Several reviews have suggested that soy food consumption may contribute to the relatively low rates of breast, colon and prostate cancers in countries such as China and Japan.65,66 Soy contains isoflavones, such as genistein and daidzein, which are thought to account for the health benefits of this legume. You will see these compounds alternatively referred to by their specific chemical name, such as genistein, as isoflavones or as phytoestrogens. The latter term arises from the fact that these isoflavones mimic some of the biological effects of the female sex hormone, estrogen. In the laboratory, genistein has well-documented activity against prostate cancer.67-71 High concentrations of genistein and other soy isoflavones cause the death of prostate cancer cells. At somewhat lower concentrations, these isoflavones arrest the growth of prostate cancer and block tumor cell invasiveness. Additionally, there are now several studies in which soy products or isolated soy isoflavones have been demonstrated to slow the growth of human prostate cancer cells in animal models.68,72,73
The mechanism by which soy isoflavones might slow the growth and/or spread of prostate cancer is unclear. Genistein, at concentrations obtainable in humans, can block the action of both the epidermal growth factor and Her-2/neu receptors. This means that genistein can theoretically block one of the major mechanisms by which prostate cancer cells become hormone-refractory. While these isoflavones act as estrogenic compounds in laboratory assays, men on soy-rich diets have only minor alterations in sex hormone levels. Finally, soy isoflavones appear to block tumor angiogenesis in laboratory models by inhibiting the growth of proliferating endothelial cells.74
There is also controversy on the best soy product to use in order to obtain high blood levels of genistein and other soy isoflavones. Genistein is absorbed much more effectively from fermented soy products such as miso, natto and tempeh than it is from soy beans, tofu or soy milk. Additionally, soy phyto-estrogen or genistein tablets or capsules currently on the market would easily permit the ingestion of several grams of soy iso-flavones per day. An additional complication is that blood levels of genistein may underestimate the levels in the prostate. When genistein levels are measured in prostatic fluid, the concentration is 5-10 times higher than in the blood.
Despite these findings, use of soy isoflavones in the treatment or prevention of prostate cancer in men is questionable. While there are quite a few epidemiology studies that show a correlation between high soy intake and a reduced risk of prostate cancer, no randomized controlled clinical trial showing that these soy products prolong the life of men with this disease has yet been published.75,76 We even lack clinical trials that show soy products arrest or slow the growth of prostate cancer in men. We do not even know the dose and schedule for the administration of soy isoflavones most likely to affect human prostate cancer. On the other hand, there appear to be other health benefits associated with the use of soy protein as a substitute for animal proteins, prompting the FDA to allow firms marketing soy protein to claim that these products have a favorable impact on the course of coronary heart disease.77
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