Retinol to Retinol-Binding Protein (RBP) Is Low in Obese Adults due to Elevated apo-RBP

Elevated serum retinol-binding protein (RBP) concentration has been associated with obesity and insulin resistance, but accompanying retinol values have not been reported. Assessment of retinol is required to discriminate between apo-RBP, which may act as an adipokine, and holo-RBP, which transports vitamin A. The relations between serum RBP, retinol, retinyl esters, BMI, and measures of insulin resistance were determined in obese adults. Fasting blood (≥8 h) was collected from obese men and women (n = 76) and blood chemistries were obtained. Retinol and retinyl esters were quantified by HPLC and RBP by ELISA. RBP and retinol were determined in age and sex-matched, nonobese individuals (n = 41) for comparison. Serum apo-RBP was two-fold higher in obese (0.90 ± 0.62 μM) than nonobese subjects (0.44 ± 0.56 μM) (P < 0.001). The retinol to RBP ratio (retinol:RBP) was significantly lower in obese (0.73 ± 0.13) than nonobese subjects (0.90 ± 0.22) (P < 0.001) and RBP was strongly associated with retinol in both...