Soy & Breast Cancer

This section is compiled by Frank M. Painter, D.C.
Send all comments or additions to:    Frankp@chiro.org

---- Original Message -----
From: Dr. Stephen G. Chaney, Ph.D.
To: SSChaney@aol.com
Sent: Monday, March 06, 2000 12:22 AM
Subject: Soy & Breast Cancer

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Recent news reports suggesting that soy might interfere with cancer treatment in people who already have breast cancer are highly misleading. This statement is based on two reports showing that soy caused a small stimulation of normal breast cancer tissue and a report that genistein, one of the phytoestrogens found in soy, stimulated the rate of growth of breast cancer cells in mice lacking both their ovaries and a functioning immune system.

The effect of soy on normal breast tissue is to be expected. Soy phytoestrogens are known to have a slight estrogenic effect. However, this effect is much weaker than the estrogen normally produced by the body. As for the other study, humans are not mice, and it is often difficult to extrapolate directly from mice to humans. In addition, these were not normal mice. Thus, if one could extrapolate from mice to humans in this case, the most one could say would be that soy might pose a risk to postmenopausal women with AIDS and breast cancer. If soy genistein were some brand new drug that had never been tested before, this one study with mice might be cause for concern. However, the effect of soy on breast cancer risk has already been evaluated in dozens of human studies. Most of these studies showed a clear protective effect. A few showed no effect. None of these studies has shown any evidence of increased breast cancer risk in women consuming soy.

Just to head off concern about future news reports, you might also be interested in the following. I recently attended a nutrition conference and saw a poster saying that genistein increased the rate of progression of prostate cancer in mice. I was curious about that so I asked the author about his study. It turns out that a previous study in rats had shown a protective effect of genistein on prostate cancer using 45 milligrams of genistein per kilogram body weight. To get the detrimental effects he reported the researcher I talked with had used 1 gram of genistein per kilogram of body weight. When I asked him if that were a physiological dose, he responded that he had no idea. In fact, the dose that he used would correspond to 45 - 80 grams of genistein per day for humans of average weight.

The richest natural sources of soy phytoestrogens (which includes Shaklee's Instant and Energizing Soy Protein) provide about 30 milligrams (remember, there are 1000 mg or millegrams in 1 gram) of genistein per serving. Most of the studies in humans have clearly shown that soy decreases the risk of prostate cancer and no studies in humans have reported any increased risk. Thus, when you read the next headline about the dangers of soy for people with prostate cancer, you'll know how to respond.

Poor science makes just as good newspaper headlines as good science. It's no wonder that the public gets confused.

Dr. Stephen G. Chaney, Ph.D.
Full Professor of Biochemistry, Biophysics and Nutrition
University of North Carolina, Chapel Hill

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Dr. Chaney has been teaching biochemistry, biophysics and nutrition to medical and dental students for 27 years. In addition he has an active program in cancer research, focusing on chemotherapy. He has authored 80 publications in peer - reviewed journals. Dr. Chaney was instrumental in establishing nutrition education as an important part of the medical school training at UNC at Chapel Hill.

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Since 3-15-2000