In a large prospective study of the health of black women, the Black Women’s Health Study (BWHS), researchers collected information about exercise habits, such as time spent exercising per week and type of exercise. They followed more than 44,000 African American women over a span of 16 years and observed whether they developed breast cancer.
They found that women who exercised vigorously for seven or more hours each week were 25 percent less likely to develop breast cancer, compared to those who exercised less than one hour each week. Examples of vigorous activity include basketball, swimming, running and aerobics. The results were similar if women walked briskly, but there was no benefit for walking at normal pace. The results did not differ by the estrogen receptor status of the breast cancer
Lynn Rosenberg, ScD, professor of Epidemiology at Boston University School of Public Health and principal investigator of the Black Women’s Health Study, states, “Although expert review panels have accepted a link between physical exercise and breast cancer incidence, most study participants have been white women. This is the first large scale study to support that vigorous exercise may decrease incidence of breast cancer in African American women.”
source : http://www.sciencedaily.com/releases/2014/08/140813103800.htm