exercise are very minor when expressed per unit of dry weight (Costill et al., 1976, 1981).
The early experiments of Pitts et al. (1944) on men marching in the desert showed that consumption of water, 0.2% saline, or 3.5% glucose to replace sweat loss resulted in lower heart rates and core temperatures and sometimes greater sweat rates than were noted in a no-fluid condition. Francis (1979) studied eight men under three different hydration treatments during, intermittent exercise consisting of eight 15-min bouts of cycling at 50% interspersed with 5-min recovery intervals. The room air temperature was 32°C, and the relative humidity was 60% to 65%. During the rest intervals, the subjects consumed either water or an electrolyte solution (20 mM sodium, 10 mM potassium, 1.3% glucose) sufficient to replace sweat losses, or no fluid at all. In the no-fluid condition, plasma volume loss after 2 h was 17.7%; when either fluid replacement beverage was consumed, no significant loss occurred. Similarly, compared with the no-fluid condition, heart rate was 15 to 20 beat/min lower, rectal temperature was 1°C lower, and plasma cortisol concentrations were significantly reduced when either fluid was consumed during exercise.
In one experiment, four marathon runners ran on a treadmill for 2 h at 70% under each of three conditions: (1) no fluid, (2) 100 ml of water every 5 min during the first 100 min, or (3) 100 ml of a glucose-electrolyte beverage (20 mM sodium, 15.3 mM chloride, 2.4 mM potassium, 4.4% glucose) every 5 min for the first 100 min (Costill et al., 1970). Compared with the no-fluid treatment, both fluid replacement regimens significantly lowered rectal temperature and reduced the concentrations of sodium and chloride in plasma.
Brandenberger et al. (1986) and Candas et al. (1986) compared the effects of five treatments: (1) no fluid, (2) water, (3) a hypotonic beverage (0.4 mM chloride, 0.04% glucose and fructose), (4) an isotonic drink (23.1 mM sodium, 16.7 mM chloride, 3.2 mM potassium, 2.0 mM calcium, 6.8% sucrose), and (5) a hypertonic sugar solution (7.55% glucose, 7.53% fructose) on temperature regulation and cardiovascular function during 4 h of intermittent cycling at a low intensity (mean = 85 watts) in a hot environment (34°C, 10°C dew point). Fluid was consumed every 10 min after 70 min of exercise in amounts calculated to replace 80% of sweat losses. Relative to the no-fluid condition, all four fluid replenishment regimes decreased rectal temperature, heart rate, plasma protein concentration,