prior dietary stabilization period in which all test subjects consumed the same daily moderate level of Na+ provided for a relatively homogeneous range of initial levels of ALD, PRA, and AVP, thus stabilizing the usual lability of these variables in young adult men. Finally, the opportunity to evaluate hormonal responses across a 10-day acclimation period permitted the addition of confirmational data to earlier findings on the effects of acclimation on the responses of these hormones to exercise in the heat (Francesconi et al., 1983, 1984).
Seventeen young adult males (mean age 19.8 ± 0.6 years) participated in this study after providing their written voluntary consent. Average height was 179.4 ± 1.6 cm, average weight was 78.5 ± 2.4 kg, and body surface area was 1.97 ± 0.03 m2. Subjects were briefed orally on the procedures, risks, and benefits of the study, as well as on their right to withdraw at any time for any reason without penalty. Prior to their participation, all subjects were medically screened and examined and found to be in good health; the mean maximal oxygen consumption for these subjects was 46.5 ± 1.1 ml per minute per kg.
The study was divided into two time intervals: (a) a 7-day dietary stabilization period under dormitory conditions and a temperature of 21°C and (b) a 10-day heat acclimation period, each day of which simulated an ''average'' 8-hour workday in a desert environment. 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 NaCl content of the diet was constant at 8 g per day for all subjects during this control interval. On day 8 of the study (day 1 of heat acclimation and work in the heat), subjects were randomly divided into two dietary groups, one of which continued to consume the moderate NaCl diet (8 g per day) and served as the control, while the second was placed on a low-salt diet (4 g per day) for the remainder of the test. Appropriate caloric consumption was sustained during the 10-day heat acclimation period for both groups as evidenced by minimal reductions in body weight over the experimental interval with no effects of diet on body weight.