Consequently, FGF21 serum concentrations are elevated in several diseases hallmarked by mitochondrial dysfunction and ER stress, such as obesity, diabetes and mitochondrial myopathies, regarded as an adaptive response by increasing mitochondrial respiration and autophagy, and attenuating ER stress and metabolic abnormalities [8, 10, 12–14]. Here, FGF21 is linked to obesity due to melanocortin 4 receptor deficiency.