May 13, 2008 -- Women who engage in regular, consistent exercise are less likely to develop breast cancer than women who are less active, according to two new studies.
Exercise has long been prescribed as a way to help ward off disease. Now experts believe physical activity may help lower a woman's breast cancer risk by reducing the production of certain hormones, such as estrogen, insulin, and insulin-like growth factor that may fuel cancer development.
Graham Colditz, MD, of Washington University School of Medicine in St. Louis, and colleagues evaluated questionnaires from more than 64,000 premenopausal women involved in the Nurses Health Study II. The women detailed their leisure-time physical activity starting from age 12 to the present.
After six years of follow-up, 550 women developed breast cancer. Researchers discovered that the women whose activity equaled 13 walking hours a week or 3.25 running hours per week had a 23% lower risk of premenopausal breast cancer compared with the less active women. The strongest association was seen with increased exercise during adolescent and young adult years (ages 12-22).
“These results suggest that consistent physical activity during a woman’s lifetime is associated with decreased breast cancer risk. Unlike many risk factors for breast cancer, physical activity is an exposure that can be modified,” the authors say in a news release.
According to background in the journal article, the current study is one of few that have examined the impact of exercise on premenopausal breast cancer.
Several studies have shown that regular exercise helps a woman reduce her risk of breast cancer after menopause. The latest study, published in the British Journal of Sports Medicine, shows that the most physically active women cut their risk of breast cancer by 25%, but the benefit varies depending on factors such as physical activity type, timing, family history of breast cancer, and body mass index (BMI).
Christine M. Friedenreich, PhD, of the Division of Population Health and Information of the Alberta Cancer Board in Calgary, Canada, and colleagues analyzed findings from 62 studies to determine how physical activity might influence breast cancer risk. They report that in more than two-thirds of the studies included in their analysis, there was a 25% reduced risk of breast cancer in the most active women compared to the least active women.
Friedenreich's team found that all activity -- including housework -- lowered a woman's risk of breast cancer, but sports and other vigorous recreational activity, such as walking or running, further reduced the risk. Women with lower BMI or postmenopausal women who played sports or engaged in regular exercise had lower risks of breast cancer compared to women with very high BMI and premenopausal women.
Fitness isn't a guarantee that you won't develop breast cancer, and the researchers are not saying lack of exercise causes breast cancer.