Impact of aerobic fitness status, menstrual cycle phase, and oral contraceptive use on exercise metabolic flexibility

Abstract

Metabolic flexibility is the ability to appropriately store or utilize substrates. Exercise such as high‐intensity interval exercise (HIIE) is also a robust test of one’s ability to shift between substrates to support metabolic demand. Menstrual cycle or oral contraceptive use may impact metabolic responses to exercise. Fitness status also plays an important role in metabolism. We assessed the impact of hormonal fluctuation and fitness status on exercise metabolism in women during HIIE. 25, healthy, active women (24.4±3.6yrs; 165.5±6.9 cm; 65.1±10.5 kg; 38.8±6.6mL/kg/min) experiencing a regular menstrual cycle (n=15) or using oral contraceptives (n=11) completed the study. Participants were split by aerobic fitness status (High: 43.7±4.7mL/kg/min; Low: 33.6±3.9mL/kg/min). Participants completed a baseline and counterbalanced HIIE sessions in the follicular (FOL) and luteal phase (LUT). FOL sessions took place 2‐7d after onset of menstruation or non‐active OC days. LUT sessions took place ~6‐8d following urine ovulation or third week of active OC pills. Exercise was performed on an electrically braked cycle ergometer. High intervals (H1‐H4) were 4x4 min intervals at the power output corresponding to 50% of the difference between the ventilatory threshold and VO2peak, separated with 3min of unloaded cycling (0W; low intervals). RPE and blood lactate assessed via finger stick were taken after each interval. There was no effect of menstrual cycle phase or oral contraceptive use on fat oxidation, carbohydrate oxidation or blood lactate. There was an effect of fitness status on fat oxidation in LUT and FOL (p=0.002). High fit women oxidized more fat during the full session (FOL: p=0.050, LUT: p=0.001), high intervals (FOL: p=0.048, LUT: p=0.001), and low intervals (FOL: p=0.032, LUT: p=0.024) during FOL and LUT, and recovery during LUT (p=0.033). Carbohydrate oxidation (g/kg; p=0.049) and AUC (p=0.024) were greater in high women during FOL, but not LUT. Blood lactate was significantly greater in low fit women during exercise interval H2 through recovery during FOL, but not LUT (p≤0.050). Total change in rate of fat oxidation (Δ g/kg/min) during HIIE intervals was greater in high vs low fitness during FOL (H2: p= 0.018, H3: p=0.004), and LUT (H1: p=0.041, H2: p=0.001, H3: p=0.001, H4: p=0.006). There was a correlation between exercise fat oxidation (g/kg) and lactate AUC (H1‐L4) during FOL (r=‐0.440, p=0.030; n=25) but not LUT (‐30 minute) (r=0.215, p=0.143; n=25). Our results reflect fitness status plays an important role in exercise fat oxidation, carbohydrate oxidation and blood lactate during HIIE in young women. High fitness women benefit from greater exercise metabolic flexibility than their low fit counterparts. These these differences may be unique to the menstrual cycle phase, with low fitness women having poorer exercise metabolic flexibility in the follicular phase.