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A few small studies have suggested that the isoflavones in soy could reduce male testosterone levels, but that was not confirmed by a recent meta-analysis of all relevant clinical trials. Based on the results of around 30 studies, researchers from the University of Minnesota concluded that soy isoflavones and protein have no effect on testosterone and other male hormones. The original studies that reported low testosterone levels were criticized for their methodological flaws. Read more about soy research at About.com.