THERMOREGULATION DURING EXERCISE IN WOMEN WHO ARE TAKING ORAL-CONTRACEPTIVES

Thermoregulation during exercise was studied in seven women who were t aking 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 dur ing 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 degrees C. A venous blood sample was drawn 30 min before each experim ent for measurement of hematocrit (packed cell volume, PCV), plasma os molality (Osm(pl)), and plasma levels of the endogenous pyrogens inter leukin-1 beta (IL-1 beta) and interleukin-6 (IL-6). Resting T-re was 0 .31 degrees C higher in P than in N (P < 0.01) and T-re remained highe r in P throughout the entire exercise period (P < 0.01). Threshold T-r e for the onset of EWL was 0.32 degrees C higher in P than in degrees 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 differe nce in PCV, Osm(p1), IL-1 beta or IL-6 between P and N. It was conclud ed that the administration of synthetic progestins in oral contracepti ves causes an upward shift in the threshold for heat loss responses, r esulting in higher body core temperatures both at rest and during exer cise. There was no evidence that these alterations in thermoregulation were mediated by changes in body fluid balance or in plasma levels of IL-1 beta or IL-6.