Metabolic Inflexibility Revisited: Muscle Substrate Oxidation Is Dissociated From Muscle Insulin Resistance

Alterations in basal metabolic substrate preference were proposed as early derangements in the development of skeletal muscle insulin resistance. Here, we report that alterations in muscle substrate oxidation are not associated with muscle insulin resistance in rats or humans. In this study we used a novel stable isotope tracer ([U-13C]glucose) liquid chromatography-tandem mass spectrometry (LCMS/ MS) method to measure the ratio of mitochondrial glucose oxidation by pyruvate dehydrogenase flux (VPDH) relative to rates of citrate synthase flux (VCS) in skeletal muscle. We found that high fat diet (HFD) fed insulin resistant rats did not have altered substrate oxidation in soleus muscle in the fasting state. Hyperinsulinemic-euglycemic clamps increased relative glucose oxidation in both normal and insulin resistant rats, although this increase was blunted in HFD fed insulin resistant rats, due to an impairment in insulin-stimulated muscle glucose transport. Additionally, we found that an acute infusion of lipid during a hyperinsulinemic-euglycemic clamp in normal rats significantly reduced mitochondrial VPDH/VCS flux in soleus muscle without any effects on insulin-stimulated peripheral glucose metabolism or muscle glucose transport. Using the same stable isotope/LC-MS/MS approach we then examined VPDH/VCS flux in insulin sensitive and insulin resistant humans and found similar relative rates of VPDH/VCS following an overnight fast and similar increases in VPDH/VCS fluxes during a hyperinsulinemic-euglycemic clamp. Taken together these results show that basal mitochondrial substrate preference in muscle is not altered in insulin resistant rats or insulin resistant humans and that acute modulation of substrate oxidation in normal rats does not affect muscle insulin sensitivity refuting the metabolic inflexibility hypothesis which stipulates that alterations in substrate oxidation as an essential step in the development of muscle insulin resistance