The Menstrual Cycle or Oral Contraceptive Pill

Abstract

1911 Substrate utilisation during exercise differs between the oral contraceptive pill (OCP) and normal menstrual cycle (NMC) such that OCP users metabolise fewer carbohydrates and oxidise a greater proportion of fat than women in the luteal phase of the NMC 1. There is, however, an increasing consensus that aerobic capacity is impaired with OCP use 2. PURPOSE: Given that exogenous female steroid hormones seem to reduce aerobic capacity but have the potential to alter energy substrate utilisation compared to endogenous female steroid hormones this study aimed to compare the effects of these hormones during the NMC and OCP, on exercise status in young (18–30 yrs), sedentary women. METHODS: Twenty-three women taking a monophasic OCP and fourteen women with normal menstrual cycles (NMC) performed incremental cycle ergometer exercise to exhaustion and 20 min steadystate exercise at 75% VO2peak. RESULTS: Maximal power output and endurance time to exhaustion were higher in the OCP group. Oxygen uptake (VO2, L·min−1) was significantly higher in the OCP group from 40% − 100% VO2peak (P < 0.05). The OCP group had a higher oxygen pulse (ml·beat−1) and a lower respiratory exchange ratio at all levels of exercise intensity (P < 0.01) in the incremental test. During both exercise tests the OCP group were deriving more energy from lipid (P < 0.01) than the NMC group and there was no difference between the groups in terms of heart rate and rate of perceived exertion. CONCLUSIONS: Exercise status defined by aerobic capacity and endurance performance is increased in young, sedentary women taking an OCP and the results suggest this could be due to a both an improvement in stroke volume and a greater dependence on fat metabolism associated with exogenous steroid hormone administration.