Metabolic Flexibility in Response to Glucose Is Not Impaired in People With Type 2 Diabetes After Controlling for Glucose Disposal Rate

Abstract

OBJECTIVE—Compared with nondiabetic subjects, type 2 diabetic subjects are metabolically inflexible with impaired fasting fat oxidation and impaired carbohydrate oxidation during a hyperinsulinemic clamp. We hypothesized that impaired insulin-stimulated glucose oxidation is a consequence of the lower cellular glucose uptake rate in type 2 diabetes. Therefore, we compared metabolic flexibility to glucose adjusted for glucose disposal rate in nondiabetic versus type 2 diabetic subjects and in the latter group after 1 year of lifestyle intervention (the Look AHEAD [Action For Health in Diabetes] trial). RESEARCH DESIGN AND METHODS—Macronutrient oxidation rates under fasting and hyperinsulinemic conditions (clamp at 80 mU/m² per min), body composition (dual-energy X-ray absorptiometry), and relevant hormonal/metabolic blood variables were assessed in 59 type 2 diabetic and 42 nondiabetic individuals matched for obesity, sex, and race. Measures were repeated in diabetic participants after weight loss. RESULTS—Metabolic flexibility to glucose (change in respiratory quotient [RQ]) was mainly related to insulin-stimulated glucose disposal rate (R² = 0.46, P < 0.0001) with an additional 3% of variance accounted for by plasma free fatty acid concentration at the end of the clamp (P = 0.03). The impaired metabolic flexibility to glucose observed in type 2 diabetic versus nondiabetic subjects (ΔRQ 0.06 ± 0.01 vs. 0.10 ± 0.01, respectively, P < 0.0001) was no longer observed after adjusting for glucose disposal rate (P = 0.19). Additionally, the increase in metabolic flexibility to glucose after weight loss was accounted for by the concomitant increase in insulin-stimulated glucose disposal rate. CONCLUSIONS—This study suggests that metabolic inflexibility to glucose in type 2 diabetic subjects is mostly related to defective glucose transport.