The first known randomised controlled trial of the effects of oral contraception on athletic performance in highly-trained women suggests that the Pill may reduce aerobic capacity to the possible detriment of performance.

The Canadian trial involved 14 athletes with regular menstrual cycles and VO2max values of more than 50ml/kg/min. Four measures of athletic performance were tested during both the follicular (days 3-8) and mid-luteal (days 4-9 after ovulation) phases of an ovulatory menstrual cycle: VO2max, anaerobic capacity, aerobic endurance and isokinetic strength.

Participants were subsequently randomly (and blindly) assigned to either a tricyclic oral contraceptive or placebo for a period of two months and retested in identical fashion between days 14 and 17 of the second month.

The key results were as follows:

• Aerobic capacity decreased by a mean 4.7% in the oral contraceptive group across the three phases of testing (with a considerable degree of individual variation), compared with a 1.5% improvement with placebo;
• There were no significant changes in other physiological variables or performance measures with oral contraceptive treatment.

‘The small decreases in VO2max that occurred in women taking oral contraceptives suggest that exogenous oestrogen may exert a deleterious effect on aerobic capacity, with potential implications for élite performance,’ conclude the researchers.

‘Further studies are necessary to delineate potential mechanisms of the change in functional aerobic capacity and body weight, and to further document whether changes in performance are reversible on discontinuation of treatment.’

Br J Sports Med 2003;37:315-320