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Soy and Breast Cancer

Posted 6/5/2013

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  There has long been expressed concern about the possible negative link between soyfoods and breast cancer recurrence risk. Since soy is a phytoestrogen, the belief has been that it may act as estrogen in the body--not a good thing for women who have ER positive breast cancers and are trying to reduce estrogen. There has even been worry that soy might diminish some of the positive effects of tamoxifen or the AIs. For some women, being told not to eat soyfoods is not an issue, but for others, it means a big dietary change.

  This new study from Oncology turns that wisdom on its head. The study following 9,514 women who had breast cancer and found a 25% decrease in recurrence over a 7.4 year period for women who did eat soy. The standard caveat about the questions re a single study remain, and I surely don't want to suggest that you run out to buy tofu and soy milk. However, if soy has been a staple in your diet, this may be something that you want to discuss with your doctor.

  Here is the abstract and a link:

Its Time for Clinicians to Reconsider Their Proscription Against the Use of Soyfoods by Breast
Cancer Patients

By Mark Messina, PhD, MS1, Bette J. Caan, DrPH2, Donald I. Abrams, MD3, Mary Hardy, MD4, Gertraud
Maskarinec, MD, PhD5 | May 13, 2013
1Department of Nutrition, School of Public Health, Loma Linda University, Loma Linda, California, 2
Division of Research, Kaiser Permanente, Oakland, California, 3Division of Hematology-Oncology, San
Francisco General Hospital, and Department ofIntegrative Oncology, UCSF Osher Center for Integrative
Medicine, San Francisco, California, 4Simms/Mann UCLA Integrative Oncology Program, Los Angeles,
California, 5University of Hawaii Cancer Center, Honolulu, Hawaii

ABSTRACT: The impact of soyfood intake on breast cancer risk has been intensely investigated. This
focus can be attributed to soyfoods being uniquely rich dietary sources of isoflavones. Isoflavones are
classified as both phytoestrogens and selective estrogen receptor (ER) modulators. The finding that
dietary genistein, the primary soybean isoflavone, stimulates the growth of existing mammary tumors
in ovariectomized athymic mice implanted with ER-positive breast cancer cells has led many
oncologists to advise their patients against the use of soyfoods. However, the clinical evidence indicates
that isoflavone exposure has little effect on markers of breast cancer risk. Furthermore, a pooled
analysis that involved 9,514 breast cancer survivors found higher isoflavone intake was associated with
a statistically significant 25% reduction in recurrence over the average 7.4-year follow-up period.
Given the clinical and epidemiologic data, our position is that clinicians should allow soyfood use by
patients for whom soyfoods already represent a normal part of their diet, and should not discourage
other breast cancer survivors from moderate consumption.


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