Responses of older and younger women to exercise in dry and humid heat without fluid replacement

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Abstract
Eight older (52 to 62 yr old) post-menopausal and 8 younger (20 to 30 yr old) women were matched with respect to body size, fatness, and maximal oxygen consumption, heat-acclimated, and then exercised at 35 to 40% maximal oxygen consumption in a warm-humid (37.degree. C, 60% relative humidity) environment. Results were compared with similar data from a hot-dry (48.degree. C, 15% relative humidity) environment. No fluid replacement was provided during either of these sessions. In each environment, the older women stored more heat as evidenced by a higher rectal temperature response, but there were no inter-group differences in skin temperature or percent maximal heart rate. Four of the older women were unable to complete either the hot-dry or the warm-humid exposure, although they completed the full 2 h during acclimation sessions when water was provided ad libitum. In the hot-dry environment, the younger women''s whole body and local sweat rates were significantly higher than those of the older women; in the warm-humid environment, there was no age-related difference in sweat rate. When local skin temperature and wettedness were artificially elevated, both groups exhibited the same pattern and rate of sweating. Percent decrease in plasma volume was greater for the older women in both conditions, but significantly so only in the warm-humid environment. It was concluded that: (i) differences in sweating rates between older and younger women are dependent upon ambient water vapor pressure; (ii) this alteration does not appear to be limited at the sweat gland level, since high local sweat rates can be artificially elicited; and (iii) the lower sweat rate of the older women may represent an inability to increase sweating due to dehydration associated with the lack of fluid replacement.