Higher circulating levels of RBP4 have been reported in visceral obesity, insulin resistance, prediabetes, T2DM, non-alcoholic fatty liver disease (NAFLD), and the metabolic syndrome (Cho et al., 2006; Jia et al., 2007; El-Mesallamy et al., 2013; Chen et al., 2017; Wessel et al., 2019). Here, RBP4 is linked to metabolic syndrome.