FGF21 and major depressive disorder: A previous study38 reported increased peripheral FGF21 levels in patients with major depression, while the association between depression and IL-6 has been well documented.39 FGF21 is a key hormonal regulator of metabolic function and nutrient preference,40 while IL-6 is a pleiotropic protein that targets multiple organs, particularly adipocytes.32 That said, both inflammatory proteins play an important role in the regulation of fat mass.31,32 Taken together, our results suggest that depression is linked to respiratory symptoms partially through dysregulated metabolic pathways.