ENDOGENOUS HORMONES SUBTLY ALTER WOMENS RESPONSE TO HEAT-STRESS

The thermoregulatory responses of menstruant women to exercise in dry heat (dry-bulb temperature/wet-bulb temperature = 48/25.degree. C) were evaluated at three times during the menstrual cycle: menstrual flow (MF), 3-5 days during midcycle including ovulation (OV), and in the middle of the luteal phase (LU). Serum concentraions of estradiol-17.beta. (E2), progesterone (Pg), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) were measured by radioimmunoassay, and these values were used to determine the dates of OV (peak LH and FSH) and LU (peak postovulatory Pg). After heat acclimation, subjects received heat stress tests (HST) consisting of a 2-h cycle-ergometer exercise at 30% of maximal O2 consumption in the heat. Rectal (Tre) and mean skin (.hivin.Tsk) temperatures, heart rate (HR), and sweat rate on the chest and thigh were recorded continuously. Total sweat loss (Msw), as indicated by weight loss, was recorded every 20 min, and equivalent water replacement was given. Steady-state excercise metabolic rate (M) was measured at 45 and 110 min. Seven of eight subjects had ovulatory cycles during experimental months. At rest, Tre was lowest at OV and significantly higher at LU. During steady-state exercise both Tre and .hivin.Tsk were lowest at OV and significantly higher at LU. There were no differences between phases in Msw, sweat rate on the chest and thigh or M. Despite higher Tre and .hivin.Tsk at LU, all subjects were able to complete the 2-h of exercise. We conlcude that 1) the menstrual cycle alters temperature regulation without obvious alterations in sweat output or steady-state metabolic rate, 2) these changes are so subtle that they easily could be missed if hormonal status is not carefully monitored concomitant with HST, and 3) the magnitude of the changes are small and do not impair a woman''s ability to work in dry heat.