Finally, it must be noted that many studies suggest that FGF21 is elevated in settings of obesity, hepatic steatosis, and other settings of metabolic stress that on the surface appear unrelated to a low-protein state [134,135,136], and thus, we lack a unifying framework that explains FGF21′s involvement in these disparate physiological or pathophysiological settings. Here, FGF21 is linked to obesity due to melanocortin 4 receptor deficiency.