From The March 2001 Issue of Nutrition Science News

Male Sex Hormones Differ With Meat and Soybeans

Prostate cancer rates are soaring in Western countries, whereas rates remain low in Asia where soy consumption is high. Could there be a connection? To find out, Raymundo Habito of Deakin University in Melbourne, Australia, used a randomized crossover design to look at sex hormones in 42 men (average age 45) eating either soy or meat protein.

The men were randomly assigned to one of two diets for four weeks, then resumed their normal diet for two weeks before switching to the alternate diet for another four weeks. Both diets provided similar amounts of energy, protein, carbohydrates, fat, fiber and alcohol. One diet included 150 g lean red meat; the other contained 250 g tofu, containing 35 g of soybean protein, as well as one serving of fish or chicken per week and additional butter and olive oil to compensate for the lower fat content of tofu compared to meat. Men on the tofu diet excreted high amounts of the isoflavones genistein and daidzein, showing they ate the tofu as directed.

Total serum values of testosterone and the estrogen estradiol were not significantly different between the two groups of men, but the testosterone:estradiol ratio was significantly higher on the meat diets. Sex-hormone-binding globulin (SHGB), which binds sex hormones and makes them inert, was 8.8 percent higher after the tofu diet than after the lean meat diet. Consequently, the amount of available testosterone as estimated by the free androgen index (total testosterone/SHGB) was 7 percent lower on the tofu diet.

Prostate cancer develops over many years, and in Asia tofu is consumed throughout the lifespan. Although the difference in sex-hormone measures was slight, it is possible that over a period of many years, minor changes in hormone levels could have significant influence on prostate cell growth. Lower levels of SHGB have been associated with a higher risk of prostate cancer. In addition to the effect of soy protein on sex hormone levels, it has been found that genistein may directly inhibit tumor growth. Therefore, it is possible that soy protects against prostate cancer through more than one mechanism.

—The British Journal of Nutrition 2000 Oct:84;557-63.