Soy is a uniquely rich source of isoflavones, which have natural estrogenic properties. Isoflavones are classified as phytoestrogens. This has created concern about how soy may affect human hormones.
Although isoflavones fall under the phytoestrogen umbrella, they are more accurately classified as selective estrogen receptor modulators (SERMs). Depending on the receptor and body tissue, SERMs can have estrogen-producing or estrogen-blocking effects. Therefore, it is not appropriate to equate isoflavones or soyfoods with human estrogen because soybeans, like all foods, have multiple biologically active molecules.