Approaches for the primary prevention of breast cancer according to period of life are discussed here briefly and are considered in more detail elsewhere. Although the major reasons for the high rates of breast cancer in affluent Western populations are largely known, this knowledge does not necessarily translate easily into strategies for breast cancer prevention.

Some risk factors (e.g., age at menarche) are well established but are difficult to modify; some (e.g., postmenopausal hormone use) are well established but have other important benefits; and others (e.g., replacement of saturated fat with monounsaturated fat) are unproven although suggested by available data and have other strong benefits that justify the strategy, with reduction in breast cancer being a possible additional benefit.

Also, known risk factors for breast cancer are modest in magnitude; relative risks are usually in the range of 1.3 to 1.8 for attainable changes. Although these relative risks are far less dramatic than those involving smoking and lung cancer, they should still be considered important. To provide perspective, the relative risk of death from breast cancer for women who do not have mammography compared with those who receive regular mammograms is approximately 1.3. Just as great emphasis and resources are given to the provision of mammography, the avoidance of a risk factor with a similar magnitude of effect should have even higher priority, because this prevents both the occurrence and need for treatment of breast cancer as well as death.

When considering primary prevention, one must remember that even small changes at the individual level can produce substantial changes in the population rates of disease.

Some strategies for prevention can be implemented by individuals themselves, but the health system, as well as governments and society as a whole, can take actions that strongly influence rates of breast cancer.

Early onset of menarche in the United States and other affluent countries is largely the result of rapid growth and weight gain of children related to an abundant food supply, excellent sanitation, and low levels of physical activity (including sitting in school). Much of this is desirable for many reasons, and no reasonable expectation exists that one could, or would, want to increase the average age at menarche to 17 years, as has been typical in rural China.

Yet generally desirable increases in physical activity, such as greater recreational activities, have been associated with modest delays in age at menarche and should thus contribute to reductions in breast cancer. The amount of time spent watching television is a major determinant of excessive weight gain by children and is therefore an appropriate focus for reducing risk of breast cancer and future cardiovascular disease. Society, through the provision of daily physical activity in schools and safe environmentsfor recreational activity, must play a major role in these efforts.

Early age at first birth substantially reduces breast cancer, but the societal trends are in the opposite direction because of delay of childbirth until after advanced educational programs are completed and careers are established. Furthermore, unplanned early pregnancies and an average of more than two completed pregnancies per woman have undesirable social and ecologic consequences.

Nevertheless, a social norm that encouraged carefully planned first pregnancies at the beginning of advanced education and career development would reduce breast cancer rates. Major behavioral changes and social supports, such as the provision of child care, would be required for this to be practical on a widespread basis; because of the complex social changes needed for it to be a practical strategy for breast cancer prevention.

At least 6 months of lactation are recommended for optimal infant health,53 and evidence suggests that this modestly reduces risk of breast cancer, particularly among premenopausal women. Improved physician counseling can encourage this practice, but changes at workplaces to allow breast-feeding and longer maternity leaves are also needed for many women to adopt this practice.

Alcohol consumption has a complex mix of desirable and adverse health effects, one being an increase in breast cancer. Individuals should make decisions considering all the risks and benefits, but for a middle-aged woman who drinks alcohol on a daily basis, reducing intake is one of relatively few behavioral changes that is likely to reduce risk of breast cancer. Taking a multiple vitamin containing folic acid greatly reduces risks of neural tube defects and may prevent coronary heart disease and colon cancer, and one study suggests this may also mitigate the excess risk of breast cancer due to alcohol. Thus, taking a multiple vitamin appears sensible for women who do elect to drink regularly.

Postmenopausal hormone use, like alcohol consumption, involves a complex trade-off of benefits and risks. From the standpoint of breast cancer risk, the optimal strategy would be to use estrogens not at all, or at most for a few years to relieve menopausal symptoms. The range of options is rapidly increasing, however, with the availability of selective estrogen receptor modulators, such as tamoxifen citrate and raloxifene hydrochloride, that can prevent the progression of osteoporosis and simultaneously reduce risk of breast cancer. Physicians need to play a key role in advising women in this rapidly evolving field.

Avoiding weight gain during adult life can importantly reduce risk of postmenopausal breast cancer as well as of cardiovascular disease and many other conditions. Individual women can reduce weight gain by exercising regularly and moderately restricting caloric intake. Health care providers play an important role in counseling patients throughout adult life about the importance of weight control. The incorporation of greater physical activity into daily life will be difficult for many persons, however, unless governments provide a safer and more accessible environment for pedestrians and bicycle riders. The provision of worksite and community exercise facilities can also contribute importantly.

Specific aspects of diet that influence risk of breast cancer are not yet well established, but available evidence generally suggests that increasing intake of vegetables can modestly reduce risk and that replacing saturated and trans fat with monounsaturated fat may reduce risk. These are reasonable strategies to pursue, because they reduce risk of cardiovascular and other diseases, and reduced risk of breast cancer may be an added benefit. Physicians can assess dietary habits and provide guidance. Governmental policies also influence diet in many ways. Providing the best current information on diet and health is one such way.

With demonstration that tamoxifen citrate, and probably other selective estrogen receptor modulators, can be effective in the primary prevention of breast cancer, chemoprevention has become an option for women at elevated risk. Many other pharmacologic agents are being evaluated and are likely to increase the number of alternatives. The availability of effective chemopreventive agents raises many questions about the optimal criteria for use of these drugs.

Evaluation of an individual woman’s risk of breast cancer has become much more important, because this risk can now be modified. Until the mid-1990s, risk had been based primarily on an evaluation of family and reproductive history and history of benign breast disease. New information on risk based on genotype, detailed histologic characteristics of benign breast disease, and serum hormone levels now allows a much more powerful prediction of risk for an individual woman. Screening for elevated estrogen levels in postmenopausal women to help identify those who would most benefit from an estrogen antagonist, as is done for serum cholesterol, may become part of medical practice. Physicians will play a key role in keeping abreast of this rapidly developing area and counseling patients appropriately.

In summary, available evidence provides a basis for a number of strategies that can reduce risk of breast cancer, although some of these represent complex decision making. Attainable changes can have an important impact on individual risk of breast cancer. However, the collective implementation of all lifestyle strategies will not reduce population rates of breast cancer to the very low levels seen in traditional poor societies, because the magnitude of the necessary changes is unrealistic or undesirable. Thus, a role will exist for hormonal and other chemopreventive interventions, which may be appropriate for women at particularly high risk and, potentially, for wide segments of the population, as few women can be considered to have very low risk. Together, the modification of nutritional and lifestyle risk factors and the judicious use of chemopreventive agents can have a major impact on incidence of this important disease.

Walter C. Willett, Beverly Rockhill, Susan E. Hankinson, David J. Hunter and Graham A. Colditz

W. C. Willett: Harvard Medical School, Boston, Massachusetts; Department of Nutrition, Harvard School of Public Health, Boston, Massachusetts
B. Rockhill: Harvard Medical School, Brigham and Women’s Hospital, Boston, Massachusetts
S. E. Hankinson: Departments of Medicine and Epidemiology, Harvard Medical School and Harvard School of Public Health, Boston Massachusetts
D. J. Hunter: Departments of Epidemiology and Nutrition, Harvard School of Public Health, Boston, Massachussetts
G. A. Colditz: Department of Medicine, Harvard Medical School, Boston, Massachussetts

References