However, when protons do not pass through ATP synthase, they flow into the mitochondrial matrix through some “shortcut paths” in the inner mitochondrial membrane to form “proton leakage.” Te energy utilization efficiency of the cell is reduced, and uncoupling protein (UCP) acts as the “shortcut path.”82 UCP2 is one of the UCP family of proteins closely associated with metabolic diseases, such as diabetes, cardiovascular diseases, obesity, and cancer.83 Here, UCP1 is linked to metabolic disease.