heat exhaustion (Taylor et al., 1944) or heat syncope (Bean and Eichna, 1943). Other experts (Consolazio, 1966; Ladell et al., 1954; Strauss et al., 1958) used field observations to derive Na+ recommendations for soldiers of 13,000 to 48,000 mg NaCl (221 to 816 mEq Na+) per day.
In contrast to those findings, laboratory studies of human heat acclimation (HA) (Armstrong et al., 1985; Conn, 1963) and dietary Na+ intake (Dahl, 1958; National Research Council, 1989b) have suggested that humans function well when consuming relatively low Na+ diets ranging from 1930 to 6000 mg NaCl per day (33 to 103 mEq Na+ per day). Unfortunately, those human studies often did not involve prolonged exercise-heat exposure on many successive days or allow ample time for dietary Na+ stabilization prior to HA. Therefore, the purpose of the current investigation was to evaluate the effects of moderate Na+ diets ([8g NaCl] 137 mEq Na+; abbreviated MNA) and low Na+ diets ([4 g NaCl] 68 mEq Na+; abbreviated LNA) on thermoregulatory, cardiovascular, hematologic, and fluid-electrolyte variables during 10 consecutive days of prolonged intermittent exercise (8 hours per day) in a simulated desert environment. This experiment was relevant to military populations because the caloric and Na+ intakes typically decrease during the initial days of deployment in a hot environment, and because the maintenance of intravascular and intracellular fluid-electrolyte balance is essential to prolonged exercise in heat.
The subjects of this investigation were 17 males who were not acclimated to heat; who gave their informed, voluntary consent to participate in the current investigation; and who underwent a medical examination. Selected physical characteristics for both treatment groups appear in Table 12-1.
During this 17.5-day study, subjects were housed 24 hours each day in a research building that contained sleeping, dining, and environmentally controlled chamber facilities. A proctor was present at all times to ensure that no subject left and that no food entered the research building. During the initial 7-day dietary equilibration period (days 1 to 7), all 17 subjects consumed MNA and were housed in an ambient temperature of 21°C. During the subsequent 10-day HA period (days 8 to 17), nine subjects continued to consume MNA and eight subjects began to consume LNA. On days 8 to 17, breakfast and dinner were consumed in the dormitory kitchen (21°C) while lunch was eaten in the tropical chamber (41°C) during the fifth rest period. Three primary meals and two snacks provided subjects with 55 percent carbohydrate, 13 percent protein, 32 percent fat, and 3600 kcal per day, in both LNA and MNA. Subjects drank assorted beverages ad libitum. when not involved in HA trials.
Upon awakening each morning (days 1 to 18), the following measures