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Soy & Cancer Prevention

July 2004

By Edward R. Rosick, DO, MPH, MS

For the past decade, a controversy has raged over whether people can reduce their risk of cancer by increasing their consumption of soy foods and soy supplements. In response to the debate, a number of studies were initiated in the 1990s to ascertain soy's effects on human health.

Over the last year, the results of these studies began to be released. While ignored by the mainstream media, the startling findings indicate that prostate and breast cancer risk could be cut in half if people only consumed more soy.

Prevention Key to Halting the Epidemic
The incidence of cancers that are not predominately caused by smoking, such as lymphoma, multiple myeloma, and brain cancer, is increasing at alarming rates.

One of every two American men will develop cancer in his lifetime, as will one of every three American women. Despite billions of research dollars spent, the epidemic of cancer continues.

A small but growing number of leading cancer researchers—such as Dr. Samuel S. Epstein, head of the National Cancer Prevention Coalition—are calling for a major change in how the federal government fights cancer.

Epstein and other advocates of cancer prevention emphasize that cancer is a multistage disease that is amenable to prevention in its earliest stages of development. One of the most widely available means of cancer prevention—yet generally overlooked by major cancer organizations and mainstream medicine—is the use of natural chemopreventive agents.

More than 250 case-control and cohort studies have been conducted on natural chemopreventive agents such as those found in fruits and vegetables. The data from these studies are overwhelming and irrefutable: the greater the consumption of fruits and vegetables, the lower the incidence of cancer. A recent article on cancer chemoprevention summarized these data as follows:

"Numerous components found in fruits and vegetables might contribute to their ability to reduce the risk of cancer, including dietary fiber, micronutrients, and various phytochemicals . . . the biomedical community needs to recognize and advocate approaches to prevent cancer [such as the use of natural chemopreventive agents] with the same enthusiasm that it currently directs toward treating it."1

Soy: A Powerful Chemopreventive Agent
Isoflavones derived from soy have shown great promise in providing natural protection against multiple types of cancer. Isoflavones are phytochemical constituents of soy, with two of the best known being genistein and daidzein. Isoflavones are believed to exert a number of positive biological effects on the human body,2 and many practitioners of integrative medicine (and even a small but growing number in mainstream medicine) now believe that consumption of soy and isoflavones can significantly reduce the risk of many chronic diseases, including cancer, heart disease, osteoporosis, and diabetes.

Significant interest in using soy and its isoflavone constituents as cancer chemoprotective agents began in the 1990s. Studies conducted in Asia found that Asian women, who consume many more isoflavones than American women, have significantly lower risks of developing breast cancer.3 Because animal studies have shown that a diet high in soy and genistein isolate can protect against mammary, colon, and skin tumors,4 it seemed reasonable to think that soy could also help prevent human cancers, and in particular, breast cancer. Yet many mainstream medical practitioners remain skeptical that something as "simple" as soy could have such a profound effect on human health.

Soy, Estrogen, and Breast Cancer
Some in the medical establishment believe that soy isoflavones have no role in preventing serious diseases such as cancer. Others feel that soy isoflavones should not be used as nutritional supplements because isoflavones act as natural estrogens and could cause many of the same problems—such as increased risk of stroke—that synthetic estrogens are now known to cause.

In fact, soy isoflavones do not simply act as "natural" estrogens. Soy isoflavones are correctly classified as selective estrogen receptor modulators. Due to their unique molecular structure, soy isoflavones can act as either estrogen receptor agonists or receptor blockers. With this ability, soy isoflavones are thought by many to confer the beneficial effects of estrogen without its potentially dangerous side effects, especially in hormonally sensitive tissues found in both the breast and endometrium.5,6

Numerous studies show the potential benefits to women of incorporating soy in their diets to help prevent breast cancer. A landmark 1991 case-control study of women in Singapore, involving 200 case subjects and 420 control subjects, found that women with the highest consumption of soy-based products had a markedly decreased risk of developing breast cancer.7 An even larger Japanese case-control study in 1995, involving 1,186 subjects and 23,163 controls, also showed that women with increased tofu (soybean curd) intake had a significantly decreased risk of developing breast cancer compared to women who consumed small amounts of soy-based products such as tofu.8 Finally, a recent and very large population-based, prospective study of 21,852 Japanese women aged 40-59 found that women with the highest intake of soy isoflavones reduced their risk of breast cancer by up to 54% compared to women with the lowest intake of soy isoflavones.9

Despite the evidence-based research showing soy isoflavones' preventive effects on breast cancer, along with epidemiological studies highlighting the much lower rates of breast cancer among Asian women who consume significant amounts of soy-based products, some doctors still caution women against using soy-based foods and supplements. They contend that because soy isoflavones have been labeled as estrogen "mimics," they could potentially worsen or even cause breast cancer. With the current knowledge that soy isoflavones act as selective estrogen receptor modulators and are not simply estrogen "mimics," these arguments simply do not hold up. As the authors of a recent study of soy isoflavones stated:

"Recommendations to avoid soy foods now being given by many health professionals to these patients [women with breast cancer] are not based on any clinical evidence to support this advice . . . the fact that an isoflavone like genistein acts more like a [selective estrogen receptor modulator] than an estrogen should be the basis for believing that soy foods [and supplements] are more likely to be beneficial for breast cancer treatment and prevention."10

In addition to being a chemoprotective supplement for breast cancer, soy isoflavones also are thought to be effective in warding off other types of cancer that afflict women, including endometrial cancer. A recent case-control study reported the effects of soy isoflavones and other phytoestrogens on the risk of developing endometrial cancer.11 The study compared 500 women aged 35-79 who developed endometrial cancer between 1996 and 1999 to 470 age-and ethnicity-matched controls. As in studies examining the effects of isoflavones on breast cancer, this study showed that women who had a higher intake of soy isoflavones had a significantly lower risk of developing endometrial cancer. Even more interesting was that the levels of isoflavones needed to provide protection against endometrial cancer were found to be much lower than the amount believed necessary to protect against breast cancer.