The French daily Le Parisien published an article today that suggested that soy could contribute to the risk of breast cancer. And I don’t agree.

The newspaper article quotes Prof. Jean-Marie Bourre from the French Academy of Medicine. “Soy is a legume that contains phytoestrogens, like oats, maize and barley, but at higher concentrations. These molecules greatly resemble oestradiol, one of the female sex hormones. ‘They are therefore potential hormone disturbers in women, and may encourage the development of hormone-dependant cancers like breast cancer,” says Jean-Marie Bourre’.”

Yes, soy does contain phytoestrogens, and it’s true that hormone replacement therapy (which is based on powerful estrogens) may contribute to an increase of at most 15 to 40 percent in the risk of developing breast cancer. The phytoestrogens found in soy-based foods are, however, roughly a thousand times less powerful than natural estrogens or those present in hormone replacement therapy. Mathematically, they cannot therefore contribute significantly to breast cancer. In addition, the genistein and daidzein in soy also act as antiangiogenics, thus contributing to limiting the growth of tumors. We’ve known for years that Japanese women who regularly consume soy products before puberty have a far lower risk of developing breast cancer than other women do. Yet another study of Japanese women has just been published showing that the higher their level of the genistein phyto-estrogen (which comes from eating soy), the greater their protection from breast cancer. (1)

In 2006, the journal of the US National Cancer Institute published a meta-analysis of all the studies performed to date on soy and breast cancer. It concluded, “ The consumption of soy may be associated with a small reduction in the risk of developing breast cancer “(2) This study also warned against using supplements that are excessively concentrated in soy isoflavones, because they may contribute to breast cancer.

In its 2003 report, the French Agency for Nutritional Health and Safety (AFSSA) recommended that women already diagnosed with breast cancer should not consume more than one portion of soy-based foods per day (soy milk, tofu etc). This recommended seems eminently reasonable, and it would be nice if members of France’s Academy of Medicine could work together with that program.

1. Iwasaki M, Inoue M, al. e. Plasma Isoflavone Level and Subsequent Risk of Breast Cancer Among Japanese Women: A Nested Case-Control Study From the Japan Public Health Center-Based Prospective Study Group. Journal of Clinical Oncology 2008;26:1677-83.

2. Trock BJ, Hilakivi-Clarke L, Clarke R. Meta-Analysis of Soy Intake and Breast Cancer Risk. J Natl Cancer Inst 2006;98(7):459-71.