They followed more than 44,000 black women for 16 years. They found those who exercised vigorously for seven or more hours a week were 25 percent less likely to develop breast cancer than those who worked out less than an hour a week.
The exercises included swimming, running, basketball and aerobics in addition to brisk walking. However, walking at a normal pace was not associated with a lower breast cancer risk.
"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," principal investigator Lynn Rosenberg, a professor of epidemiology at Boston University School of Public Health, said in a university news release.
The study was published online recently in the journal Cancer Epidemiology, Biomarkers & Prevention.
The U.S. Office on Women's Health has more about exercise.