Thirst As A Marker Of Hydration Status During And After Exercise In The Heat

Abstract

PURPOSE: The purpose of this study was two-fold: 1) examine the perception of thirst as a potential marker of hydration status during and after exercise in the heat 2) investigate changes in thirst perception following consumption of a bolus of fluid post exercise. METHODS: Twelve men (mean ± SD; age, 23±4 y; body mass, 81.4±9.9 kg; height, 182±9 cm; body fat, 14.3±4.7%) completed two 180-minute bouts of exercise on a motorized treadmill in a warm environment (ambient temperature, 35.2±0.6°C; RH, 30.0±5.4%; WBGT, 26.6±1.1°C) followed by a 60-minute recovery period. Participants completed a euhydrated (EUH) and hypohydrated (HYPO) trial, where fluid losses were minimized or fluid consumption was restricted, respectively. During recovery, participants were randomly assigned to a fluid replacement (FL) or no fluid replacement (NFL) group where they were given 10-minutes to consume water ad libitum at a volume matching fluid losses. Perception of thirst was measured using a 9-point scale and was measured periodically during and after exercise. A three-factor (trial x condition x time) repeated-measures ANOVA was used to measure differences in thirst perception post exercise between EUH-FL, EUH-NFL, HYPO-FL and HYPO-NFL. RESULTS: %BML was significantly greater in HYPO (2.7±0.7%) than EUH (0.4±0.6%) trials (p<0.001). Fluid replacement after exercise did not influence %BML between HYPO (HYPO-FL, 2.8±0.9%; HYPO-NFL, 2.6±0.6%) and EUH (EUH-FL, 0.2±0.7%; EUH-NFL, 0.6±0.5%) trials (p=0.330). During exercise, thirst perception was significantly greater in HYPO trials than EUH trials beginning at minute 90 of exercise (p<0.05). Beginning at minute 5 during the recovery period, thirst perception was significantly greater in HYPO-NFL than EUH-FL, EUH-NFL and HYPO-FL (p<0.05). CONCLUSIONS: During exercise, changes in perception of thirst were reflective of increased fluid losses, especially when %BML >2%. However, receiving a bolus of fluid post exercise when hypohydrated (%BML >2%) negated differences in perception of thirst between those euhydrated and hypohydrated. Thirst could have limited utility in guiding hydrated practices when attempting to quickly reestablish euhydration following prolonged exercise in the heat.