Heat acclimation, aerobic fitness, and hydration effects on tolerance during uncompensable heat stress

Abstract

The purpose of the present study was to determine the separate and combined effects of aerobic fitness, short-term heat acclimation, and hypohydration on tolerance during light exercise while wearing nuclear, biological, and chemical protective clothing in the heat (40 degrees C, 30% relative humidity). Men who were moderately fit [(MF); <50 ml . kg-1 . min-1 maximal O2 consumption; n = 7] and highly fit [(HF); >55 ml . kg-1 . min-1 maximal O2 consumption; n = 8] were tested while they were euhydrated or hypohydrated by approximately 2.5% of body mass through exercise and fluid restriction the day preceding the trials. Tests were conducted before and after 2 wk of daily heat acclimation (1-h treadmill exercise at 40 degrees C, 30% relative humidity, while wearing the nuclear, biological, and chemical protective clothing). Heat acclimation increased sweat rate and decreased skin temperature and rectal temperature (Tre) in HF subjects but had no effect on tolerance time (TT). MF subjects increased sweat rate but did not alter heart rate, Tre, or TT. In both MF and HF groups, hypohydration significantly increased Tre and heart rate and decreased the respiratory exchange ratio and the TT regardless of acclimation state. Overall, the rate of rise of skin temperature was less, while DeltaTre, the rate of rise of Tre, and the TT were greater in HF than in MF subjects. It was concluded that exercise-heat tolerance in this uncompensable heat-stress environment is not influenced by short-term heat acclimation but is significantly improved by long-term aerobic fitness.

Induction and decay of short-term heat acclimation

“…The adaptive effects of medium to long-term heat acclimation (>8-12 d) have received much research attention (Nielsen et al 1993;Regan et al 1996;Nielsen et al 1997;Cheung and McLellan 1998;Weller and Harrison 2001;Patterson et al 2004).. However, many of the important adaptations to heat stress are cardiovascular, and occur relatively rapidly (<7 d).…”

“…Therefore, the use of short-term heat acclimation regime (Weller and Harrison 2001;Patterson et al 2004) has been adopted in this study. The use of permissive dehydration during acclimation contradicts the existing fluid replenishment guidelines for heat acclimation, which recommend the maintenance of good hydration status during exposure to heat stress conditions (Armstrong and Maresh 1991;Convertino et al 1996;Cheung and McLellan 1998;Casa et al 2000). However, the reality for many people undergoing acclimation bouts is that some level of dehydration is normal, if not frequently inevitable (Noakes et al 1988;Greenleaf 1992).…”

The Management of Heat Stress for the Firefighter: A Review of Work Conducted on Behalf of the Toronto Fire Service

“…Fluid restriction [39][40][41] and hypohydration 40) have been shown to have detrimental effects on tolerance time while wearing protective clothing. Dehydration of one to three percent of body mass is associated with decrements in both psychological 42,43) and physiological [44][45][46] performance.…”

“…This observation is similar to the response observed between trained and untrained individuals 25) , where the untrained individuals experienced a greater cardiovascular strain at a given T re due to lower blood and stroke volumes 59) . In fact, fluid replacement during work in the heat has been deemed as important as hydration status prior to work 40) . In addition, failure to replace fluid loss can negate advantages obtained from acclimation 40) and/or aerobic fitness 47) placing a high fit or acclimated individual at higher risk of heat-related illness.…”

“…In fact, fluid replacement during work in the heat has been deemed as important as hydration status prior to work 40) . In addition, failure to replace fluid loss can negate advantages obtained from acclimation 40) and/or aerobic fitness 47) placing a high fit or acclimated individual at higher risk of heat-related illness. From a health and safety perspective, fluid replacement allows higher core temperatures to be tolerated during weight-bearing activity.…”