As a novel fibroblast growth factor (FGF) family member identified in 2000 [136], FGF21, has emerged as an important regulator and orchestrator of glucose and lipid metabolism and energy homeostasis [137,138] with the potential to treat obesity, T2DM, and meta-inflammation [139,140,141,142]. This evidence concerns the gene FGF21 and obesity due to melanocortin 4 receptor deficiency.