Soy, friend or foe in breast cancer?

There is still some confusion about the estrogenic effect of soy. Oncologists still advise women with estrogen-positive breast cancer to avoid soy. Some women are told that soy foods may have a negative effect on their tamoxifen treatment. Women with risk factors for breast cancer often believe they should avoid soy foods as part of preventing cancer.

However, these recommendations and fears are not supported by clinical studies. In fact, soy’s image as a healthy food in the West was largely triggered by the much lower incidence of breast cancer in Asian women, where soy usually is eaten daily.

Instead, soy is avoided because of data from animal studies, and flawed animal studies at that. In these studies, scientists implanted breast cancers into mice without thymus glands and ovaries.  The mice were then fed different diets. The breast cancers grew in mice eating soy but shrank in mice on a soy-free diet.  Scary data but it appears to have been the result of using mice without thymus glands. Soy does not trigger tumor growth in mice with a more intact immune system (which requires a normal thymus gland).

We, of course, should instead pay greater attention to epidemiology and the human studies on soy and breast cancer: Asian women, eating traditional diets and daily soy foods, had a much lower incidence of breast cancer than women on a Western diet. This benefit was lost in women who emigrated and adopted a more Western diet.  And, unfortunately, the incidence of breast cancer in China is rising rapidly today, attributed to a move away from the traditional Asian diet and pollution.

There are no human studies showing that a diet rich in soy increases the incidence of cancer. In some soy studies, no benefit is seen on breast cancer prevention but there is no evidence of harm. Usually there is either a strong showing of benefit or at least a trend toward benefit. The latest review of all human studies again suggests that soy in fact helps prevent breast cancer. This protective effect is strongest in younger women. In fact, soy eaten early in life reduced the risk of getting breast cancer by 28 to 60 percent.

Moreover, clinical studies now show a benefit even in women diagnosed with breast cancer. A Chinese study of 5033 women found that those who ate the most soy had the lowest rate of breast cancer recurrence and a lower mortality. In fact, women eating more soy but not taking tamoxifen did better than women taking tamoxifen but eating little soy.

A recent German review of soy and breast cancer sums it up: Oncologists need no longer warn their patients against eating soy. And young women (teens) should be advised to eat soy foods because it has a significant ability to prevent breast cancer from forming later in life. There is no need to quit eating soy foods if you are at high risk for breast cancer, have breast cancer (in remission or under treatment), or are taking tamoxifen. There is every reason to include traditional soy foods in your daughter’s diet, if you have a daughter.

However, opinion is divided about the safety of soy as an isoflavone supplement, soy protein powder, or in the form of highly processed, chemicalized, and sweetened soy foods.  And even if those are safe, it is worth noting that traditional soy foods reduced the incidence of breast cancer where as a more Western diet of baked goods and processed soy foods did not.

Photo: “. . . and now there’s Soya”  by Wadem (Wade M)

NOTE: You are welcome to use my blog’s original images and content for non-commercial purposes if you attribute the work to me (Kathy Abascal) and link back to the blog. This work is licensed under a Creative Commons Attribution-NonCommercial 3.0 United States License.


About kathyabascal

Herbalist with a background in neurobiology, biochemistry, and law. Teacher of the TQIDiet, how to quiet inflammation with food.
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2 Responses to Soy, friend or foe in breast cancer?

  1. Oda says:

    I know of a friend who started spotting almost daily while drinking soy milk. It completely stopped when she stopped drinking the soy milk. Flax (organic whole or milled as well as organic flax oil), also high in phytoestrogens would always make my breast tenderness worse each sycle. Our bodies were telling us that it was not beneficial to eat either. Perhaps it is for some women, but is wasn’t for us 🙂

    • kathyabascal says:

      We are variations on a theme and not all foods agree with all people. We live in a complex world and I think you might be surprised at how many xenoestrogens your diet may contain and how they too may be affecting your hormone cycles. Phytoestrogens are only part of the picture. Moreover, phytoestrogens come in many forms: Resveratrol in red wine for instance. . .

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