During exercise, obese individuals oxidize less glycogen and more fat than their lean counterparts, but the shift in substrate use may be mediated by insulin resistance rather than body fat per se. In addition, individuals with Type 2 diabetes are not resistant to contraction-mediated glucose uptake during exercise, but in vivo studies uncomplicated by hyperglycemia are lacking. The purpose of this study was to compare blood glucose uptake and the balance between carbohydrate and fat utilization during exercise in insulin-resistant (IR) and insulin-sensitive (IS) women of equivalent body fatness and maximal oxygen consumption (VO2 max). Twelve overweight sedentary women were divided into two groups with similar body mass index (IR = 28.5 +/- 1.6, IS = 27.5 +/- 1.9), lean mass (IR = 42.4 +/- 1.8 kg, IS = 41.5 +/- 1.9 kg), and VO2 max (IR = 29.7 +/- 3.5 ml.kg(-1).min(-1), IS = 30.7 +/- 3.9 ml.kg(-1).min(-1)) but a markedly different composite insulin sensitivity index (IR = 3.0 +/- 0.7, IS = 7.7 +/- 0.9). Blood glucose kinetics and substrate oxidation were assessed by stable isotope dilution and indirect calorimetry during 50 min of treadmill walking at 45% VO2 max. Total carbohydrate oxidation and estimated muscle glycogen use were significantly lower in the IR group. Blood glucose uptake was the same in the IR and IS groups. These data suggest that insulin resistance, independent of body fat, spares muscle glycogen and shifts substrate oxidation toward less carbohydrate use during exercise. Insulin-resistant individuals with normoglycemia appear to have no defect in blood glucose uptake during exercise.