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May 2010 Soy and tofu appear healthy, but isolated soy protein not

The relationship between soy products and breast cancer is a hotly debated topic. It is important to separate two groups of effects: whole soy beans versus isolated soy protein. On the one hand, natural soy grains and related products like tofu show positive health effects in various cancer studies on human populations [1, 2, 3]. On the other hand, isolated soy protein may be more risky.

Main text:
Even though various studies show positive health effects in various cancer studies on human populations [1, 2, 3], there is a question whether the cancer protective effects in these studies are caused by high soy consumption or by the fact that these individuals generally consume less animal products (meat, fish, eggs, dairy). Still, most analyses indicate that soy contains many healthy nutrients and micronutrients, quite like other beans. And Asian women eating more soy products had better cancer survival [1].

However, isolated soy protein is increasing getting bad press, recently. Think for example of animal studies showing adverse tumor-related effects [4, 5]. However, these mice were given high dose of isolated soy protein. Studies investigating (animal test based) claims about harmful hormone receptor effects showed neutral effect of soy. In addition, the breast tissue density, as biomarker of breast cancer risk followed in clinical studies, didn't change after regular soy consumption [2, 3].

Next, there are the findings in humans of raised IGF-1 factor (cancer risk factor) after consuming soy protein sausages, burgers and similar products. In a study on consumption of 40gr of isolated soy protein per day (amount found in one soy chicken patty and two soy burgers, or four soy breakfast patties) the IGF-1 factor significantly increased and was almost twice as powerful as cow-milk protein in raising IGF-1 levels [6].

A large, recent study again supports soy consumption and indicates that conflicting concerns may have been unfounded. Latest research on association of soy consumption after a breast cancer diagnosis, published in 2009 by Xiao Ou Shu MD, PhD and her colleagues, found that regular, moderate consumption of soy foods lower the risk of death and cancer recurrence in women who've had breast cancer [7]. Their study included almost 5,000 Chinese women who had been previously diagnosed with breast cancer between 2002 and 2006. The women were aged 20 to 75, with the majority of women between 40 and 60 at the time of diagnosis. The researchers collected information on cancer diagnosis and treatment, lifestyle factors (including diet) and disease progression at 6, 18, 36 and 60 months after diagnosis. The results showed that higher soy intake (up to 11 grams of soy protein which is equivalent to about one-fourth of a cup of tofu daily) produced lower mortality.

Beside these findings, the authors pointed out that Chinese woman tend to get their soy from natural sources, such as tofu, edamame or unsweetened soy milk, instead of the processed soy milk, burgers and sausages. Furthermore, women who were also engaged in healthier lifestyle overall, including more exercise and high vegetable intake, had better prognosis.

Finally, according to Dr Shu breast cancer patients can be assured that consuming soy food won't hurt them and may even reduce the risk of recurrence. But when discussing the pros and cons of soy, consumers should be aware that there are differences in the quantity and quality of soy in popular food products. Try to get it in natural sources, not from processed food.

[1] Boyapati, S.M., X.O. Shu, Z.X. Ruan, Q. Dai, Q. Cai, Y.T. Gao, et. al. (2005). Soyfood intake and breast cancer survival: a follow up of the Shanghai Breast Cancer Study. Breast Cancer Res Treat. 92 pp. 11-17.

[2] Maskarinec, G., Y. Takata, A. A. Franke, A. E. Williams, S. P. Murphy (2004). A 2-year soy intervention in premenopausal women does not change mammographic densities. J Nutr. 134 pp. 3089-94.

[3] Nagel, G., U. Mack, D. Von Fournier, J. Linseisen (2005). Dietary phytoestrogen intake and mammographic density - results of a pilot study. Eur J Med Res. 10 pp. 389-94.

[4] Allred C. D., N. C. Twaddle, K. F. Allred, T. S. Goeppinger, D. R. Doerge, W. G. Helferich (2004). Soy processing influences growth of estrogen-dependent breast cancer tumors. Carcinogenesis 25 pp. 1649-57.

[5] Allred, C.D., K. F. Allred, Y.H. Ju, S. M. Virant, W. G. Helferich (2001). Soy diets containing varying amounts of genistein stimulate growth of estrogen-dependant (MCF-7) tumors in a dose-dependant manner. Cancer Res. 61 pp. 5045-50.

[6] John McDougall, MD. http://www.youtube.com/watch?v=mHYFOJBU434

[7] Shu, X. O., Y. Zheng, H. Cai, K. Gu, Z. Chen, et. al. (2009). Soy food intake and breast cancer survival. JAMA 302 pp. 2437-43. Abstract of the study can be found on http://jama.ama-assn.org/cgi/content/abstract/302/22/2437

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