Evidence that kidney function but not type 2 diabetes determines retinol-binding protein 4 serum levels

Abstract

OBJECTIVE-It has been suggested that retinol-binding protein 4 (RBP4) links adiposity, insulin resistance, and type 2 diabetes. However, circulating RBP4 levels are also affected by kidney function. Therefore, the aim of this study was to test whether RBP4 serum levels are primarily associated with kidney function or type 2 diabetes. RESEARCH DESIGN AND METHODS-RBP4 serum concentration was determined by enzyme-linked immunosorbent assay in 126 nondiabetic and 104 type 2 diabetic subjects. The study population was divided according to estimated glomerular filtration rate (eGFR) into the following groups: eGFR >90 ml/min per 1.73 m2 (n = 53), 60-90 ml/min per 1.73 m2 (n=90), 30-60 ml/min per 1.73 m2 (n=38), and <30 ml/min per 1.73 m2 (n=49). Each group was subdivided into nondiabetic and type 2 diabetic subjects. RESULTS-RBP4 serum concentration was elevated (2.65 vs. 2.01 μmol/l; P < 0.001) and eGFR was reduced (56 vs. 74 ml/min per 1.73 m2; P < 0.001) in type 2 diabetic vs. nondiabetic subjects, respectively. By stratifying for eGFR, no more differences in RBP4 serum concentration were detectable between type 2 diabetic and nondiabetic subjects. A linear regression analysis revealed an influence of eGFR (r=-0.477; P < 0.001) but not A1C (r=0.093; P=0.185) on RBP4 serum concentration. CONCLUSIONS-Existing human data showing elevated RBP4 levels in type 2 diabetic patients may be the result of moderate renal insufficiency rather than support for the suggestion that RBP4 links obesity to type 2 diabetes. © 2008 by the American Diabetes Association.