Soybeans and soy products contain isoflavones, a type of phytoestrogen that can lead to a rise in estrogen (i.e., estradiol) in your body, which in turn cuts your T levels. Soy also causes a boost in levels of the stress hormone cortisol. A recent study from the University of Connecticut found that men who took a soy supplement (soy protein) for 14 days had a reduction in serum testosterone. An exception to these soy activities is tempeh and other fermented soy foods. Fermentation significantly reduces levels of isoflavones and also results in soy foods that provide beneficial bacteria (probiotics), which is helpful because the biological impact of soy isoflavones are influenced by the activity of bacteria in the gut.