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FLUID REPLACEMENT AND HEAT STRESS
they are consumed, yet salt supplementation has been recommended (Hubbard et al., 1986; Leighthead and Lind, 1964; Hubbard and Armstrong, 1988) during the initial 3-5 days of heat exposure because the kidneys require 3-5 days (and the sweat glands require 5-10 days) of heat exposure to maximally conserve Na+ (Hubbard et al., 1986). Symptoms and casualties of heat syncope and heat exhaustion are greatest during this period and decrease notably after day 5 (Hubbard and Armstrong, 1988). This can best be explained by the fact that the primary adaptations during the initial 3-5 days of heat acclimatization are cardiovascular, and emphasizes the need for adequate salt intake to maintain extracellular fluid and plasma volumes.
Dasler et al. (1973) have published the only study that has examined extremely high salt consumption during heat acclimatization. They found that subjects exhibited impaired heat acclimatization responses when they ate high levels of salt (22.5-30 g of NaCl per day). These responses included cardiovascular impairment; decreased optimal work capacity; and increased excretion of K+, bicarbonate, and other anions. This impaired heat acclimatization response may have been due to inadequate water intake by their subjects, because the water requirement increases approximately 1 liter for each 5 g of NaCl added to the diet (Baker et al., 1963). In addition, Knochel and Vertel (1967) have implicated salt loading as a possible factor in the production of potassium deficiency, rhabdomyolysis, and heat injury. These two reports impact on the evaluation of carbohydrate-electrolyte replacement beverages, because a soldier could theoretically exceed the NaCl intake in the study of Dasler et al. (1973) if he ate three MRE (13 g of NaCl per day), ate three MRE salt packets (12 g of NaCl per day), and drank 10 liters of Gatorade (12 g of NaCl per day, full-strength) in a 24-h period. In Alitz's, and Donovan's units, these theoretical NaCl totals would be 31 g and 27 g, respectively.
CASE REPORT A: TEN HEATSTROKE PATIENTS
During the past 2 years, we evaluated the time course and extent of recovery in prior heatstroke patients. Ten active-duty male soldiers came to our laboratory for 14-day investigations of their thermoregulatory and heat-acclimatization abilities, as well as for evaluations of blood chemistry values and fluid-electrolyte balance. This work has been published elsewhere (Armstrong et al., 1989), showing that it is useful to consider the events of the day on which these men experienced heatstroke. Nearly all of these soldiers experienced heatstroke (verified by rectal temperature of >106°F, elevated serum enzymes, and altered mental status) at Fort Benning, Georgia, which presents a hot, humid environment at mid-day. In most