Obese, insulin-resistant patients have been shown to have metabolic inflexibility. The goal of this study was to examine the effect of insulin administration on energy metabolism in lean, type 1 diabetic (DM1) patients. Eleven DM1 patients without vascular complications and 11 healthy controls (C) were examined. We performed a 2-step hyperinsulinemic euglycemic clamp (240 minutes; period 1: 1 mU. kg(-1). min(-1) and period 2: 10 mU. kg(-1). min(-1)) combined with indirect calorimetry during basal period B (B, -45 to 0 minutes), period 1, and period 2 of the clamp. The metabolic clearance rates of glucose (MCR) were lower in DM1 compared with C in period 1 (12.54 +/- 3.38 v 17.41 +/- 6.18 mL. kg(-1). min(-1); P <.02), as well as in period 2 (21.63 +/- 6.47 v 26.61 +/- 4.45 mL. kg(-1). min(-1); P <.05). Basal respiratory quotient (RQ) was lower in DM1 compared with C (0.72 +/- 0.04 v 0.75 +/- 0.04; P <.03). Insulin administration was accompanied by an increase in RQ in both groups, which was lower in DM1 compared with C (period 1: +0.09 +/- 0.04 v +0.11 +/- 0.07; P <.001; period 2: +0.13 +/- 0.04 v +0.16 +/- 0.04; P <.001). Glucose oxidation did not differ between the groups in period B; however, it was lower in DM1 compared with C in periods 1 (1.17 +/- 0.67 v 3.28 +/- 1.11 mg. kg(-1). min(-1); P <.003); and 2 (2.10 +/- 0.64 v 3.28 +/- 0.93 mg. kg(-1). min(-1); P <.009). Lipid oxidation was higher in DM1 in all periods compared with C; period B (3.28 +/- 0.77 v 1.16 +/- 0.55 mg. kg(-1). min(-1); P <.001), period 1 (1.10 +/- 0.41 v 0.67 +/- 0.54 mg. kg(-1). min(-1); P <.05), and period 2 (0.99 +/- 0.29 v 0.52 +/- 0.58 mg. kg(-1). min(-1); P <.01). The groups did not differ in protein oxidation. In conclusion, DM1 patients with secondary insulin resistance (IR) are characterized by metabolic inflexibility manifesting itself by smaller increases in RQ and glucose oxidation after insulin administration during the euglycemic clamp.