Does Soy Increase Your Breast Cancer Risk?

A lot of women ask me this question: Does soy cause breast cancer or increase your risk?

It’s a good question that shows just how much misinformation is out there about soy and breast cancer.

Soy contains phytoestrogens. These plant molecules are similar to estrogen and can react with the estrogen receptors on breast cancer cells. About 80% of all breast cancers are estrogen-positive. That means that the hormone estrogen will stimulate these cancers to grow faster.

So here’s the big question. Will phytoestrogens do the same thing? A recent review article sheds some light on this important topic.

The article points out that not only does soy not promote breast cancer, it actually decreases the risk. The authors point out that there have been “considerable” studies indicating that the more soy a woman has in her diet, the lower her risk for developing breast cancer. So it seems pretty obvious that at least statistically soy prevents breast cancer. But is there any direct scientific evidence?

One study looked at a group of healthy women who did not have breast cancer. In that study, the women were given an isoflavone supplement. Isoflavone is one of the strongest phytoestrogens in soy. The researchers were looking to see if there was any increase in breast cell proliferation from taking the isoflavone supplement. Breast cell proliferation is an effect of estrogen that stimulates breast cancer growth.

The researchers found that, unlike estrogen, the phytoestrogen isoflavone had no breast cell proliferation effect. In other words, it might be similar to estrogen, but it does not have the same effects on breast tissue. But what about actual before-and-after studies in women who already have breast cancer? There are three clinical studies that address this question.

Two of them looked at the effect of isoflavone supplements in women who had documented breast cancer. A third study looked at the effect of soy protein supplements in women who were undergoing breast biopsy or definitive surgery for breast cancer. In the authors’ own words, “Breast cell proliferation was assessed in each study, and in none was it shown to be increased. Further, for the most part, there was no evidence of any estrogenic effects of isoflavones on breast tissue.” And that’s not all.

The authors also further stated, “Studies also indicate that isoflavones from either soy or red clover appear not to affect breast tissue density.” Breast tissue density is a risk factor for getting breast cancer. One of these studies was a very large, three-year experiment with women who had an increased breast cancer risk. Even in this study, there was no evidence that soy isoflavones increased the risk of breast cancer. They conclude by indicting conventional synthetic hormone therapy when they state, “These findings [on soy and soy products] are certainly suggestive of safety, and contrast with those of conventional combined hormone therapy [the synthetic hormones premarin and provera], which increases breast cell proliferation, breast tissue density, and breast cancer risk.”

So the data is pretty clear. Studies show that women who eat soy are 30% less likely to come down with breast cancer. Experimental studies in women who have breast cancer show that soy products do not stimulate cancer growth. Even the American Cancer Society states that patients with breast cancer can safely consume soy and soy products.

So if you have breast cancer, even if it’s an estrogen-positive cancer, there is no need for you to be concerned about eating soy or taking soy-based supplements. In fact, if you don’t have breast cancer, as this report shows, you will reduce your risk by as much as 30% by eating soy and taking a soy supplement.

Yours for better health,

Frank Shallenberger, MD