Glucose kinetics and substrate oxidation during exercise in the follicular and luteal phases

The purpose of this investigation was to determine whether plasma glucose kinetics and substrate oxidation during exercise are dependent on the phase of the menstrual cycle. Once during the follicular (F) and luteal (L) phases, moderately trained subjects [peak O₂ uptake (V̇o₂) = 48.2 ± 1.1 ml·min^-1·kg^-1; n = 6] cycled for 25 min at ∼70% of the V̇o₂ at their respective lactate threshold (70%LT), followed immediately by 25 min at 90%LT. Rates of plasma glucose appearance (R_a) and disappearance (R_d) were determined with a primed constant infusion of [6,6-²H]glucose, and total carbohydrate (CHO) and fat oxidation were determined with indirect calorimetry. At rest and during exercise at 70%LT, there were no differences in glucose R_a or R_d between phases. CHO and fat oxidation were not different between phases at 70%LT. At 90%LT, glucose R_a (28.8 ± 4.8 vs. 33.7 ± 4.5 μmol·min^-1·kg^-1; P < 0.05) and R_d (28.4 ± 4.8 vs. 34.0 ± 4.1 μmol·min^-1·kg^-1; P < 0.05) were lower during the L phase. In addition, at 90%LT, CHO oxidation was lower during the L compared with the F phase (82.0 ± 12.3 vs. 93.8 ± 9.7 μmol·min^-1·kg^-1; P < 0.05). Conversely, total fat oxidation was greater during the L phase at 90%LT (7.46 ± 1.01 vs. 6.05 ± 0.89 μmol·min^-1·kg^-1; P < 0.05). Plasma lactate concentration was also lower during the L phase at 90%LT concentrations (2.48 ± 0.41 vs. 3.08 ± 0.39 mmol/1; P < 0.05). The lower CHO utilization during the L phase was associated with an elevated resting estradiol (P < 0.05). These results indicate that plasma glucose kinetics and CHO oxidation during moderate-intensity exercise are lower during the L compared with the F phase in women. These differences may have been due to differences in circulating estradiol.