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The Truth about Soy: Busting the Myths

By Jessica Tong, RD on January 28 2019 | News, Nutrition & Recipes

In today's era of endless content, there is an abundance of sources discussing what to include, or not to include, in one's diet. Like so many other popular products, soy can be a common topic of contention. People often hear about the many benefits of soy, while also coming across claims such as the possibility that soy affects fertility, or that soy increases the risk of breast cancer.

The latest edition of Canada’s Food Guide recommends soy beverages, tofu, soybeans and other soy products as part of a healthy diet because they are good sources of protein and vitamins A, D, and B12. To better understand whether or not soy deserves to be part of your healthy diet, here is what the evidence tells us, and can moderate your body's hormones as opposed to add to them. .


What are phytoestrogens?

Phytoestrogens, or plant estrogens, are substances that occur naturally in plants and plant-based foods such as soy foods. They have a similar chemical structure to our own body's estrogen (one of the main female hormones that regulates menstrual cycles), and are able to bind to the same receptors that our own estrogen does. However, they behave differently.


Foods containing phytoestrogens


soy foods


Isoflavones are a class of phytoestrogens. Soybeans and soy products are the richest whole-food sources of isoflavones. However, concentrated forms of soy or isoflavones, such as soy protein isolate in sports supplements, contain much more isoflavones.




What are the effects of isoflavones on men's and women's health?


Men's Health

The estrogen-like effects of isoflavones have raised concerns about the impact of soy consumption in men due to anecdotal claims that they can affect men’s fertility. 

Research (from a meta-analysis, a randomized cross-over trial, and evidence-based reviews) found no statistically significant effect of soy consumption from different sources (including isolated soy protein, soy flour, soy milk, tofu, and pure soy isoflavones) on levels of follicle-stimulating hormone (a measure of infertility in men), testosterone (high levels are a risk factor for prostate cancer; low levels are associated with conditions causing infertility), and other reproductive indicators.  

One randomized cross-over trial indicated that consuming high isoflavone (61.7 mg) soy protein isolate for eight weeks did not significantly affect semen volume in a sample of 32 healthy adult males.


Women's Health

Soy foods have been suggested to increase the risk of breast cancer for a while. However, ongoing studies show that eating a moderate amount of soy foods does not increase risk of breast cancer – in fact, a 2009 study found that consuming soy foods decreased the risk of death and recurrence in women with breast cancer and another 2015 study found that phytoestrogens contained in soy foods inhibited the growth of breast cancer cells. 

A moderate amount is one to two servings a day of whole-soy foods. One serving is:

  • 3 oz or 90 g tofu
  • 1 cup organic soy milk
  • ½ cup cooked edamame beans

While isoflavones may act like estrogen, they are not the same as female estrogens. They also have anti-estrogen properties. Isoflavones can block the natural female estrogens from binding to the estrogen receptor. They also have antioxidant and anti-inflammatory properties that can help to reduce growth of cancer cells.


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