FGF21 and obesity due to melanocortin 4 receptor deficiency: Thus, the inverse relationship observed between FGF21 and FGF19 in obesity and diabetes—upregulation of FGF21 and downregulation of FGF19—(Gallego‐Escuredo et al., 2015) did not occur in healthy elderly individuals, despite the significant increase in FGF21 (Figure 1a).