It has been known since the early 1970s that breast cancer is somehow related to estrogen. In addition, women who have their ovaries removed early in life and are therefore exposed to much less estrogen that other women, rarely have breast cancer. These clues all relate estrogen to breast cancer. In recent years, menarche in American women has been occurring at earlier and earlier ages. The declining age of menarche in U.S. women increases their lifetime exposure to estrogen, which possibly increases their risk of breast cancer.
Some scientists say simply, improved nutrition.
Others say it may be caused by industrial chemicals in our food that mimic hormones and interfere with the internal mechanisms that regulate our bodies. Dr. Patricia Whitten at Emory University in Atlanta, Ga., has examined historical records extending back 200 years in several countries. Dr. Whitten's concludes that improved nutrition cannot account for all the changes in human reproduction that are observable in the historical record. The debate over menarche has real consequences in the kinds of research that scientists are doing. On the other hand, those who believe improved nutrition provide sufficient explanation for the change in menarche tend to ignore external influences, such as environmental chemicals.