Oxidation of long chain fatty acids (FAO) is largely determined by carnitine palmitoyl transferase (CPT-1)-mediated entry of the fatty acids into mitochondria. An important regulator of CPT-1 is the allosteric inhibitor malonyl CoA produced by the enzyme acetyl-CoA carboxylase 2 (ACC2). We have previously shown that global deletion of ACC2 increased whole body and skeletal muscle FAO, as well as skeletal muscle glycogen content, without changing glucose tolerance, body weight or fat mass in young mice (4-5 months)¹. The aim of the current study was to determine if this phenotype persisted in older mice and to determine the effect of ACC2 gene deletion on insulin sensitivity using a euglycemic-hyperinsulinemic clamp. At 9-12 months of age, male wild type (WT) and ACC2^-/- mice (n=8-10 mice/group) were of similar body weight (33.5 ± 0.4g and 33.6 ± 0.8g respectively) and fat mass (16.2±1.1% and 19.4±2.0% respectively by DEXA). WT and ACC2^-/- displayed identical glucose tolerance. The older ACC2^-/- mice showed increases in whole body FAO (24hr average RER=0.95±0.02 and 0.92±0.02 for WT and ACC2^-/- respectively, p<0.05) and skeletal muscle glycogen content (WT 10.2±0.9μmol/g; ACC2^-/- 16.5±1.4μmol/g, p<0.05) without any detectable change in energy expenditure. In line with the glucose tolerance results, hyperinsulinemic/euglycemic clamp studies (n=7-8 per genotype) revealed no difference in insulin action between groups, with similar glucose infusion rates (WT 36.6±2.4; ACC2^-/- 38.6±3.5 mg/min/kg). There was also no difference in glucose uptake into skeletal muscle (WT 10.0±1.7; ACC2^-/- 9.0±0.9 μmol/min/100g) or epididymal fat (WT 5.6±1.2; ACC2^-/- 4.4±0.5 μmol/min/100g). These results demonstrate that: deletion of ACC2 can increase whole body FAO without any measurable change in energy expenditure. Over the long-term, increasing FAO by deletion of ACC2 is not sufficient to alter body composition or insulin sensitivity.