Drinking and water balance during exercise and heat acclimation

Abstract

To describe the interactions between drinking and plasma volume, ions, osmolality (Osm), vasopressin (PVP) and renin activity (PRA) during exercise and heat-induced dehydration, voluntary H2O intake (16.degree. C) was measured in a group of 5 men (21-24 yr) undergoing acclimation during ergometer exercise (75 W) in a warm environment for 2 h/day (Tdb [dry-bulb temperature] 39.8.degree. C, Twb [wet bulb] 30.0.degree. C, rh [relative humidity] 50%) for 8 consecutive days. A control group of 5 men (19-22 yr) underwent a similar regimen in a thermoneutral environment (Tdb 23.8.degree. C, Twb 16.9.degree. C, rh 50%). Fluid intake in the control group varied between 129 and 232 ml/h during the 2 h of exercise; it increased from 450 ml/h on day 1 to .apprx. 1000 ml/h on days 5-8 in the acclimation group. The average level of negative H2O balance was .apprx. 400 ml (52% of total replacement) in the control group and decreased from 900 ml on day 1-800 ml (60-30% [P < 0.05] of replacement) on days 5-8 in the acclimation group. Increased drinking during acclimation was characterized by a progressively shortened time to the 1st drink (from 26-11 min, NS [nonsignificant]) and a threefold increase to 9.5 drinks/exposure (P < 0.001). Mean volume/drink increased from 96 ml in control to 174 ml (P < 0.01) during acclimation. Changes in plasma Na concentration, Osm, and PVP were minimal during both experiments; as exercise hypovolemia and PRA were significantly greater during acclimation. Reduction in body fluid volumes and the renin-angiotensin II system appear to be more closely associated with the control of drinking during dehydration induced by exercise in heat than is the Na+-osmotic-PVP pathway.