- The use of phytoestrogens as a natural source of estrogen has been rapidly accepted by consumers for various remedial or preventive purposes, including breast cancer prevention. Phytoestrogens exhibit mixed weak estrogen agonist/antagonist properties and antioxidative activity. They may play a significant inhibitory role during the initiation and promotional phases of cancer development. Experimental and physiologic studies provide much of the evidence on the inverse association between breast cancer risk and phytoestrogen intake. Research in humans has been limited to observational (case-control) epidemiologic studies and is far from conclusive. A critical evaluation through controlled trials of phytoestrogens' breast cancer-protective role needs to be performed before they are adopted as chemopreventive agents.