The relationship between soy intake and breast cancer risk has been rigorously investigated for more than 20 years.1 Initial interest in this relationship was based on the low breast cancer incidence rates in soyfood-consuming countries, especially Japan, and rodent data showing that soy inhibits chemically-induced mammary cancer.2,3 While several putative chemopreventives were identified in soybeans, one class, the isoflavones, has been the focus of most research.
Isoflavones are diphenolic compounds with a very limited distribution in nature (the soybean is the only commonly consumed food to contain nutritionally-relevant amounts).4 They exhibit both hormone-dependent and hormone-independent effects under various experimental conditions.5,6
Although epidemiologic studies generally show that Asian women who frequently consume soyfoods are less likely to report having breast cancer than Asian women who consume relatively little soy, evidence now indicates that to derive protection against this disease, soy must be consumed early in life.7-9 The hypothesis that early soy intake is protective against breast cancer, which was first proposed in 1995, is consistent with a considerable amount of evidence that early life events in general have a profound impact on the risk of developing breast cancer later in life.10,11
Despite the low breast cancer rates in soyfood-consuming countries, concern has arisen that the estrogen-like effects of isoflavones, and therefore also soyfoods, could be harmful to breast cancer patients.12 However, prospective epidemiologic studies from China and the United States not only show soyfoods to be safe for breast cancer patients, but suggest soy consumption improves their prognosis.13-16
The most recent study to address this issue was conducted by researchers from the Affiliated Hospital of Inner Mongolia Medical College, Hohhot, China.17 For this study, trained interviewers conducted face-to-face interviews using a structured questionnaire to collect information on dietary habits and potential confounding factors. After a median follow up of 52.1 months (range, 9-60 months), a total of 79 breast cancer related deaths were recorded among the 616 breast cancer patients enrolled in this study. The hazard ratios (hazard ratios are similar to relative risks; a hazard ratio of 1.5 indicates risk was increased by 50% whereas a value of 0.5 means risk was reduced by half) for estrogen receptor (ER) negative (ER-) breast cancer for women in the first through fourth isoflavone intake quartiles were 1.0, 0.83, 0.76 and 0.78, respectively. More pronounced protective effects were noted in response to ER positive (ER+) breast cancer, with hazard ratios for the first through fourth isoflavone intake quartiles being 1.0, 0.77, 0.66 and 0.59, respectively. The reduction in risk was statistically significant for both types of cancers. The cutoffs (mg/d) for the intake quartiles were28.83, respectively. Thus, an intake of approximately one serving of soyfoods per day was associated with a 22 to 41% reduction in mortality.
While the number of participants in this most recent study was relatively small, the results are consistent with two studies conducted in the United States and two from China.13-16 Further, they are consistent with the 2012 position of the American Cancer Society, which is that “For the breast cancer survivor, current evidence suggests no adverse effects on recurrence or survival from consuming soy and soy foods, and there is the potential for these foods to exert a positive synergistic effect with tamoxifen.18
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