An immune signal promotes the production of energy-burning “beige fat,” according to a new study published August 5, 2021 in the open-access journal PLOS Biology by Zhonghan Yang, PhD, of Sun Yat-Sen University, Guangzhou, China, and colleagues. The finding may lead to new ways to reduce obesity and treat metabolic disorders. The article is titled “IL-25–Induced Shifts in Macrophage Polarization Promote Development of Beige Fat and Improve Metabolic Homeostasis In Mice.” The beige color in beige fat comes from its high concentration of mitochondria, the cell’s powerhouses. Mitochondria burn high-energy molecules like fats and sugars with oxygen, releasing energy. Normally, that energy is stored as ATP, the energy currency that the cell uses for almost all its activities. But in beige fat, mitochondria accumulate a protein called “uncoupling protein-1” that limits ATP production, generating heat instead.