Oral Contraceptives Exert Influence on  Exercise Physiology

birth control or oral contraception

Oral contraceptives have been found to hinder aerobic performance, but a new study suggests low-dose monophasic oral contraception may not have an effect on endurance performance.

Athletes put in countless hours in practice to achieve minute, but consequential improvements in performance. Therefore, reports that oral contraception may compromise athletic performance have steered many elite female athletes away from using oral contraception. The studies are not conclusive; some studies demonstrated that oral contraception causes measurable declines in performance, while others found no significant difference.  

Three studies published between 2000 and 2003 indicated an oral contraceptive mediated decline in maximal aerobic capacity. However, a study published this month using low-dose monophasic oral contraceptives as the form of birth control found no differences in aerobic capacity (Maximal fat oxidation, but not aerobic capacity, is affected by oral contraceptive use in healthy women, 2014. Laurie Isaco, et al.). Low-dose monophasic oral contraceptives are popular today (e.g. microgestin) and provide a constant dose of estrogen and progesterone over the menstrual cycle.

The researchers looked at twenty-one recreationally active women who were either taking monophasic oral contraception or who were not taking any oral contraception. They found that women on oral contraceptives showed no measurable difference in aerobic capacity. Furthermore, there was no difference in cardiorespiratory parameters between the women on oral contraceptives and women not on oral contraceptives at maximal aerobic capacity. However, the maximum lipid oxidation rate was higher in women taking oral contraceptives. The women on oral contraceptives had a higher %VO2max (percentage of maximum aerobic capacity) at which the maximum lipid oxidation rate was reached (also referred to as Lipoxmax). The figure below demonstrates the effect of low-dose monophasic oral contraceptives on lipid oxidation during exercise.

Influence of oral contraceptive use (OC+ or OC−) on (A) intensity of exercise at which maximum lipid oxidation rate occurs (Lipoxmax, expressed as %VO2max) and (B) maximal lipid oxidation rate (MLOR) in healthy women. Data is displayed as mean ± SD. **, *** is p<0.01 and p<0.001, respectively. MLOR is milligrams of lipids oxidized per minute per kg of fat-free mass (FFM).

The increased lipid oxidation is difficult to interpret in regards to athletic performance. Endurance athletes generally have higher lipid oxidation rates at sub-maximal paces to conserve carbohydrates. Carbohydrates provide more oxygen efficient energy and their depletion is responsible for “the wall” marathoners hit late in the marathon race. As the figure below demonstrates, women on the low-dose monophasic contraception showed no significant difference in VO2Max. However, there is a decreased VO2Max trend in the women on oral contraceptives relative to the women not on oral contraceptives.

Influence of low dose monophasic oral contraceptives (OC+ and OC-) on VO2Max (maximum ml of oxygen consumed in a minute, a measure of aerobic capacity). Data displayed as mean ± SD. NS is no significant difference between groups.

The authors warn against extrapolating this data to trained athletes. It would also be interesting to use a different assay to measure aerobic capacity. For example, higher lipid oxidation may decrease VO2Max, which reflects a race of 7-10 minutes, but might it have the opposite effect at longer races where carbohydrate depletion is a constraint?

The question remains, does oral contraception hinder endurance performance? The best oral contraceptive option for women appears to be low-dose monophasic oral contraception. The jury is still out on whether the low-dose monophasic oral contraception hinders endurance, but any effect would be relatively minor.

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