To Soy or Not to Soy?

Soy foods often make the health headlines. However, confusion still remains as to whether the isoflavones found in soy foods contain anti-carcinogenic (anti-cancer) properties or the opposite, that is, do they stimulate breast cancer growth through their estrogen-like properties.

Let’s take a closer look…

As you may know, estrogen is an essential hormone necessary for the normal development and growth of breasts and child-bearing organs. However, a woman’s risk for breast cancer is associated with a lifetime exposure to estrogen due to its role in stimulating breast cell division.

Soy foods contain isoflavones which have a very similar chemical structure to estrogen. The isoflavones compete with estrogen to bind to the estrogen-receptor (ER) sites on breast cells.  With increased consumption of soy and increased circulation of isoflavones they end up binding to the ER sites, thereby preventing the binding of estrogen. What researchers have found is that the isoflavones exhibit a WEAKER effect on breast tissue than natural estrogen. Therefore, isoflavones end up preventing the estrogens from interacting with breast cells and this reduces the cancer risks associated with excess estrogen.

Nevertheless, studies have been mixed in terms of showing empirical evidence that soy consumption reduces the risk of breast cancer. Well… finally some good news.

The largest study to date on the influence of soy foods on breast cancer outcomes was published in the American Journal of Clinical Nutrition in May of this year.  This study followed 9514 breast cancer survivors from both the US and China.  They found that post-diagnosis soy food consumption of 10 mg or more isoflavones per day was associated with a 29% reduced risk of cancer recurrence compared to those who consumed less than 4 mg per day. They also found that the more you ate, the more your risk reduced. Those women in the highest percentile of isoflavones consumption were 36% less likely to have their breast cancer recur compared to those in the lowest percentile.

The Nitty Gritty:

These statistically significant results were based on those with ER-negative breast cancer. However, isoflavones consumption did also show a protective effect in women with ER-positive breast cancer, it was just not as statistically significant. Traditionally nutritional advice has steered woman away from soy with ER-positive breast cancer. So this study has set the stage for change in this regard.

Finally, some concern have been raised previously as to whether soy consumption should be avoided in those breast cancer survivors taking tamoxifen. This study showed that isoflavones do NOT interfere with tamoxifen and those taking tamoxifen showed a significant inverse association for isoflavones intake and breast cancer recurrence.

The Bottom Line:

This study focussed on women who already had breast cancer. However, previous studies have shown that early consumption of soy foods, may reduce breast cancer risk from developing later in life.  This is likely due to the fact that the breast is most sensitive to dietary influences during its development stage through childhood and adolescence. 

Therefore who should eat natural soy foods? Barring those with an allergy to soy:

  • If you are a young or teenage girl – you should eat soy
  • If you don’t eat soy foods now – you may wish to start
  • If you have been diagnosed with either ER-positive or ER-negative breast cancer and ARE on tamoxifen or are NOT on tamoxifen – you can and perhaps should eat some natural soy

“Natural Soy”

Try to avoid soy products that contain “soy protein isolate”. This is a highly processed form of soy and the studies conducted on the benefits of soy used natural, minimally processed products. These include: tofu, tempeh, edamame, soy nuts and soy milk. Soy supplements have not been extensively studied and therefore sticking to moderate portions of natural soy food is far preferable. If the budget allows, try to buy organic soy. 85% of the soybeans grown in the US today are genetically modified. Organic foods are automatically non-GMO foods.  Brands such as So Nice, Earth’s Balance or Canadian Mackellar Farms Edamame all use organic, non-GMO soybeans.

No Magic Bullet

There are a multitude of factors that can help reduce your risk of breast cancer or its recurrence; however, there is no one “magic bullet”.

The 2012 study is very positive in that it clarifies the protective effects of consuming soy isoflavones even for those with breast cancer. However, it should be noted that the higher intakes of isoflavones were also associated with positive life-style factors including, regular exercise, higher consumption of cruciferous vegetables (like broccoli, kale, cauliflower & cabbages), a lower BMI and non-smoking status.

So by all means, add a serving or two of natural soy to your daily routine, but don’t forget the morning swim, eating your greens, managing your stress levels and a good night’s sleep.

The Recipe of the Week:

Stir Fry Tofu & Greens

Serves 4.


1 tablespoon light olive oil

1 small onion or 3 scallions chopped

2 cloves garlic, minced

1 tablespoon fresh ginger root, grated

12 ounces firm tofu, cut in cubes

3 tablespoons reduced-sodium soy sauce (or tamari for gluten free option)

1/2 cup snow peas, trimmed and cut diagonally in half

1 cup kale leaves – washed, trimmed of woody stalk and torn into bite size pieces

2 tablespoons chopped cilantro leaves

1 teaspoon dark (roasted) sesame oil


In a wok or large skillet, heat the olive oil, and quickly sauté the onion, garlic, and ginger, stirring constantly with a large spoon ( about 2 minutes)

Add the kale and snow peas – continue stirring for 2 minutes.

Add the tofu and soy sauce – warm it through (1 – 2 minutes)

Add the cilantro and sesame oil. Mix together and serve.

Can be served on its own or on top of a ½ cup (per person) of wild or brown rice.


Nechuta SJ, Caan BJ, Chen WY, Lu W, Chen Z, Kwan ML, Flatt SW, Zheng Y, Zheng W, Pierce JP, Shu XO. Soy food intake after diagnosis of breast cancer and survival: an in-depth analysis of combined evidence from cohort studies of US and Chinese women. Am J Clin Nutr. 2012 Jul;96(1):123-32. Epub 2012 May 30.

Messina M, Hilakivi-Clarke L., Early intake appears to be the key to the proposed protective effects of soy intake against breast cancer. Nutr Cancer. 2009;61(6):792-8.


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