Soy Intake During Childhood and/or Adolescence and Adult Breast Cancer: An Examination of the Early Soy Intake Hypothesis.

Breast cancer is one of the leading causes of cancer incidence and mortality in many countries worldwide although there is considerable geographic variation. Diet is thought to impact risk of developing breast cancer but identifying specific dietary factors involved in the etiology of this disease has proven difficult. The two primary factors that initially led to an interest in soy are the historically low breast cancer incidence and mortality rates in Japan and the uniquely high concentration of isoflavones in soybeans and foods derived from this legume. Isoflavones bind to both estrogen receptors although preferentially to estrogen receptor-β. Prospective cohort studies indicate that isoflavone intake is associated with a reduced risk of developing breast cancer, but randomized controlled trials in which the impact of soy and isoflavones on markers of breast cancer risk has been evaluated are not supportive of this protective association. It may be that for isoflavones to reduce risk, intake needs to occur during childhood and/or adolescence. The notion that consuming soy early in life reduces risk of adult breast cancer, herein referred to as the "early soy intake hypothesis" (ESIH), was proposed >30 years ago. The results of rodent studies and retrospective observational studies that examined incidence and/or markers of breast cancer risk support the ESIH. However, a lack of randomized controlled trials precludes a clear recommendation for soy consumption during childhood and/or adolescence specifically for breast cancer prevention. Although soy foods provide high-quality protein and a variety of nutrients and can be part of a healthy diet for young and adolescent girls, more research is needed to advance the ESIH.