This brief report discusses the relationship of select adipokines, serum retinol-binding protein-4 (RBP-4), leptin, and adiponectin to intramyocellular lipid (IMCL) and intrahepatic lipid (IHL). The data confirms that fasting serum RBP-4, adiponectin, and leptin levels: 1. are associated with peripheral insulin sensitivity, 2. are abnormal in the first-degree relatives of type 2 diabetic parents, and 3. correlate with soleus IMCL content as well as IHL content. This report also reveals that when compared to fasting serum adiponectin and leptin concentrations, RBP-4 is a stronger marker of insulin resistance in lean, nondiabetic, insulin-resistant individuals. Study findings also show that RBP-4 and leptin are better independent predictors of IMCL content than body fat content, whereas adiponectin is an independent predictor of IHL content along with sex and body mass index. Accordingly, the authors suggest that serum RBP-4, leptin, and adiponectin concentrations reflect ectopic fat accumulation in humans. They also draw attention to a new finding of their study: serum RBP-4, even if unassociated with body adiposity, correlates with excessive fat accumulation in nonadipose tissues such as the skeletal muscle and liver. The authors conclude that the sites where these adipokines may be involved in insulin resistance remain uncertain, but their effects may be mediated by ectopic fat accumulation in both the liver and skeletal muscle.
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