SIRT1 and obesity due to melanocortin 4 receptor deficiency: CD38 (−/−) mice are protected against high-fat-diet-induced obesity, increased lipid droplets in the hepatocytes and skeletal muscle, and glucose intolerance by enhancement of energy expenditure via increased mitochondrial biogenesis, which is related to the activation of Sirt1 and peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) [93] (Figure 4a).