Soy & Cancer PreventionJuly 2004
By Edward R. Rosick, DO, MPH, MS
Soy Counteracts Prostate Cancer in Men
Most recently, in a study published in the journal Cancer Science in March 2004, researchers found that soybean products, as well as other Far Eastern dietary staples such as fish and tofu, were associated with a decreased risk of prostate cancer in Japanese men.13 Specifically, men who consumed the greatest amounts of soybeans and tofu were 47% and 53% less likely, respectively, to develop prostate cancer than those who consumed the smallest amounts. Furthermore, in men who consumed the greatest amount of the fermented soybean natto, the prostate cancer incidence was reduced a remarkable 75%.
Early animal studies found that this difference is most likely attributable to the cancer-inhibiting effects of soy isoflavones. In 1999, Harvard Medical School researchers studying mice with transplanted human prostate cancer cells found that those fed a diet high in soy isoflavones experienced tumor regression compared to mice fed a non-soy diet.14 The study authors noted that "our data suggest that dietary soy products may inhibit experimental prostate tumor growth through a combination of direct effects on tumor cells and indirect effects on tumor neovasculature."
Newly published animal research lends strong support to this hypothesis. In a study published in January 2004, University of Colorado researchers fed inositol hexaphosphate (IP6), a dietary phytochemical found in cereals, soy, legumes, and other fiber-rich foods, to mice that had been injected with prostate cancer cells. Compared to controls, the mice fed IP6 exhibited suppressed hormone-refractory prostate cancer growth by as much as 66% when compared to littermates without the IP6-enriched diet.15
In another study published in March 2004, Japanese researchers evaluated the effects of Fuji-flavone, a commercial isoflavone supplement derived from soybean products, on rat prostate carcinogenesis. Rats fed a diet containing Fujiflavone showed a significantly lower incidence of prostate carcinomas than those fed a soy-free diet, leading the researchers to conclude that "intake of dietary isoflavones can be promising for prevention of human prostate cancer."16
While animal-based studies are important in advancing scientific knowledge, not all results from animal studies are directly transferable to humans. Fortunately, human studies (in addition to the previously noted epidemiological reports) also support the theory that soy isoflavones can protect men against prostate cancer. One of the largest of these, a prospective study conducted between 1976 and 1992 on 12,395 men in Loma Linda, California, found that men who drank a glass of soy milk more than once a day reduced their risk of prostate cancer 70% compared to men who did not drink soy milk.17 Even after adjusting for various statistical confounders, the 70% reduction held firm, leading the authors to conclude that "our study suggests that men with high consumption of soy milk are at decreased risk of prostate cancer. This may also be the case for men who frequently consume other soy products with intact content of isoflavones . . ."
Finally, in a case-control study published in January 2004, Japanese researchers sought to ascertain whether a high serum concentration of phytoestrogens reduces the risk of prostate cancer. The researchers collected lifestyle information and serum samples from more than 14,000 Japanese men in 1988-90, who were tracked until 1999. Phytoestrogens and sex hormones stored in serum were measured in 2002, and 52 case subjects and 151 controls were identified. This study clearly established that elevated serum levels of all three phytoestrogens assessed—genistein, daidzein, and equol—imparted a strong protective effect against prostate cancer.18 Men with the highest circulating levels of genistein, daidzein, and equol reduced their risk of prostate cancer by 62%, 57%, and 66%, respectively.
Positive Effects on Skin Aging, Cholesterol
Multiple studies now show that soy isoflavones also reduce harmful levels of cholesterol and the resulting atherosclerosis. Asian men, who consume much more soy than Americans, have a sixfold lower mortality rate for coronary heart disease compared to American men. The difference also is seen in Asian women, who have an eightfold lower mortality rate from heart disease compared to their US counterparts.
Animal studies have confirmed that soy isoflavones have significant effects in lowering harmful LDL cholesterol,20 as have studies in humans. In two recent studies at the University of Minnesota, women aged 18-70 who were given soy isoflavone supplements saw a marked decrease in their LDL cholesterol.21,22 An even more recent Canadian study examined the effects of soy isoflavones on cholesterol and homocysteine levels, as well as on blood pressure, in 41 men and women.23 In this randomized crossover study, patients were fed a low-fat dairy-based diet, a diet low in soy isoflavones, or a diet high in soy isoflavones. After three months, the men and women who ate either the low- or high-isoflavone diet had significantly lower LDL cholesterol and homocysteine levels, as well as a reduction in blood pressure, compared to those who ate the low-fat dairy-based diet. This led the study authors to conclude that "soy protein foods, regardless of their isoflavone content, may improve many lipid and nonlipid risk factors for [coronary artery disease] and thus justify the use of soy foods as part of a dietary strategy to reduce [coronary artery disease] risk."
A Simple Food with Multiple Benefits
With strong scientific evidence showing that soy isoflavones can help prevent breast, endometrial, and prostate cancer, as well as protect both men and women against heart disease, physicians should not hesitate to educate their patients on the need to incorporate soy in their diets. Soy isoflavones not only protect against the two most common and deadly killers in America today, but clinical research suggests that they also may protect against diabetes and obesity.24,25
A small, nondescript legume, soy nevertheless continues to provide us with a bountiful harvest of health benefits.
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