More workouts for women may mean less risk of breast cancer. (Women's Health).

Sternfield B, Jacobs MK, Quesenberry CP, Gold EB, Sowers M. 2002. Physical activity and menstrual cycle characteristics in two prospective cohorts. Am J Epidemiol 156(5):402-409.

Numerous studies have shown that women who exercise regularly are at decreased risk for breast cancer, compared to women who lead a more sedentary lifestyle. However, the reasons for this difference have not been fully determined. One possible explanation could be an overall healthier lifestyle among the women who exercise. Another may be that regular vigorous exercise can lead to longer menstrual cycles (and in extreme cases, usually involving competitive athletes, missed menstrual cycles). This results in decreased exposure of breast tissue to circulating estrogens and progesterone at various stages of the menstrual cycle. Because breast cancer is a hormonally mediated disease, NIEHS grantee Ellen B. Gold from the University of California at Davis School of Medicine and colleagues investigated the role of exercise-induced changes in menstrual cycle in two large cohorts of women of reproductive age. The study was also funded in part by the National Institute for Child Health and Human Development and the Semiconductor Industry Association.

The study found that physical activity was associated with increased menstrual cycle length. The magnitude of this association decreased as body mass index increased, suggesting that the greatest benefit is seen in women of average size. This finding also supports the benefits of promoting regular, even vigorous, physical exercise.

The study was limited in that it relied on self-reporting to assess the level of physical activity; some women probably overreported this factor. However, the cohorts were large enough that such measurement errors were unlikely to have biased the results, say the authors. In addition, both cohorts were ethnically and racially diverse, which may allow for greater generalization of the results to the general population of women.

To build upon these new data, the investigators say, future studies should aim at assessing the effects of exercise on both the control of the cyclical hormonal feedback that governs ovulation and the reduction of hormone levels.