Retinol-binding protein 4 and insulin resistance.

To the Editor: Graham et al. (June 15 issue)1 report that serum levels of retinol-binding protein 4 (RBP4) correlated with the magnitude of insulin resistance in various subjects. However, RBP4 and plasma insulin levels were dissociated in subjects who did not have an improvement in insulin sensitivity after exercise.1 Thus, we believe that the importance of RBP4 should be assessed in more diverse groups. We assessed serum RBP4 levels in 473 subjects with normal glucose tolerance (fasting blood glucose level, <110 mg per deciliter) who were randomly selected from the Suita Study, an epidemiologic cohort study.2 Serum RBP4 levels correlated with triglyceride and uric acid levels and tended to correlate with glycated hemoglobin values and levels of high-density lipoprotein (HDL) cholesterol (Table 1). However, RBP4 levels did not correlate with the fasting blood glucose level, waist-to-hip ratio, body-mass index (BMI), systolic blood pressure, or fasting insulin level. Even in the 38 subjects with a family history of diabetes, RBP4 did not correlate with the fasting insulin level. Thus, it seems unlikely that RBP4 will be useful for assessing the risk of type 2 diabetes in Japanese people. The biologic action of RBP4 in relation to the action of insulin will need to be clarified to explain this inconsistency. Naoyuki Takashima, M.D. Hitonobu Tomoike, M.D., Ph.D. Naoharu Iwai, M.D., Ph.D.