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they were compared to six younger men (aged 21 to 29) who were matched for height, weight, and body surface area, but not percent body fat, demonstrated that the older men were less able to respond to a single 3-hour period of thermal dehydration than the younger men (Miescher and Fortney, 1989). Rectal temperatures in the older men increased more rapidly while sweat rates were not significantly different. In addition, plasma volume decreased and plasma osmolality increased to a larger extent in the older men. Within 30 minutes of rehydration, plasma volume and osmolality had returned to normal in the young men, while the older men took 60 minutes to restore plasma osmolality, and 90 minutes to restore plasma volume. Since these older subjects were not considered ''extremely'' fit, did not have similar ratios of body fat compared to the younger subjects, and since the protocol did not call for work or exercise during the 3 hour period, it is possible that the differences noted were due to these factors and not age per se.

Military researchers have measured thermoregulatory responses and acclimation in two groups of nine men who were matched for body weight, surface area, percent body fat, and maximal aerobic power, but with average ages of 21 and 46 (Pandolf et al, 1988). Initially, the older group demonstrated increased performance time with decreased rectal and skin temperatures and increased body sweat loss. After acclimation (measured after 10 days), no differences were seen between the two groups in thermoregulatory responses, including sweating rate, or performance time. The authors also noted that those in the older group who engaged in regular weekly aerobic activity were better able to initially respond to the thermal stress, although such differences were not evident after heat acclimation.

An additional study, compared a group of eight sedentary men, average age 34, with six "moderately active" older men, average age 57 (Smolander et al., 1990). The men in both groups walked on a treadmill at 30 percent for up to 3.5 hours in thermoneutral, warm-humid, and hot-dry environments. There was little difference in the ability of the older men to tolerate the protocol when compared with the younger men. The authors concluded that the ability to exercise in hot environments may not necessarily be associated with calendar age but more importantly with factors such as physical activity habits and aerobic capacity.

Based on these studies, it appears that for the age group of the active military, it is important to take into consideration the level of fitness of military troops regardless of age, particularly when going into a hot environment in which significant work is initially expected.

Effect on Electrolyte Balance

Although electrolytes are lost with sweat, these losses, except in some extreme cases, are usually not large enough to affect performance capacity

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