Discharged members of the US Armed Services are at an increased risk of metabolic disease which is exemplified by the prevalence of type 2 diabetes mellitus (T2DM) affecting an estimated 1/3 of all VHA patients. A key defect in the etiology of T2DM is the inability of insulin to suppress hepatic glucose production, or hepatic insulin resistance. Alterations in hepatic lipid metabolism precede hepatic insulin resistance and are regulated largely by mitochondrial fatty acid oxidation (β-oxidation) and, in particular, the ability to maintain effective metabolic flexibility under different dietary states. Previous work has implicated mechanistic target of rapamycin (mTOR) as a mediator of this process through the regulation of β-oxidation. However, our preliminary work found an interesting dichotomy; inhibition of mTOR promotes β-oxidation of fatty acids when there is a prevalence of saturated fatty acids substrates available but in contrast impairs β-oxidation when unsaturated fatty acids are the primary dietary lipid sources. That is, unsaturated fatty acid catabolism by β- oxidation is not complete in the context of low mTOR singaling. β-oxidation of unsaturated fatty acids requires accessory enzymes to desaturate for use as mitochondrial substrates. Because the development of insulin resistance is linked to dysregulation in metabolic flexibility, we propose that mTOR-mediated regulation of this process is a key to maintaining hepatic insulin sensitivity and preventing metabolic disease. The long-term goal of this proposal is define a relationship that could be central to the development of hepatic insulin resistance. This metabolic dysfunction is highly prevalent among Veterans and is a significant long-term healthcare issue due to increased risk of developing additional pathologies associated with this condition, including non-alcoholic fatty liver disease and hepatocellular carcinoma. Treatment and prevention options will significantly reduce the health burden of Veteran patients as well as Veterans Health Administration costs associated with treatment. Our overall hypothesis is that mTOR regulates the response to dietary fatty acids through its regulation of β-oxidation accessory enzymes and that dysfunction in this pathway leads to hepatic insulin resistance. Our rationale for this study is that understanding how this pathway regulates nutrient usage under metabolic stress will serve as a means to define new therapeutic targets to be utilized for treatment and prevention of metabolic disease in Veterans. We test this hypothesis using both pharmaceutical and genetic manipulation of mTOR signaling and the rate limiting β-oxidation accessory enzyme 2,4 Dieonyl-CoA reductase (DECR1) in primary hepatocytes and mouse models in experimental aims that link this pathway with mitochondrial energetic function and metabolism. In aim 1, we test whether mTOR signaling has direct impact on the activity of DECR1 with a functional outcome on fat oxidation. ...