Ask an Expert: Soy and flax in hormone-receptor-positive breast cancer survivors

Q. If a hormone-positive breast cancer survivor wants to be proactive in her cure, should she avoid/limit as many phytoestrogens as possible, or just soy and flax, which seem to be high in these estrogen mimics?

Answer from Miles Hassell, M.D., director of Providence Integrative Medicine Program at Providence Cancer Center: 

A good case can’t be made for breast cancer survivors to limit dietary phytoestrogens. Doing so would require the elimination of a high proportion of everyday, healthy whole foods, and there is no hint from the medical literature to suggest that such a step would be wise. I’ll deal with the two prominent sources of phytoestrogens, soy and flax. 

1. Traditional soy foods: Soybeans, soy milk, tofu, miso, tempeh. 

Soy foods may contain selective estrogen receptor modulators, which influence the activity of estrogen in the breast and potentially could have an adverse effect in breast cancer survivors. However, soy foods also have a number of non-hormonal effects that may be favorable. Studies in humans have failed to show that eating soy in adulthood has much effect one way or another on the development of breast cancer, or on survival. Therefore, it appears that about one serving or less of traditional soy foods daily is safe and reasonable. 

I would suggest avoiding concentrated soy extracts, such as isoflavone supplements, as the value or risk of these is much less clear. Cell studies have shown that soy extracts may inhibit the effect of tamoxifen, but several animal studies suggest that soy extracts combined with tamoxifen offer increased benefit over tamoxifen alone. In the face of this conflicting data, the recommendation to restrict soy intake to natural foods, at traditional levels, seems reasonable. 

Breast cancer survivors should also be aware of some striking anecdotal reports of outstanding success in women with advanced breast cancer who used very high doses of soy foods. This is an area that clearly needs definitive studies. 

2. Flaxseed meal and oil. 

Just as soy is a significant food staple in much of the world, flaxseed used to be a staple food in northern Europe prior to the industrial revolution. Studies have shown consistent, favorable outcomes in animals and humans treated with flaxseed, both for estrogen-positive and -negative tumors. 

Flaxseed meal appears to have better results than flax oil. Both golden and brown flax are available. Brown flaxseed is equal in nutritional value to golden flax, and much cheaper. Aim for one or two tablespoons of flax meal daily. 

I can find no data that suggest a need for caution in the use of flax by breast cancer survivors. Furthermore, I discussed this question with Paul Goss, M.D., an oncologist in Toronto who has conducted some of the major studies with flaxseed in breast cancer. Dr. Goss was unaware of any likelihood of harm with the use of flaxseed in breast cancer. 


1. Messina, Loprinzi. “Soy for breast cancer survivors: A critical review of the literature.” J Nutr 2001;131:3095S-3108S. 

2. Thompson et al. “Biological effects of dietary flaxseed in patients with breast cancer.” Breast Cancer Res. Treat. 2000; 64(1):50,A157

3. Chen et al. “Dietary flaxseed inhibits human breast cancer growth and metastasis and down-regulates expression of insulin-like growth factor and epidermal growth factor receptor.” Nutrition and Cancer 2002;43:187-92 

Update March 2006 

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