The main aim of the study was to investigate the effect of menstrual cycle phases on substrate oxidation during steady state intensity exercise in adult females with regular menstrual cycle and on oral contraceptive (OC). Twenty-four healthy endurance and strength trained females, with regular menstrual cycle phases (n= 15; Age 35.6±4.2; height 163.9±5.9 cm; body mass 58.1±5.2 kg; VO2peak 50.3±3.6 ml/min·kg) or on oral contraceptives (n=9; Age 30.4±4.5; height 163.9±9.0 cm; body mass 58.1±6.7 kg; VO2peak 52.4±4.2 ml·min-1·kg-1) participated in the study. All participants performed a graded maximal exercise test to determine their peak oxygen consumption (VO2peak). Participants then exercised at the speed corresponding to 75% of VO2peak for 40 minutes on a treadmill in each menstrual cycle phase: regular menstrual cycle group (early follicular phase, mid-follicular phase and luteal phase) and OC group (hormonal phase and non-hormonal phase). There were no differences in the respiratory exchange ratio of each phase, in regular menstrual cycle phase group (mean±SEM): early-follicular phase 0.89±0.01, mid-follicular phase 0.87±0.01 and luteal phase 0.88±0.01 (p>0.05). There were also no differences in respiratory exchange ratio for the participants using oral contraceptive: hormonal phase 0.89±0.01 and non-hormonal phase 0.91±0.01 (p>0.05). However, we found that OC may influence fat oxidation (p=0.018) during the hormonal phase. Our preliminary results suggest that menstrual cycle and oral contraceptive do not influence substrate oxidation in females with regular menstrual cycle phases. Regarding the few disparities, more research is needed to understand how sexual hormones influence substrate oxidation in female.
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