Context The impact of progestins on exercise performance in women has not been previously studied.

Objective To examine the effect of a synthetic progestin on aspects of exercise status in young women.

Design, Patients, Setting 23 young, healthy, habitually sedentary women participated in a single-blind, randomized, counter-balanced, cross-over study in a university-based laboratory setting.

Intervention Two monophasic OCP's were administered where the dose of the synthetic progestin, norethisterone, was 2-fold different but the dose of the synthetic estrogen, ethinyl estradiol, was constant. During each month of OCP aspects of exercise status were assessed during incremental exercise to exhaustion, steady-state sub-maximal exercise and with a performance test.

Main outcome measures Peak oxygen uptake ^·VO_2peak), respiratory exchange ratio (RER), time to exhaustion, lactate concentrations and total work done.

Results Peak heart rates were 95% of age-predicted values with both OCP preparations whereas ^·VOO_2peak was 30% above age-predicted values. Peak post-incremental exercise plasma lactate concentrations exceeded those reported for males and females whereas the RER was below expected values throughout both incremental and steady-state exercise. The effects on ^·VOO_2peak and RER were increased with the higher dose progestin OCP, as were exercise time to exhaustion and total work done.

Conclusion Synthetic progestins in OCP formulations can have a significant affect on the exercise status of young, sedentary women possibly through an effect on stroke volume, and a shift in the principal energy substrate used during exercise from carbohydrate to lipid.