Gender differences in glucose kinetics and substrate oxidation during exercise near the lactate threshold

The purpose of this investigation was to determine plasma glucose kinetics and substrate oxidation in men and women during exercise relative to the lactate threshold (LT). Subjects cycled for 25 min at 70 and 90% of O₂ uptake (Vo₂) at LT (70 and 90% LT, respectively). Plasma glucose appearance (R_a) and disappearance (R_d) were determined with a primed constant infusion of [6,6-²H] glucose. There were no significant differences in glucose R_a between men [22.6 ± 1.9 and 39.9 ± 3.9 μmol·kg fat-free mass (FFM)^-1·min^-1 for 70 and 90% LT, respectively] and women (22.3 ± 2.7 and 33.9 ± 5.7 μmol·kg FFM^-1·min^-1 for 70 and 90% LT, respectively). Similarly, there were no significant differences in glucose R_d between men (21.2 ± 1.9 and 38.1 ± 3.7 μmol·kg FFM^-1·min^-1 for 70 and 90% LT, respectively) and women (21.3 ± 2.8 and 33.3 ± 5.6 μmol·kg FFM^-1·min^-1 for 70 and 90% LT, respectively). Although there were no differences between genders in the relative contribution of carbohydrate (CHO) to total energy expenditure, the relative contribution of muscle glycogen to total CHO oxidation (75.8 ± 3.2 and 64.2 ± 8.0% for men and women, respectively, at 70% LT and 75.1 ± 2.6 and 60.1 ± 11.2% for men and women, respectively, at 90% LT) was lower in women. Consequently, the relative contribution of blood glucose to total CHO oxidation was significantly higher in women. These results indicate that although plasma glucose R_a and R_d are similar in men and women, the relative contribution of muscle glycogen and blood glucose is significantly different in women during moderate-intensity exercise relative to LT.