Matthews CE, et al. - Given several prospective studies have suggested that increased 2-hydroxylation of estrogens could reduce postmenopausal breast cancer risk, researchers assessed the impacts of exercise on estrogen metabolism in 153 postmenopausal women randomized to 12-months of moderate-vigorous exercise and 153 controls. They also examined links with cardiorespiratory fitness measured by treadmill. They found that estrogen metabolism was not altered by 12-months of moderate-vigorous exercise or increased fitness in a way that might attenuate breast cancer risk. Overall, they concluded that, changes in estrogen metabolism are unlikely to be mediators of the protective impact of exercise on postmenopausal breast cancer.