Question: What is the role of the relative rates of fatty acid delivery from lypolysis and of mitochondrial oxidative phosphorylation in explaining variations in intramyocellular lipid content and insulin resistance?
Population: Insulin-resistant subjects (three men and 11 women) and insulin-sensitive controls (five men and seven women) who were young [26 (±7) and 28 (±7) years, respectively], lean [body mass index (BMI) of 23 (±2) and 21 (±2) kg/m2, respectively], sedentary and in general good health were studied. Insulin resistance was defined as an insulin sensitivity index1 <4.0, and a family history of type 2 diabetes (DM); insulin-sensitive controls had to have an insulin sensitivity index >6.3.
Design and methods: Between 2001 and 2003, 150 individuals were screened through questionnaires and an oral glucose tolerance test (OGTT) to select subjects at extremes of insulin resistance and insulin sensitivity. 1H magnetic resonance spectroscopy (MRS) was performed to assess intracellular triglyceride content within liver and muscle cells. Subjects then either underwent (i) 31P MRS, to assess mitochondrial phosphorylation rates (a proxy of muscle ATP synthesis rates) and the ratio of inorganic phosphate-to-creatine phosphate (which reflects the ratio of oxidative to glycolytic muscle fibers); or (ii) a hyperinsulinemic-euglycemic clamp, to assess the glucose infusion required to respond to a constant insulin influx (a measure of the body’s responsiveness to insulin). Two of the insulin-sensitive controls were excluded after they were found to be insulin resistant by the clamp study. Whole body energy expenditure, and fat and glucose oxidation were measured through indirect calorimetry. In vivo lipolysis was assessed through microdialysis probes. Subjects were instructed to maintain regular diets with adequate carbohydrate intake (> 150 g per day) and refrain from exercise, other than routine walking, for the 3 days before the clinic visit. Women were examined during the follicular phase to avoid the potential impact of ovarian hormones on glucose metabolism.
Results: (i) Oral glucose tolerance test: levels of glucose at baseline and during the OGTT were within the normal range for both insulin-resistant and insulin-sensitive subjects, but were consistently higher in the former group. Both groups had similar levels of plasma fatty acids and glucagon. (ii) Hyperinsulinemic-euglycemic clamp: a60% lower glucose infusion rate was required in insulin-resistant subjects to maintain a euglycemic state relative to insulin-sensitive subjects, largely due to a 70% reduction in non-oxidative glucose clearance in the former group. No difference was found between groups in the rate of glucose or fat oxidation; however, the basal metabolic rate was lower in insulin-resistant subjects relative to insulin-sensitive subjects. (iii) Intracellular triglyceride content: an80% increase in intramyocellular but not intrahepatic triglyceride content was found in insulin-resistant subjects relative to insulin-sensitive subjects. (iv) Mitochondrial phosphorylation rates were decreased by 30% and the ratio of inorganic phosphate-to-creatine phosphate was decreased by 20% among insulin-resistant subjects relative to insulin-sensitive subjects. (v) Similar levels of glycosylated hemoglobin, adiponectin, tumor necrosis factor α, interleukin-6, and resistin were found in both groups.
Conclusion: Insulin-resistant subjects have a marked elevation in intramyocellular lipid levels perhaps due to an inherited defect in mitochondrial oxidative phosphorylation rates.