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Researchers at the University of SouthernCalifornia found that women who did high-intensity sessions, such asswimming or running, for at least five hours per week had a 20 to 30percent lower risk of breast cancer than women who did more moderateactivities (like walking) for less than 30 minutes a week. "Strenuousexercise can lower production of estrogen and progesterone," says leadresearcher Leslie Bernstein, Ph.D. "Increased levels of these hormoneshave been associated with a higher risk of breast cancer." To make sureyou're hitting the mark, spend half of your weekly cardio workouts atRPE 6 or 7 (see below for RPE chart).