Acclimatization to dry heat: active men vs. active women.

Physiological responses to exercise in dry heat were compared between six active men [maximum O2 consumption (VO2max), 51.4 +/- 1.2 ml . kg-1 . min-1] and four active women (VO2max, 47.2 +/- 1.3 ml . kg-1 . min-1) before, during, and after heat acclimatization. Subjects cycled a maximum of 2 h at 40% VO2max at 45 degrees C dry-bulb temperature, 23 degrees C wet-bulb temperature for 11 days. Prior to acclimatization there were no sexual differences for performance time, rate of increase of rectal temperature (delta Tre), or sweat rate per degree C increase of rectal temperature (msw/delta Tre). Sweat rate (msw) was greater for the men than for the women. Although there was no difference in the rate of increase of heart rate (delta HR), HR for the women was maintained 15-20 beats . min-1 higher than for the men. Acclimatization occurred for both sexes as indicated by reduced Tre and HR and increased msw and performance time. With acclimatization the women had longer performance times than the men. Even though the men still had greater msw, delta Tre was also greater; therefore msw/delta Tre for the men was less than for the women. Neither HR nor delta HR was different between the sexes. Throughout, resting hematocrit for the women was less than for the men; no changes in hematocrit were observed during exercise or with acclimatization. Plasma protein concentration increased during exercise on all days; no changes in plasma osmolality were observed. It is concluded that active women perform exercise of equal relative intensity in dry heat as well as active men. Moreover active women acclimatized to heat at a faster rate or to a greater extent than did active men.