Prostate cancer is the most common cancer among U.S. men and the second most common cause of cancer death among American males.1 Globally, however, the incidence of and death from prostate cancer varies greatly.2 When men from low-risk countries move to high-risk countries, prostate cancer rates increase among the succeeding generations. This observation indicates that differences in prostate cancer risk are due to environmental factors such as lifestyle habits rather than genetic differences among populations. To this point, evidence suggests that diet may play a role in both the risk for prostate cancer and its prognosis in men who have the disease.3,4,5
In 1990, participants at a workshop sponsored by the U.S. National Cancer Institute identified isoflavones as chemopreventive (anti-cancer) agents.6 Isoflavones are diphenolic compounds with limited distribution in nature (the soybean is the only commonly consumed food to contain nutritionally-relevant amounts) that exhibit both hormone-dependent and hormone-independent effects under various experimental conditions.7,8,9
In 2009, a meta-analysis of 24 epidemiologic studies found that prostate cancer risk was significantly reduced by 24% in high-soy consumers compared to low-soy consumers.10 In addition, subanalysis of the data revealed that risk was reduced by 48% among Asian men whereas there were no differences in groups among the Western populations. It’s not surprising that soy consumption had no discernible effect among Westerners because few Western men in these studies would have typically consumed enough soy to exert physiological effects.11 The protective effects noted in the meta-analysis were supported by a more recent analysis of the epidemiologic literature by Korean researchers.12
Numerous investigators have examined the impact of isoflavones on blood levels of prostate specific antigen (PSA). PSA levels are commonly and routinely screened as a means of detecting prostate cancer and can also be used to assess treatment efficacy although the predictive ability of PSA has recently been challenged.13,14 Based on a review of studies published in 2006, there is no evidence that soy or isoflavones affect PSA levels in healthy men with normal PSA levels.15 However, this does not rule out benefits of isoflavones since clinical studies indicate that, in healthy men with low PSA levels, it is possible to reduce prostate cancer risk without affecting PSA.16
In contrast to the results found in healthy men, four of the eight trials involving men with prostate cancer, all of which were included in the previously cited review, showed that isoflavones slowed the rise in PSA levels, although no study reported an absolute decrease.15 Five other studies, published more recently, support the findings of this review. One of these studies was conducted for one year, two were six months in duration, one was three to six weeks and the other was conducted for two years.17,18,19,20,21 The results of the two-year trial are especially notable because all of the subjects had failed conventional treatment—that is, their PSA levels continued to rise—having undergone surgery and radiation with or without therapy to decrease testosterone levels.21 In half the subjects, PSA levels favorably responded to the consumption of three servings of soyfoods (primarily soymilk) per day.
Finally, a one-year clinical trial found that an isoflavone supplement reduced risk for progressing to prostate cancer among older Japanese men at high risk of developing the disease. In this study, men with precancerous conditions were given either a placebo or 60 mg of isoflavones daily. At study termination, 34 and 21 percent of the men in the placebo and isoflavone groups developed cancer, respectively. Even greater differences were observed among the older men in this study; among those =65 years of age, 57 and 28 percent of the men in the placebo and isoflavone groups developed cancer, respectively.22
In addition to helping prevent the development of prostate cancer and inhibit tumor growth, there is speculative but intriguing evidence from both animal and human studies suggesting that isoflavones may also be useful for stopping metastasis. For example, a study published in 2009 reported that levels of an enzyme that allows cells to invade tissues—matrix metalloproteinase-2–was markedly reduced in prostate cancer patients given genistein.23
Although no definitive conclusions can be made at this time, research findings suggest that soybean isoflavones and isoflavone-rich soyfoods may prevent the development of prostate cancer and aid in the treatment of this disease by inhibiting the spread of prostate tumors and slowing tumor growth. These findings also suggest that the consumption of two to three servings of traditional soyfoods (e.g., 1 cup soymilk, 3-4 ounces of tofu) is sufficient to derive the proposed benefits.
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