Abstract
The purpose of this study was to examine the perception of thirst as a marker of hydration status following prolonged exercise in the heat. 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 min bouts of exercise on a motorized treadmill in a hot environment (35.2 ± 0.6 °C; RH, 30.0 ± 5.4%), followed by a 60 min recovery period. Participants completed a euhydrated (EUH) and hypohydrated (HYPO) trial. During recovery, participants were randomly assigned to either fluid replacement (EUHFL and HYPOFL; 10 min ad libitum consumption) or no fluid replacement (EUHNF and HYPONF). Thirst was measured using both a nine-point scale and separate visual analog scales. The percent of body mass loss (%BML) was significantly greater immediately post exercise in HYPO (HYPOFL, 3.0% ± 1.2%; HYPONF, 2.6% ± 0.6%) compared to EUH (EUHFL, 0.2% ± 0.7%; EUHNF, 0.6% ± 0.5%) trials (p < 0.001). Following recovery, there were no differences in %BML between HYPOFL and HYPONF (p > 0.05) or between EUHFL and EUHNF (p > 0.05). Beginning at minute 5 during the recovery period, thirst perception was significantly greater in HYPONF than EUHFL, EUHNF, and HYPOFL (p < 0.05). A 10 min, ad libitum consumption of fluid post exercise when hypohydrated (%BML > 2%), negated differences in perception of thirst between euhydrated and hypohydrated trials. These results represent a limitation in the utility of thirst in guiding hydration practices.
Highlights
The complexities surrounding the turnover of body water complicates any single measure of hydration status qualifying as a standard for assessment [1]
Follow-up testing revealed that, during recovery, mean thirst perception (TH) was significantly greater in HYPONF than EUHFL (p < 0.001, Effect size (ES) = 6.44), EUHNF (p < 0.001, ES = 5.05), and HYPOFL (p = 0.002, ES = 3.70) (Figure 1A)
There were no significant differences in feelings of thirstiness (p = 0.052), pleasantness toward drinking water (p = 0.211), dryness in the mouth (p = 0.072), taste in the mouth (p = 0.12), fullness (p = 0.099), and sickness (p = 0.145) between trial, recovery condition, and time; it can be observed that feelings of thirstiness and dryness in the mouth trended toward significance in this three-way interaction
Summary
The complexities surrounding the turnover of body water complicates any single measure of hydration status qualifying as a standard for assessment [1]. Methods for assessing hydration status utilize urinary and hematologic measures, among others; these methods are not without limitations regarding accuracy and applicability in all settings [1,2,3,4]. Many of these assessment methods require expensive laboratory instrumentation and/or expertise in these techniques, limiting real-world applicability for all persons. Nutrients 2019, 11, 2689 of 1–2% body mass loss [6,9] is influenced by hyperosmolality, hormonal responses (arginine vasopressin (AVP) and angiotensin II (Ang II)), and peripheral osmoreceptors [10,11,12], but is highly variable within individuals [5]. Non-homeostatic influences, such as beverage taste, availability, individual drinking habits, and timing with meals, dictate daily fluid intake [8]
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