Energy released from oxidation of carbohydrates and lipids generates a proton gradient across the mitochondrial inner membrane that can be used for ATP synthesis (coupled mitochondrial respiration) and thermogenesis (uncoupled mitochondrial respiration)1. While most attention has been focused on the role of mitochondrial respiration in ATP production, it is estimated that in endotherms the majority of the proton-motive force is used for heat generation to maintain a stable body temperature2,3. Studies have identified uncoupling protein UCP1 in cellular thermogenesis by transporting protons from the intermembrane space into the matrix of mitochondria4,5