FGF21 and obesity due to melanocortin 4 receptor deficiency: FGF21 is activated by activating transcription factor 4 (ATF4) induced by autophagy deficiency [95], protecting mice against diet-induced obesity [96] by enhancing the mitochondria oxidative efficiency [97], increasing fatty acid utilization, promoting lipid excretion [98], and lowering the level of blood glucose and triglycerides [99].