Ways That Women May Possible Reduce Their Risk of Breast Cancer
Authors: L. A. Brinton
Journal: JNCI Journal of the National Cancer Institute (1994)

Despite the fact that breast cancer has been one of the most investigated tumors, it is estimated that only about 55% of the disease is explained by recognized risk factors (1). An additional frustration is that most of the identified risk factors are not readily preventable. These risk factors include relationships of risk with familial susceptibility, menstrual characteristics, reproductive behavior, and socioeconomic status. This lack of understanding about primary prevention of the disease has led to several therapeutic interventions, such as tamoxifen (2) and gonadotropin-releasing hormone agonists (3), which may not be without side effects. Thus, questions have arisen as to whether there are other ways by which women can reduce their risk of developing breast cancer. Much attention has focused on possible dietary means of lowering risk, but most studies to date have yielded inconclusive findings (4). Restriction of alcoholic beverage intake is generally viewed as beneficial, although the levels of consumption that are associated with risk as well as the biologic means of the action of alcohol remain to be elucidated. Whether the restriction of dietary fat can lower risk remains controversial. Analytic studies have not generally supported a relationship, but it remains unclear the extent to which this lack of association reflects difficulties in detecting effects in populations with homogeneous diets, a focus on the wrong periods of consumption (recent rather than adolescent diet), measurement imprecision, or a failure to account for interactive effects of other nutrients. Some recent evidence suggests that breast cancer risk may be reduced by vitamin A, carotenoids, vitamin C, vitamin E, or fiber intake. Thus, there is certain to be increased attention on the role of other nutrients and dietary patterns, which hopefully will generate further clues to dietary interventions. In this issue of the Journal, Bernstein et al. (5) present fairly convincing evidence that high levels of physical activity can lead to substantial reductions in breast cancer risk. These findings lend credence to the landmark research of Frisch et al. (6), who found reduced risk of subsequent breast cancer in girls who were athletes during college. Many remained skeptical about the results of this earlier study because of its focus on prevalent cancers and its inability to completely control for other breast cancer risk factors. Thus, the results of Bernstein and her colleagues are important in showing that physical activity is a seemingly independent predictor of risk. However, like most other studies in women, the measures of physical activity were not validated, highlighting the need for further studies in the area of exposure assessment. Nonetheless, the results are provocative, especially since physical activity has been shown to modify levels of endogenous hormones that could affect breast tumorigenesis. Studies to elucidate the hormonal consequences of physical activity will be important to furthering our understanding of preventive mechanisms. Bernstein et al. (5) contend that the effect of physical activity is independent from that of body size, but disentanglement is difficult because of the influences of physical inactivity on the development and maintenance of excess body mass. Although increased body mass has now been fairly consistently shown to increase the risk of the development of postmenopausal breast cancer, there has been a surprising lack of attention on weight loss as an intervention for lowering breast cancer risk. This may result from the complexities associated with body mass as a predictor of breast cancer risk, since large body mass has actually been associated with reductions in the risk of premenopausal breast cancer. Although this association has been suggested to reflect difficulties in detecting breast lesions in young, heavy women, several studies (7,8) suggest that this is not the sole explanation. Effects of obesity on endogenous hormones have been offered as an alternative explanation, with particular attention focused on adverse effects of high levels of progesterone in thin, premenopausal women (9). Further complicating the interpretation of the relation between anthropometric factors and breast cancer are findings that the timing of weight change as well as the distribution of body fat are predictive of risk. Several studies (7,JO) have emphasized the importance of changes in adult weight, possibly reflecting its role in adult adiposity. Further, there is accumulating evidence that fat distribution may predict breast cancer risk among postmenopausal women, with abdominal obesity being more hazardous than peripheral accumulation (//). Insights into the mechanisms of action of these risk factors may come from further studies of the interrelationships between fat patterns with endogenous hormones. Other modifiable risk factors for breast cancer unfortunately remain elusive. A possible factor that is re-emerging is breastfeeding (12,13). Patterns of breastfeeding that are most protective are unclear. Also, some evidence suggests that lactation protects mainly against the development of premenopausal breast cancer. Nonetheless, given the widespread potential for this intervention, it is clear that further exploration of its preventive effects on breast cancer should be pursued. It is noteworthy that a recent study (14) has shown that breastfeeding may not