Thermoregulation during exercise in women who are taking oral contraceptives

Abstract

 Thermoregulation during exercise was studied in seven women who were taking oral contraceptive pills for 3 weeks of each month. The subjects were studied once in the 3rd week of taking the pill (P) and once during the following week when they took no pill (N). Rectal temperature (T _re), heart rate (f _c) and evaporative water loss (EWL, ventilated capsule technique) were measured while they walked on a treadmill for 60 min at 4.8 km · h⁻¹ at a 10% gradient. Ambient temperature was 22 °C. A venous blood sample was drawn 30 min before each experiment for measurement of hematocrit (packed cell volume, PCV), plasma osmolality (Osm _pl), and plasma levels of the endogenous pyrogens interleukin-1β (IL-1β) and interleukin-6 (IL-6). Resting T _re was 0.31 °C higher in P than in N (P < 0.01) and T _re remained higher in P throughout the entire exercise period (P < 0.01). Threshold T _re for the onset of EWL was 0.32 °C higher in P than in N (P < 0.01). Exercise f _c was 6.5 beats · min⁻¹ higher in P than in N (P < 0.01). There was no significant difference in PCV, Osm _pl, IL-1β or IL-6 between P and N. It was concluded that the administration of synthetic progestins in oral contraceptives causes an upward shift in the threshold for heat loss responses, resulting in higher body core temperatures both at rest and during exercise. There was no evidence that these alterations in thermoregulation were mediated by changes in body fluid balance or in plasma levels of IL-1β or IL-6.