Diet, Soy & Breast Cancer Risk

Diet, Soy & Breast Cancer Risk

Last week’s column reviewed new clinical research findings suggesting that higher levels of soy-derived isoflavones in the diet, and in the blood, may significantly reduce the risk of prostate cancer. This week, I will be discussing a new research study that makes similar claims regarding the prevention of breast cancer.

Breast cancer in women is, in several important ways, analogous to prostate cancer in men. Like prostate cancer, most breast cancer cells are fueled by the body’s sex hormones (androgens stimulate prostate cancer growth, while estrogen in women stimulates breast cancer growth). Prostate cancer is the number one cause of cancer in men, and breast cancer is the number one cause of cancer in men. Approximately 192,000 cases of each of these cancers will be diagnosed in 2009, with prostate cancer accounting for about one-fourth of all cancers afflicting men, while breast cancer also represents about one-fourth of all cancers that affect women. Prostate cancer is the second most common cause of cancer death in men, and, analogously, breast cancer is the number two cause of cancer death in women. More than 27,000 men will die of prostate cancer in 2009, while more than 40,000 women will succumb to breast cancer this year.

In last week’s column (Diet & Prostate Cancer Risk), I reviewed an innovative clinical research study that assessed both the dietary intake of soy-derived isoflavones and the concentration of these dietary nutrients in the blood. Isoflavones belong to a larger group of dietary compounds that are, collectively, referred to as phytoestrogens, as these nutrients are able to stimulate (albeit weakly) chemical receptors for the sex hormone estrogen. In a newly published clinical study, in the American Journal of Clinical Nutrition, the results of a large prospective Chinese women’s health study add to previous similar research in suggesting that a diet rich in soybean-derived products, especially when consumed before and during adolescence, may be associated with a decreased overall risk of developing breast cancer later in life.

In this prospective public health study, more than 73,000 Chinese adolescent girls and women were followed for an average of 7.4 years. All of the participants in this very large study completed validated dietary surveys, and the incidence of new breast cancers among this very large group of Chinese women was then compared to their self-reported intake of soy-based foods.

The results of this study were rather striking, and were highly statistically significant when comparing the incidence of breast cancer among women with the highest levels of soy intake versus the women with the least soy intake. In this epidemiological study, the women who reported the highest regular dietary intake of soy-based foods were, overall, nearly 60 percent less likely to be diagnosed with premenopausal breast cancer during the course of this clinical research trial. This dramatic reduction in the risk of premenopausal breast cancer was observed in both the women who reported high levels of soy-derived foods in their diet and in the women who frequently consumed foods that were generally rich in isoflavones.

While this particular study did not measure isoflavone levels in the blood, as was done in the prostate cancer study that I reviewed last week, its results, nonetheless, mirror the findings of other similar epidemiological studies.

As with all survey-based disease prevention studies, of course, there is the potential for significant bias in this particular research study, and studies such as these cannot, by themselves, prove a “cause-and-effect” mechanism behind the clinical findings that they reveal. However, such studies, when conducted prospectively and with a high degree of scientific integrity, can still suggest potentially important disease prevention strategies. When the results of studies such as these appear to be especially compelling, they should then be followed-up by large, prospective randomized, placebo-controlled clinical research studies that provide so-called “Level I” clinical evidence.

Previous studies that have supported a role for soy consumption in breast cancer prevention have suggested that the increased consumption of soy-derived isoflavones before and around the time of adolescence is critically important, when the female breast is actively developing, and that this apparent protective effect of soy foods against breast cancer diminishes greatly after breast development is completed in early adulthood. (At the same time, however, the weakly estrogenic effects of dietary isoflavones have raised concerns about soy intake among women with a prior history of breast cancer, although there is no convincing scientific data, yet, showing that dietary isoflavones increase the risk of breast cancer recurrence.)

Another caveat that must be mentioned regarding the findings of this particular study is that the potentially beneficial effects of soy-derived foods, and of dietary isoflavones in general, in preventing breast cancer were observed in a homogeneous population of Asian women. Even if high levels of isoflavones in the diet really are protective against breast cancer, as this epidemiological study strongly suggests, it is still unclear whether or not this putative cancer prevention benefit applies equally to non-Asian women as well. Once again, only well-balanced, prospective, randomized, placebo-controlled clinical studies can confirm or contradict the findings of this very large Chinese public health study. Fortunately, there are several such studies underway at this time.

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