Soy is Safe for Women to Take Long-Term

There has been a lot of controversy about whether soy is safe. I believe that soy is very safe, especially when used in moderation. There have been many, high-quality studies showing soy’s safety for short-term use. Now there is a reliable study showing that soy is safe for postmenopausal woman when taken long-term. 

Many midlife women take soy supplements and eat foods containing soy to alleviate menopausal symptoms. They prefer this to taking traditional hormone therapy.

Soy contains phytoestrogens, which are plant compounds that have structural similarity with estrogen. Soy contains an active substance called soy isoflavones, which is very unique because it adapts to the body’s needs for estrogen.

People who worry about soy’s safety are concerned about its estrogenic activity. Certain tissues, like breast and uterine, have estrogen receptors.  If these receptors are over-stimulated, it may increase your risk of cancer.

Integrative physicians believe that, because these phytoestrogens are only weakly estrogenic, they will actually bind to your estrogen receptors and prevent your own body from producing too much estradiol. Estradiol is a form of estrogen that can over-stimulate these receptors. Therefore, soy’s phytoestrogens may confer a protective effect against breast cancer.
Soy is Safe for Women to Take Long-Term
In February 2011, the results of a two-year, double-blind, placebo controlled study was published in the American Journal for Clinical Nutrition. Four hundred three (403) healthy, postmenopausal women ages 40 to 60 were assigned to receive either: 80 mg soy hypocotyl, 120 mg soy hypocotyl, or a placebo. Numerous tests, including blood tests, mammograms, Pap smears, and uterine ultrasounds (given to about a third) were administered at the start of the study (for a baseline), after one year, and after two years.

Head researcher Dr. Francene Steinberg reported these encouraging findings:

•    Breast density, fibroids and uterine thickness, and Pap smears results remained the same.
•    Thyroid hormones remained the same.
•    Blood pressure and other heart-related markers remained the same.
•    All blood tests came back within normal ranges.
•    A slight increase in immune function.

Only one woman developed breast cancer and one woman developed uterine cancer during the two-year study. This is considered well below what one would have expected in the general population with no intervention at all.  In addition, there were slight increases in blood urea nitrogen, a test of kidney function.

I feel very comfortable advising my patients to eat three to four servings of non-GMO soy foods per week. It is also safe to take 80 mg of soy isoflavones every day. However, if you have concerns, despite this article and other literature showing soy’s safety, then feel free to talk with your own physician for their recommendation.

Steinberg, F.M., Clinical outcomes of a 2-y soy isoflavone supplementation in menopausal women, Am J Clin Nutr., 2011 Feb;93(2):356-67. Epub 2010 Dec 22.

Am J Clin Nutr. 2011 Feb;93(2):356-67. Epub 2010 Dec 22.