Listen to Dr. Chih-Hao Lee, a professor in the Department of Molecular Metabolism at Harvard T.H. Chan School of Public Health, whose team discovered that the metabolic benefits of endurance exercise are triggered by IL-13, a molecule released by immune cells residing in the muscle tissue. IL-13, speculated to have evolved to adapt to the energy stress during human parasitic infections, triggers efficient use of energy during muscle contraction resulting in increased endurance capacity, mitochondrial biogenesis, glucose tolerance, and fatty acid oxidation. “Furious Freak” by Kevin MacLeod is licensed under CC BY 4.0.