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October 1, 2012 at 5:00 AMComments: 0 Faves: 0

Can Soy Cause Breast Cancer?

By Jessica Corwin MPH RDN More Blogs by This Author

Soy beans can be a nutritious addition to any diet, offering protein, fiber, and antioxidants. However, soy also offers "isoflavones," which may behave similarly to estrogen in our body and  potentially increase our risk for hormone dependent cancers such as breast and ovarian. Therein lies the controversy of whether soy is safe or detrimental to our diets.

Soy Health Benefits

Complete Protein. Amino Acids. Fiber. Antioxidants. Potassium.

As a complete protein, supplying all the essential amino acids our bodies rely on, soy is a hot commodity, offering benefits for vegetarians and omnivores alike. In fact, when replacing animal proteins with soy, evidence suggests we may reduce our risk of cardiovascular disease - particularly when those animal proteins are rich sources of saturated fat. This is likely due to the fact that soy doesn't contain LDL (which raises saturated fat and cholesterol) as meat does, and it provides fiber and antioxidants, both of which contribute to a healthier cardiovascular system. Furthermore soy products contain potassium, which helps reduce the risk of high blood pressure and is also anti-inflammatory.

Soy and Isoflavones

As I mentioned earlier, soy contains isoflavones, a type of phytoestrogen, or plant hormone, that acts similarly to our own human hormone estrogen. Some research has found an increased risk of estrogen-dependent breast cancers with a diet heavy in added soy. Yet, the totality of the clinical and epidemiologic evidence indicates that traditional soy foods can be safely consumed by women with breast cancer - a point reinforced by Eastern societies where soy has been a dietary staple for over 1,000 years, and both fermented and unfermented soy foods are remain prevalent in their diets today. Despite the ubiquity of soy in their diet, breast cancer rates are far lower than ours... Confusing, right?

How can other nations consume soy without any association with breast cancer, while here in the U.S. the association remains? Once you consider the soy food sources being eaten here in the States versus Asian countries, the answer becomes slightly more clear. Asian citizens' dietary soy intake is primarily made up of whole soy foods such as edamame, tempeh, natto, and tamari, whereas American soy consumption consists of bits and pieces of processed soy (think soy isolates, soy protein powder, or soy fortified protein bars) with a splash of whole foods added in among the minority of our population (think vegans and vegetarians).

Soy Research

Before we dive into the research, a quick note about research studies on diet and health. Conclusive results are best found from long-term human studies with large population groups, yet as these are also far more expensive to conduct, the more typical research available is derived from small, short studies, many of which rely on animal subjects. It's also important to point out that much of the research hones in on total soy protein intake rather than total soy food intake. These two aspects can drastically influence the difference in the data.

From the clinical trials and epidemiological studies I've read, the risk is not found when consuming WHOLE soy foods (e.g. edamame, miso, nato, tofu, tempeh, etc.), but when we have soy protein isolates added to our food - something that is happening far more than you may have noticed. Check out the ingredient statement, especially if the product is high in protein!


Whole sources of soy may offer a healthy synergistic effect on our bodies, but we don't know what will happen by extracting only one piece of the soy (i.e. soy protein isolate). This is why I encourage eating as close to natural as possible (advice that makes sense with nearly any food, as processing tends to remove nutrients while adding sugar and fat). While soy foods such as tofu and tempeh are not exactly WHOLE foods, they include the whole soy bean rather than only the random pieces and parts (i.e. soy isolates).

Perhaps due to the very dietary differences discussed above, one study conducted by the American Cancer Research Institute found that Caucasians following a typical Western diet had a cancer mortality rate 65% higher than Asian-Pacific Islanders over the 4-year period of 1998-2002.  This study and others suggest that if we are to base our dietary soy recommendations on the traditional Asian diet, we are likely able to consume as many as 3 servings of whole soy foods per day along with a balanced diet without concern. I simply encourage you to stick with the most natural and/or fermented soy food choices, while also keeping an eye on those ingredient statements as you scan for soy additives.

To Soy or Not to Soy?

Most health experts seem to agree that including soy in our diet won't increase our risk of cancer, thyroid disease, or infertility.

Further, it may surprise you to know that some of the greatest studies have actually found that soy intake may even offer a protective benefit against breast cancer! Check out this article for one of the most thorough reviews of soy research available. 

How to Eat Soy Safely

Eat 1-3 Servings Daily. Opt for Whole Sources. Avoid Soy Derivatives and GMO Soy.

The Asian diet averages 1-2 servings of natural soy foods daily, and the majority of soy consumption tends to be fermented soy, indicating greater health benefits from soy foods such as tempeh, natto, tamari, and pickled tofu. Even those who are adamantly against soy consumption will agree that fermented soy should be categorized separately due to the natural health benefits associated with the fermentation process.

The American Cancer Research Institute states that women, even those with breast cancer, can safely enjoy 2-3 servings of soy in their diet per day. Note that they also discourage the use of soy supplements in general and taking extra steps to cut down on soy intake if receiving anti-estrogen treatments for breast cancer.

If you do your best to keep an eye on your total soy intake by reading ingredient statements on processed foods and keeping track of the whole soy foods in your diet, you will be well on your way to taking control of your health. Knowledge is power, and the simple awareness of how much soy is already found in your diet will provide you with the knowledge of whether or not you may want to cut back.  

A single serving may consist of:

  • 1/2 cup Tofu
  • 1/2 cup Shelled Edamame
  • 1-1/8 cup In-Shell Edamame
  • 1 cup Soymilk

For those of you concerned about genetically modified foods, some data suggests that up to 93% of the American soy available is a GMO product. To avoid GM soy, choose USDA Certified Organic or those brands which state they are GMO free such as Silk Soy Milk.   

Bottom Line?

Please understand that adding or subtracting one single food or beverage won't magically make or break your risk of cancer. What will make a difference? Following an overall healthy and balanced diet and making the choice to be active each and every day.

Oh, and as for me? I will continue to enjoy Light Chocolate Silk Milk, tofu, and edamame in my diet - while doing my best to pay attention to the soy being added to my snack bars and protein bars (including Luna and Clif bars).

How do you like YOUR soy?

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