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13. Endocrinological Responses to Dietary Salt Restriction During Heat Acclimation
Pages 259-276

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From page 259...
... or 6 g per day of NaCl (Cone, 1949~. Of course, the latter view is supported by the considerable populations that flourish in extremely hot global environments despite dietary NaCl intakes of from 2 to 6 g per day (Ladell, 1957; Orr and Gilks, 1931~.
From page 260...
... When troops are rapidly deployed from garrison to field conditions, their normal salt intake in many cases drops precipitously due to the altered salt content of the issued field rations and the generally reduced total ration consumption, especially during the first several days of the deployment. If the mobilization destination is a desert or jungle environment, the problem is compounded by enhanced salt losses in sweat, especially during the early stages of heat acclimation.
From page 261...
... During the complete 17-day interval, test volunteers were dedicated to the study and supervised 24 hours per day by test investigators who had total dietary control. During the 7-day dietary stabilization period, all volunteers consumed approximately 4000 kcal per day, which was adequate to sustain initial weights; the NaC1 content of the diet was constant at 8 g per day for all subjects during this control interval.
From page 262...
... After completing the daily chamber exposure, subjects returned to their dormitory setting until the regimen was repeated the next morning. Blood Sampling During the dietary stabilization period, on days 1, 4, and 7 a 6-ml sample of blood was removed by venipuncture from a superficial arm vein by a trained phlebotomist using aseptic techniques.
From page 263...
... . Plasma renin activity was estimated by the quantitation of angiotensin I in EDTA plasma.
From page 264...
... 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 < .051. By day 11, the marked effects of the low-salt diet on circulating ALD were manifested in significant (p < .01)
From page 265...
... on plasma levels of aldosterone. During the dietary stabilization period (days 1 to 7)
From page 266...
... In examining first the PRA of both groups during the dietary stabilization period, there were no significant differences in plasma levels between groups on day 1. In progressing from day 1 to day 4, a trend occurred toward elevations in PRA in both groups, but neither achieved statistical significance.
From page 267...
... Further, even after dietary manipulation and consecutive days of exercise in the heat, the data indicated that throughout the period of heat acclimation there were no significant effects on AVP of either the dietary manipulation or the recurrent exercise in the hot environment. DISCUSSION Because the endocrinological variables under consideration in the current experiments are significantly affected by dietary salt consumption (McDougall, 1987)
From page 268...
... These results suggest that the chronicity of both the heat acclimation regimen and the low-salt diet engender temporally related endocrinological accommodations to these experimental manipulations. Thus, the attenuated hormonal responses over time may be related to the following: increased plasma volume of heat acclimation (Bonner et al., 1976)
From page 269...
... Moreover, when the physiological strain of exercise in the heat has been reduced after full acclimation and expansion of plasma volume have been achieved, these hormonal responses are obviated. In the current experiments, subjects on the low-salt diet gradually achieved the same state of acclimation as the control group over the 10-day acclimation period as suggested by significant reductions in heat- and exerciseinduced heart rate, rectal temperature, urinary Na+, and perceived exertion.
From page 270...
... The endocrinological adaptations occurring especially during the first several days of the dietary-heat-work regimen may be important to the physiological adaptations and electrolyte equilibria needed for achieving acclimation. These hormonal responses moderated during the latter portions of the experimental regimen, which indicates a dietary and acclimation steady-state characterized by greatly reduced physiological strain to the test volunteers.
From page 271...
... Berlyne 1974 Plasma renin activity after acute heat exposure in nonacclimatized and naturally acclimatized man.
From page 272...
... Kuoppasalmi, and H Adlerkreutz 1976 Plasma renin activity, angiotensin II, and aldosterone during intense heat stress.
From page 273...
... Increased plasma renin activity was probably due to diminished splanchnic blood flow, as Rowell (Rowell, 1986) has shown; decreased plasma volume or reduced renal blood flow ordinarily elicits a very immediate response in terms of renal tubular biosynthesis and release of plasma renin.
From page 274...
... In fact, at one time we infused hyperoncotic albumin to test volunteers to increase plasma volume in a matter of hours versus the six or eight days that Dr. Armstrong showed were required here in this study.
From page 275...
... FRANCESCONI: No, I don't. The hypohydration study which I have referenced actually had 12 women in that group of 24 test subjects and I saw no differences in hormonal responses and I don't think Dr.


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