Hormones and breast and endometrial cancers: preventive strategies and future research.

A number of hormonal approaches for prevention of endometrial and breast cancers have been proposed. Because of the hormonal responsiveness of both tumors, much attention has focused on effects of exogenous hormone use. Although estrogens in hormone replacement therapy increase the risk of endometrial cancer, the disease is substantially reduced by long-term use of oral contraceptives. The issues with breast cancer are more complex, mainly because of a variety of unresolved effects. Long-term estrogen use is associated with some increase in breast cancer risk, and certain patterns of oral contraceptives appear to predispose to early-onset disease. With respect to estrogens, preventive approaches for both tumors would include use for as limited periods of time as possible. Addition of a progestin appears to lower estrogen-associated endometrial disease, but its effect on breast cancer risk remains less clear. Additional studies on effects of detailed usage parameters should provide useful insights into etiologic mechanisms. Other preventive approaches for endometrial cancer that may work through hormonal mechanisms include staying thin, being physically active, and maintaining a vegetarian diet. Breast cancer risk may possibly be reduced by extended periods of breastfeeding, restriction of intake of alcoholic beverages, remaining thin later in life, and being physically active. Additional research is needed to clarify the biologic mechanisms of these associations. The bridging of epidemiology with the biologic sciences should clarify many unresolved issues and lead to better preventive approaches.