The expression of FGF21 is induced by peroxisome proliferator-activated receptors (PPARs)-α and -γ in various stresses, such as starvation, ketogenic diet, amino acid deprivation, high-fat diet (HFD) or obesity, low-carbohydrate levels, autophagy deficiency, and mitochondrial stress (26, –28). This evidence concerns the gene FGF21 and obesity due to melanocortin 4 receptor deficiency.