Hypothesized Anticancer Effects of Soy: Evidence Points to Isoflavones as the Primary Anticarcinogens

Abstract

Soybeans contain several putative anticarcinogens. To determine which anticarcinogen is most likely responsible for the hypothesized anticancer effects of soy specifically in regard to breast and prostate cancer, Japanese dietary data and findings from epidemiologic studies were first used to estimate the amount of soy associated with anticancer effects. On the basis of this information, the experimental data for likely candidates were evaluated. One hundred g of tofu was selected as a basis for estimating the contribution of soy to the total Japanese dietary intake of the various anticarcinogens in soybeans. Data suggest that tofu contributes only minor amounts of phenolic acids (<1%), phytosterols (<5%), and vitamin E (-10%), modest amounts of protein (10-13%), omega-3 fatty acids (-15%), and phytate (-20%), but large amounts of saponins (80%), the Bowman-Birk Inhibitor (BBI) (-90%), and isoflavones (100%). The evidence that saponins inhibit breast and prostate cancer risk is unimpressive. The BBI clearly has exciting potential as a chemopreventive agent but data specifically for breast and prostate cancer are not strong. In contrast, several lines of evidence suggest isoflavones reduce breast (primarily via early exposure) and prostate cancer risk. Thus, if soy consumption reduces cancer risk, protective effects are likely due solely to isoflavones. However, this conclusion does not preclude the possibility that other soybean anticarcinogens, when consumed in amounts greater than that found in 100 g tofu, exert anticancer effects in humans.