Effect of physical activity on women at increased risk of breast cancer: results from the E3N cohort study.

PURPOSE There is a need to investigate the type, duration, frequency, and intensity of physical activity that are critical to reduce the risk of breast cancer, and if this relation differs among subgroups of women. METHODS We analyzed the relation between physical activity and breast cancer incidence between 1990 and 2002 (n=3,424 cases), among 90,509 women of the French E3N cohort, ages between 40 and 65 years in 1990. We gave special attention to effect modification by body mass index (BMI), family history of breast cancer, parity, and hormone replacement therapy (HRT). RESULTS A linear decrease in risk of breast cancer was observed with increasing amounts of moderate (P(trend)<0.01) and vigorous (P(trend)<0.0001) recreational activities. Compared with women who reported no recreational activities, those with more than five weekly hours of vigorous recreational activity had a relative risk of 0.62 (0.49-0.78). This decrease was still observed among women who were overweight, nulliparous, had a family history of breast cancer, or used HRT. Compared with the whole cohort, among nulliparous women, the reduction of risk observed was of a higher magnitude, although the test for heterogeneity did not reach statistical significance. CONCLUSION A risk reduction of breast cancer was particularly observed with vigorous recreational activity. Further investigations are needed to confirm that intensity is an important variable to consider in risk reduction and to identify the precise biological mechanisms involved in such a risk reduction.