported in the December 2009 issue of
the Journal of the American Medical Association by researchers at
Vanderbilt University and Shanghai Institute of Preventive Medicine (Shu
et al, JAMA, 302: 2437-2443, 2009).
It was a large, well designed, study that enrolled 5042 Chinese women
aged 20 to 75 years old who had been diagnosed with breast cancer and
followed them for an average period of 3.9 years.
The women were divided into four groups based on the soy content of
their diet (ranging from 5 grams/day to 15 grams/day). The results were
clear cut.
Breast cancer survivors with the highest soy intake
had 25% less chance of breast cancer recurrence and 25% less chance of
dying from breast cancer than the women with the lowest soy intake.
The effect was equally strong for women with estrogen receptor-positive
and estrogen receptor negative cancers, for early stage and late stage
breast cancer and for pre- and post-menopausal women.
In short this was a very robust study.
The study also showed that soy protein intake did not interfere with
tamoxifen,
a drug that blocks the binding of estrogen to its receptor on cancer
cells. Tamoxifen is used for both for treating estrogen-responsive
breast cancer and preventing its recurrence. In this study, the
reduction in the risk of breast cancer recurrence & death was just
as great whether the breast cancer survivors were taking tamoxifen or
not.
In fact, tamoxifen was protective only for women with low soy intake. It
conferred no extra protection for the women at the highest level of soy
intake because the soy isoflavones were also blocking the binding of
estrogen to its receptor.
Other Clinical Studies
If that were the only published clinical study to test the soy-breast
cancer hypothesis, I and other experts would be very cautious about
making definitive statements. However, at least f
our more clinical studies have
been published since then, both in Chinese and American populations.
The studies have either shown no significant effect of soy on breast
cancer recurrence or a protective effect.
None of them have shown any detrimental effects of soy consumption by breast cancer survivors.
A meta-analysis of all 5 studies was published earlier this year (Chi et
al, Asian Pac J Cancer Prev., 14: 2407-2412, 2013). This study combined
the data from 11,206 breast cancer survivors in the US and China.
Those with the highest soy consumption had a 23% decrease in recurrence and a 15% decrease in mortality from breast cancer.
The Bottom Line:
What does this mean for you if you are a breast cancer survivor?
1)
There are many reasons to include soy protein foods as part of a healthy diet.
Soy foods are one of the highest quality vegetable protein sources and
provide a great alternative to many of the high fat, high cholesterol
animal proteins in the American diet.
2) I personally feel that
these studies are clear cut enough that breast cancer survivors no longer need to fear soy protein as part of a healthy diet.
3)
The responsible websites agree with this assessment.
For example, WebMD and the American Institute for Cancer Research
(AICR) both say that breast cancer survivors need no longer worry about
eating moderate amounts of soy foods.
4) The irresponsible websites (I won’t name names, but you know who they
are) are still warning breast cancer survivors to avoid soy completely.
As a scientist I really have problem with people who are unwilling to
change their opinions in the face of overwhelming scientific evidence to
the contrary.
5) Of course, some of those bloggers have now shifted their arguments to
say that it is unfermented or genetically modified soy that causes
breast cancer. Those statements are equally bogus - but that’s another
story for another time.
6) Finally, I want to emphasize that the published studies merely
show that soy does not increase the risk of breast cancer and is safe to
use for breast cancer survivors.
None of those studies suggest that soy is an effective treatment for breast cancer. The
protective effects of soy are modest at best. If you have breast
cancer, consult with your physician about the best treatment options for
you.
These statements have not been evaluated by the Food and Drug
Administration. This information is not intended to diagnose, treat,
cure or prevent any disease.
