EFFECTS OF WITHHOLDING WATER BEFORE OR DURING PROLONGED EXERTION

Early Experiments

In 1944, Pitts et al. reported data from a series of experiments in which men walked on a treadmill for 1 to 4 h at 5.6 km/h up a 2.5% grade with or without fluid replenishment (Table 3-1). The environmental temperatures ranged from 32° to 38°C, the relative humidity was 35% to 83%, and the subjects were allowed to rest for 10 min each hour. When the subjects drank nothing during the walks, their rectal temperatures and pulse rates were usually higher and their sweat rates were lower than

Table 3-1 Mean Heart Rates, Rectal Temperatures, and Sweat Rates after 4 h walking at 38°C and 35% relative humidity (n = 3-7)

Fluid Type

Heart Rate (beat/min)

Rectal Temp. (°C)

Sweat Rate (liter/h)

None

154

38.9

0.76

Water ad libitum

143

38.4

0.74

Water each 15 min at sweat rate

132

38.3

0.08

0.2% NaCl each 15 min at sweat rate

131

38.3

0.81

3.5% Glucose each 15 min at sweat rate

126

38.1

0.71

Source: Pitts et al. (1944).

when water, 2% saline, or 3.5% glucose in volumes equivalent to sweat loss were consumed every 15 min or when water was consumed in quantities that just satisfied thirst. These early experiments demonstrated that progressive dehydration during prolonged exertion in the heat can adversely affect cardiovascular function, as reflected by elevated heart rates, and temperature regulation, as indicated by high rectal temperatures and reduced sweat rates. More recent investigators have confirmed these observations (Candas et al., 1988).

The experiments of Pitts et al. (1944) also suggested that thirst was not an adequate stimulus for the subjects to replace all of the water they lost as sweat. This was confirmed several years later in an experiment reported by Brown (1947), in which military recruits attempted to complete a 34-km hike in a desert environment at temperatures ranging from 30° to 33°C with or without free access to water. Without water, 7 of 13 subjects became



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