Objective - Sirtuin 1 (SIRT1) is implicated in the regulation of mitochondrial function, energy metabolism, and insulin sensitivity in rodents. No studies are available in humans to demonstrate that SIRT1 expression in insulin sensitive tissues is associated with energy expenditure and insulin sensitivity. Research Design And Methods - Energy expenditure (EE), insulin sensitivity, and SIRT1 mRNA adipose tissue expression (N=81) were measured by indirect calorimetry, euglycemic hyperinsulinemic clamp, and quantitative RT-PCR in 247 non-diabetic offspring of type 2 diabetic patients. Results - High EE during the clamp (r=0.375, P = 2.8x10(-9)) and high DeltaEE (EE during the clamp - EE in the fasting state) (r=0.602, P = 2.5x10(-24)) were associated with high insulin sensitivity. Adipose tissue SIRT1 mRNA expression was significantly associated with EE (r = 0.289, P = 0.010) and with insulin sensitivity (r = 0.334, P = 0.002) during hyperinsulinemic euglycemic clamp. Furthermore, SIRT1 mRNA expression correlated significantly with the expression of several genes regulating mitochondrial function and energy metabolism (e.g. PGC-1beta, estrogen-related receptor alpha, nuclear respiratory factor -1, mitochondrial transcription factor A), and with several genes of the respiratory chain (e.g. including NADH dehydrogenase (ubiquinone) 1alpha subcomplex, 2, cytochrome c, cytochrome c oxidase subunit IV, and ATP synthase). Conclusions - Impaired stimulation of EE by insulin and low SIRT1 expression in insulin sensitive tissues are likely to reflect impaired regulation of mitochondrial function associated with insulin resistance in humans.