HYPERHYDRATION - TOLERANCE AND CARDIOVASCULAR EFFECTS DURING UNCOMPENSABLE EXERCISE-HEAT STRESS

This study examined the efficacy of glycerol and water hyperhydration (1 h before exercise) on tolerance and cardiovascular strain during un compensable exercise-heat stress. The approach was to determine whethe r l-h preexercise hyperhydration (29.1 ml H2O/kg lean body mass with o r without 1.2 g/kg lean body mass of glycerol) provided a physiologica l advantage over euhydration. Eight heat-acclimated men completed thre e trials (control euhydration before exercise, and glycerol and water hyperhydrations) consisting of treadmill exercise-heat stress (ratio o f evaporative heat loss required to maximal capacity of climate = 416) . During exercise (similar to 55% maximal O-2 uptake), there was no di fference between glycerol and water hyperhydration methods for increas ing (P < 0.05) total body water. Glycerol hyperhydration endurance tim e (33.8 +/- 3.0 min) was longer (P < 0.05) than for control (29.5 +/- 3.5 min), but was not different (P > 0.05) from that of water hyperhyd ration (31.3 +/- 3.1 min). Hyperhydration did not alter (P > 0.05) cor e temperature, whole body sweating rate, cardiac output, blood pressur e, total peripheral resistance, or core temperature tolerance. Exhaust ion from heat strain occurred at similar core and skin temperatures an d heart rates in each trial. Symptoms at exhaustion included syncope a nd ataxia, fatigue, dyspnea, and muscle cramps (n = 11, 10, 2, and 1 c ases, respectively). We conclude that 1-h preexercise glycerol hyperhy dration provides no meaningful physiological advantage over water hype rhydration and that hyperhydration per se only provides the advantage (over euhydration) of delaying hypohydration during uncompensble exerc ise-heat stress.