Physical activity may not seem to have much to do with the health of your breast tissue, but research shows that women who exercise are less likely to develop breast cancer than their sedentary peers. One of the largest such surveys found an 18% drop in risk of the disease for women who walked briskly for 75 to 150 minutes each week compared with women who were less active. While doctors can't explain exactly how exercise reduced cancer risk, they believe it may have something to do with how much body fat a woman has; fat cells tend to release more estrogen, which is known to promote some types of breast cancer.