Effects of overhydration on man's physiological responses to work in the heat

The question was asked whether men could work in the heat with less physiological strain if they drank water in excess of expected fluid losses than if they merely replaced their losses as they worked. Thirty volunteer soldiers walked on 2 successive days for 90 min at 3.5 mph on a level treadmill, at a temperature of 120/80 F dry bulb/ wet bulb. Each man drank 2,000 ml water before the walk on one day and no water before the walk on the other; 1,200 ml were drunk during the walk on both days. Overhydration resulted in significantly lower rectal temperatures and pulse rates and significantly higher sweat rates than did the control state. Two matched groups of six men each were then acclimatized to heat by daily 100-min walks under the conditions described above. One group was overhydrated during each day of the acclimatizing period; the other was not. Overhydration did not affect the pattern of acclimatization to heat; conversely, ac climatization to heat did not alter the above-described acute response t...