INS and endothelial dysfunction: Increased serum FGF21 may compensate for endothelial dysfunction in retinopathy, with defects in FGF21 expression or activation reducing insulin sensitivity, liver fatty acid oxidation, and triglyceride clearance (Kharitonenkov et al., 2005; Yang et al., 2012; Holland et al., 2013; Rusu et al., 2017).