Due to the chronicity of the heat acclimation regimen and the need to complete eight work-rest cycles on a given day, all subjects were unable to complete the 80 treadmill walks (224 km). The main reasons for not completing all trials included foot blisters, inner thigh skin chafing, heat rash, and leg muscle pain. However, there was no significant difference in the proportion of the maximal possible walks completed between the two groups (control = 82.7 percent, low-salt = 75.4 percent). Further, as noted previously, if subjects could do only a portion of the total walks on a given day, they remained in the chamber and maintained the same rigorous nutritional, hydrational, and psychological testing requirements of the study as their walking counterparts. For these reasons the endocrinological data of subjects who did not walk the full complement of trials on all days were included in the mean values reported as well as the statistical analyses.
Figure 13-1 illustrates the effects of the low-salt diet and recurrent exercise in the heat on plasma levels of aldosterone. During the dietary stabilization period (days 1, 4, and 7) there were no significant differences (p > .05) noted between the control and low-salt groups on any of the days; in fact, the two groups displayed remarkably consistent between-group values in this sometimes labile variable. The slight elevation in levels of both groups on day 4 may have been in response to the dietary stabilization intake of NaCl (8 g), which probably represented a decrement in salt intake for most of these young adult men in comparison to their normal garrison consumption (approximately 11 to 15 g per day; Szeto et al., 1987). This increment nearly achieved statistical significance (for example, on day 1, low-salt, mean = 18.3 ng per dl; on day 4, low-salt, mean = 33.2 ng per dl, minimal critical difference of the means necessary for significance = 15.1 ng per dl, p > .05).
On the first day of both heat acclimation and dietary manipulation (eighth experimental day) plasma ALD levels at T1, T2, and T3 were not significantly different between groups (p > .05). However, the effects of exercise in the hot environment are noted in the elevated plasma ALD levels in both groups at T2, which achieved statistical significance by T3 in both the control and the low-salt groups (p < .05). By day 11, the marked effects of the low-salt diet on circulating ALD were manifested in significant (p < .01) increments at all sampling times when compared to the control levels. In the low-salt group, the increment observed between T1 and T2 narrowly failed to achieve significance (difference of means = 45.73, minimal difference for significance = 46.46 ng per dl).
A strikingly similar pattern emerged on day 15. Thus, at each of the sampling times, plasma ALD in the low-salt group was significantly elevated (p < .01) when compared to the respective mean of the control