McMaster researchers have found one more reason to exercise: working out triggers influential stem cells to become bone instead of fat, improving overall health by boosting the body’s capacity to make blood. The body’s mesenchymal stem cells are most likely to become fat or bone, depending on which path they follow. Using treadmill-conditioned mice, a team led by the Department of Kinesiology’s Gianni Parise has shown that aerobic exercise triggers those cells to become bone more often than fat. The exercising mice ran less than an hour, three times a week, enough time to have a significant impact on their blood production, says Parise, an associate professor. In sedentary mice, the same stem cells were more likely to become fat, impairing blood production in the marrow cavities of bones. The research appears in a new paper published by the Journal of the Federation of American Societies for Experimental Biology.