The Effect of Fluid Balance on Exercise Performance in the Heat

Introduction: In dehydrated individuals thirst seems to disappear after ingestion of small amount of water before full rehydration. This phenomenon has been linked to oropharyngeal receptors. However, some researchers suggest that drinking to satisfy thirst is enough for optimal performance. It is well established that water deficit greater than 2% of body weight decreases exercise performance in the heat. No study has ever examined the effect hydration on exercise performance in the absence of thirst via oropharyngeal stimulation. Purpose: The purpose of this study is to examine the effect of dehydration on exercise performance and thermoregulation during exercise in the heat independently of thirst. Method: Five competitive male cyclists (age, 31.6±4.9 y, weight, 74.7±3.7 kg, height, 180.9±4.3 cm, VO2peak, 57.5±28 mL∙min-1∙kg-1) were performed 2 hours cycling in 35°C and 30% relative humidity after that they completed 5K time trial. Two experimental trials were performed: on Dehydration without Thirst (DEH): drinking 25 ml water every 5 min with no infusion in the nasogastric tube, and the other was euhydration not thirst (EUH): drinking 25 ml water every 5 min while infusing in the nasogastric tube enough fluid to match sweat losses. Results: Sweat rate during 5K time trial in the EUH trial (2.2±0.7 L/h) was higher than that in the DEH trial (1.9±0.1 L/h). In the EUH trial, % body weight loss (-0.1±0.3 %) was lower than that in the DEH trial (-2.2±0.2 %) during 2 hours cycling. The finishing time of the 5K in the EUH trial was faster than that in the DEH trial (12.9±0.5 and 13.4±0.5 min). Core temperature at the end of the 5K time trial in the EUH trial was lower than that in the DEH trial (39.0±0.1 and 39.5±0.8 ˚C). Mean skin temperature during the 5K time trial in the EUH trial was higher than that in the DEH trial. In both trials, there was no difference in the degree of thirst. Conclusion: In the EUH group, subjects completed faster the 5K time trial than in the DEH, probably due to lower thermoregulatory strain and better cardiovascular function

The Practical Application of Heat Acclimatization Induction and Intermittent Exercise-Heat Exposures in Endurance Athletes

Background: Heat acclimation (HA) and heat acclimatization (HAz) are impactful strategies to mitigate negative impact of exercise performance in the heat. However, there is no practical strategy to prevent decay following HAz and HA. Purpose: The purpose of this study was to investigate the effect of HA following HAz or dual heat acclimatization (DHA) on endurance performance and the effect of heat training (HT) on endurance performance following DHA. Methods: Twenty-six endurance athletes (mean (M)±SD; age, 35±12yrs; body mass, 72.8±8.9kg; height, 178.7±6.3 cm; VO2max, 57.3±6.7ml·kg-1·min-1) completed five 4km time trials (TT) (baseline-unacclimatized, test#1; post-HAz, test#2; post-HA/DHA, test#3; 4 weeks post-DHA, test#4; 8 weeks post-DHA, test#5) in the heat (M±SD; ambient temperature [Tamb], 35.5±0.7 °C; relative humidity [%RH], 46.3±2.2%; Wet Bulb Globe Temperature (WBGT), 29.2±0.7 °C). After test#1, participants performed self-directed summer training followed by test#2. Then, they completed a five-days of a HT over eight days in the heat (M±SD; Tamb, 39.2±0.4 °C; %RH, 51.1±2.6%; WBGT, 33.2±0.7 °C). During the HA sessions, participants exercised to induce hyperthermia for 60 minutes, which is defined as hyperthermic zone HA (HZHA, 38.50°C and 39.75°C). Participants were then divided into three groups; maximal heat training group (HTMAX), minimum heat training (HTMIN), and the control group (HTCON). HTMAX completed a total of sixteen visits and HTMIN completed a total of eight visits over the course of eight weeks. The exercise used for the HT matched the HA sessions. Percent 4km time change (TTp) was calculated based on test#3 results. Results: TTp was significantly faster at test#3 compared to test#1 (M±SD; 4.8±10.1 %, p=0.024) and test#2 (M±SD; 3.1±7.4 %, p=0.040). TTp was significantly faster in HTMAX (M±SD; -4.2±5.4 %) compared to HTMIN (M±SD; 1.9±6.5 %, p=0.044) and HTcon (M±SD; 10.7±17.0 %, p=0.024) at test#5. There were no differences of TTp in HTMIN between test#3, test#4 (M±SD; 0.95±5.55%), and test#5 (M±SD; 1.93±6.45%). Conclusions: These results indicated that HT twice per week demonstrated improvement after 8 weeks following DHA, while HACON lost adaptations in 4 weeks and even greater losses in 8 weeks. HT once per week may maintain adaptations for 4 weeks and potentially for 8 weeks

Performance Improvements from Heat Acclimation, Heat Acclimatization

Heat acclimation/acclimatization (HA/HAz) are important heat mitigation strategies that help develop heat tolerance from prolonged and repeated exposure to a hot environment, subsequently improving exercise performance in the heat. PURPOSE: To assess and quantify the magnitude of performance benefits of short- (STHA), medium- (MTHA), and long-term HA (LTHA) in endurance-trained athletes. METHODS: A literature search was conducted in PubMed, SPORTDiscus, Scopus, and Cochrane-Library, with data from 23 studies extracted for analysis. Subgroup analysis distinguished differences in performance and thermoregulatory adaptations between short-, medium- and long-term HA interventions. RESULTS: HA produced significant improvements in time trial performance (Effect size [95% confidence intervals] 0.72 [0.42– 1.03]), with LTHA displaying the most significant performance time decrease (-15.29%). MTHA and STHA showed a slight reduction in time trial performance time (-4.28% and -4.40%, respectively). Mean power output during exercise in the heat increased by 7.2% following MTHA, which was greater than STHA (-3.4%). HA showed a significant, small reduction in mean resting skin temperature (Tsk) (0.34 [0.00–0.68]) and core temperature (Tc) (0.40 [0.16–0.63]). Subgroup analysis demonstrated that mean Tsk reduction was more significant in the STHA (-0.35 ± 0.32°C) compared to MTHA (-0.24 ± 0.40°C), whereas Tc showed the greatest decrease in temperature from LTHA (0.66 [0.40–0.92]). CONCLUSION: Results indicate a noticeable improvement in endurance performances in the heat, with a trend towards longer-duration protocols eliciting the greatest performance adaptations. Findings show that long-term HA/HAz results in improved endurance performance in the heat which is influenced by thermoregulatory adaptations that increase thermal tolerance in hot and humid environments. These findings are important for athletes and their support teams to evidence-inform and individualize HA prescription

Cardiovascular Adaptations Induced by Heat Acclimation/Acclimatization in Endurance Trained Athletes

Heat acclimation/acclimatization (HA) is the improvement in heat tolerance that comes from gradually increasing intensity or duration of work in a hot background. HA reduces heat-mediates performance decrements while providing protection against exertional heat illness. PURPOSE: To assess and quantify the magnitude of cardiovascular adaptations induced by heat acclimation/acclimatization in endurance trained athletes. METHODS: A literature search was conducted using PubMed, SPORTDiscus, Scopus, and Cochrane- Library, with data from 23 studies being chosen for analysis. Subgroup analysis determined the differences in cardiovascular adaptations between short (≤7days; MTHA), medium (8-13 days; MTHA), and long-term HA (≥14 days; LTHA). RESULTS: HA (Heat Acclimation/Acclimatization) offered a moderate reduction in mean heart rate (HR) during exercise in the heat when established within short duration timeframes. Collectively, HA produced a non-significant, small reduction in resting HR (Effect size [95% confidence intervals], 0.23 [-0.08–0.55], p \u3e 0.05). MTHA displayed a trivial reduction HR (-2 ± 2 bpm). STHA produced a small, but slightly greater effect (-3 ± 1 beats.min-1) Overall, HA generated a significant, moderate increase in resting plasma volume (0.62 [0.38–0.86], p \u3c 0.05). MTHA increased resting plasma volume to the greatest extent (5.6 ± 3.8%), demonstrating a large, significant effect (p \u3c 0.05). LTHA displayed a greater effect compared to STHA (0.66 and 0.46, respectively), although the percentage increase in the resting plasma volume was similar (3.7 and 3.8 ± 1.6%, respectively). CONCLUSION: HA offered a moderate reduction in mean HR during exercise. Increased heat exposure could offer greater cardiovascular adaptation potential. Cardiovascular adaptions are essential to athletic performance and health. Overall, HA produced a significant, moderate increase in resting plasma volume. These findings may explain the more prominent HR adaptations over longer HA observations across multiple studies. The moderate increase in plasma volume could be the cause of effectual regulation of blood pressure which consequently lowered resting heart rate

Assessing the Most Effective Heat Adaptation Method for Endurance Performance in the Heat: A Systematic Review and Meta-Analysis

The consequences of increasing global warming and the globalization of elite sport have produced increased exposure to episodes of extreme heat for athletes at major sporting events. Heat acclimatization and acclimation (HA) are widely used heat mitigation strategies that aim to help athletes combat the effects of heat-stress on athletic performance and susceptibility to exertional-heat illness. PURPOSE: To assess and quantify the effect of different HA methods on the magnitude of performance and physiological adaptations in endurance trained athletes METHODS: A literature search was conducted in PubMed, SPORTDiscus, Scopus, and Cochrane-Library, with data from 23 studies gathered for analysis. Subgroup analysis determined differences in adaptations between controlled intensity, controlled hyperthermia, passive heating, self-paced (acclimatization), and self-regulated (acclimation). RESULTS: Collectively, HA had a significant, moderately beneficial effect on improving time-trial performance in the heat (Hedges’ g = 0.72 [0.42-1.03], p \u3c 0.05). Self-paced (acclimatization) elicited greatest improvement in time-trial performance (Hedges’ g = 1.40 [0.82-1.97], p \u3c 0.05) Furthermore, heat acclimatization generated the greatest reductions in mean heart rate during endurance exercise in the heat (Hedges’ g = 0.71 [0.22-1.19], p \u3c 0.05). CONCLUSION: Heat acclimatization appears to be favorable for decreasing mean heart rate and improving time-trial performance in the heat. These findings are relevant for athletes and their supports teams when implanting HA protocol proximal to competition to reduce the magnitude of heat-mediated performance decrements

Thirst Sensation Does Not Effect Reaction Time But Decreses Mood in Men

Exercise in hot environments results in dehydration accompanied by thirst sensation, a vital signal for fluid homeostasis. While cognitive performance and mood have been studied with exercise in hot environments and in dehydration states, no studies have investigated the effect of inducing thirst on cognitive performance and mood. PURPOSE: To investigate the effect of inducing thirst on cognitive performance and mood. METHODS: Twelve recreationally active men (mean±SE age: 29±3.6 years; body mass: 74.7±2.3 kg; height, 179.4±2.0 cm; maximal oxygen consumption [VO2max]: 49.8±1.9 ml·kg−1·min−1) performed 90 mins of cycling at 55% VO2max in a environmentally controlled chamber (ambient temperature, 34.9±0.2°C; relative humidity, 30.3±0.3%; wind speed, 3.4 miles ×h-1) followed by a 12 km cycling time trial. Two experimental conditions were performed with: subjects drank 25 mL of water every 5 minutes (NT) and infused 25 mL of isotonic saline every 5 minutes via intravenous intravenous tube (T). to maintain hydration across conditions. Thirst was measured every 5 minutes with the visual analog scales. Additionally, rectal temperature (Trec), skin temperature (Tsk) were recorded every 5 minutes. The Environmental Symptoms Questionnaire (ESQ), Profile of Mood States (POMS), a modified flanker task, and body mass were performed and recorded at the beginning (Pre) and the end (Post) of each trial. The flanker task assesses reaction time with congruent and incongruent conditions. Incongruent measures executive function while congruent trials measure simple reaction time. RESULTS: There was no significance between body mass loss and USG (p\u3e.05) demonstrating similar hydration states between thirst intervention. Trec and thirst were significantly higher in T compared to NT after 15 minutes and throughout the 90 minutes of exercise and 12 km time trial (p.05). Subjects also reported more fatigued after exercise (Pre: 2.0±0.6, Post: 13.8±1.2, p.05). CONCLUSION: After inducing thirst with exercise in hot environments, subjects were more fatigued and experienced trouble concentrating, however, reaction time was not affected . More research is necessary, but the results of this study suggest strategies to mitigate thirst are important to maintain mood during physical performance, however, does not affect cognitive performance

Differential Effects of Oral vs. Intravenous Fluid Administration on Bioelectrical Impedance During Dehydration Induced by Exercise and Heat

There is continued debate regarding optimal evaluation of hydration. Bioimpedance analysis has been utilized to evaluate hydration status, but there is limited information regarding the ability of this technology to detect physiological changes occurring during acute dehydration. PURPOSE: To evaluate whether bioimpedance spectroscopy (BIS) detects changes in bioelectrical resistance (R) in response to dehydration induced by exercising in the heat, assess whether these changes are related to body mass changes, and determine if the route of fluid administration during the dehydration protocol influences these observations. METHODS: Twelve males (mean ± SD; age: 28.6 ± 12.4 y; body mass: 74.7 ± 7.9 kg; height: 179.4 ± 7.0 cm; VO2max: 49.8 ± 6.6 mL/kg/min) completed two randomized experimental trials, each consisting of 90 minutes of continuous cycling exercise at 55% VO2maxfollowed by a 12 km time trial in the heat (ambient temperature: 34.9 ± 0.6 °C; relative humidity: 30.3 ± 0.9 %; wind speed: 3.4 mile×h-1). During each trial, fluid was administered either orally (DRINK) or intravenously (IV). During the DRINK trial, participants drank 25 mL of water every 5 minutes. During the IV trial, participants received 25 mL of isotonic saline solution through their IV catheter every 5 minutes. Nude body mass and BIS data were collected before and after trials to assess hydration status. Data were analyzed using Pearson’s correlations and paired t-tests with p-values corrected via false discovery rate. RESULTS: Body mass decreased, without differences between conditions (IV: -2.3 ± 0.5%; DRINK: -2.4 ± 0.9%; p=0.85). However, significant differences were observed for changes in predicted R at zero frequency (R0; IV: -3.6 ± 4.6%; DRINK: 1.3 ± 5.6%; p=0.02) and R at 50 kHz (R50; IV: -3.2 ± 4.1%; DRINK: -0.2 ± 4.1%; p=0.04), without differences in predicted R at infinite frequency (R∞; IV: -2.4 ± 6.1%; DRINK: -1.1 ± 3.7%; p=0.45). In the IV condition, significant correlations between body mass changes and R changes were observed for R0 (r=-0.80; p=0.002), R50 (r=-0.85; p\u3c0.001), and R∞ (r=-0.84; p\u3c0.001); however, no correlations were observed in the DRINK condition (r=-0.06 to 0.13; p≥0.69 for each). CONCLUSION: Differences between oral and intravenous fluid administration were seemingly detected by bioelectrical resistance at low-to-moderate, but not high, frequencies. With intravenous administration, negative correlations between changes in body mass and changes in R at all frequencies were observed, unlike with oral fluid administration. These findings suggest a potential sensitivity of bioimpedance technologies for monitoring intravenous fluid administration in the context of acute dehydration. However, additional investigation is needed to confirm their utility during distinct fluid loss scenarios and to confirm if these technologies are useful in the context of oral intake of fluids varying in composition

Mild Dehydration Led to Increased Difficulty Falling Asleep

Sleep is fundamental process that benefits health and overall quality of life which can be affected by various aspects of daily living such as dehydration. A study has yet to investigate the impacts of euhydration and mild dehydration on sleep. PURPOSE: The purpose of this study was to examine effects of euhydration, mild-dehydration, and ad libitum drinking on sleep. METHODS: Eighteen male participants (mean±SD; age, 23±4y; height, 175.8±5.7cm; weight, 80.1±9.7kg) reported to the laboratory with different hydration status for 4 consecutive mornings(Day 1, baseline; Day 2, euhydrated; Day 3, mild-dehydrated; Day 4, ad libitum drinking). Hydration status was monitored by first morning urine specific gravity (USG) and plasma osmolality. Sleepwas measured using the Karolinska sleep diary (KSD). RESULTS: USG (baseline, 1.024±0.006; euhydrated, 1.018±0.007; mild-dehydrated, 1.030±0.003; ad libitum, 1.021±0.008, pCONCLUSION: When subjects were mildly dehydrated, sleep duration was longer while it was more difficult to fall asleep. More research is necessary, but the results of this study suggest it may be important to maintain euhydration to fall asleep easier

The Effect of Heat Acclimatization and Heat Acclimation on Endurance Trained Athlete Substrate Utilization

Heat acclimatization (HAz) and Heat Acclimation (HA) are important strategies to induce thermoregulatory adaptations to mitigate negative impact of heat stress. However, despite improving endurance performance, few studies explore their impact on substrate utilization. PURPOSE: To investigate the effect of HAz and HA on endurance athlete substrate utilization during submaximal exercise in the heat. METHODS: Fourteen endurance-trained male athletes (mean ± SD; age, 33 ± 9 years; body mass, 70.9 ± 10.1 kg; height, 177.7 ± 6.4 cm; VO2max, 59.3 ± 7.4 ml·kg−1·min−1; % body fat, 8.5% ± 3.9%) participated in this study. Participants performed 60 mins bouts of submaximal exercise (58.9 ± 2.2% vVO2max) in the heat (ambient temperature [Tamb], 35.5 ± 0.2°C; %relative humidity [%RH], 46.4% ± 1.3%; wet bulb globe temperature [WBGT], 29.3 ± 0.3°C; wind speed 4.0 ± 0.1 km·h−1). Prior to heat exposure (baseline), following HAz (post-HAz), and post-HA. During 60 min exercise, oxygen consumption (VO2) and respiratory exchange ratio (RER) were measured at the beginning (5-10 min), middle (30-35 min), and ending stages (55-60 min) of the protocol. Following the baseline, participants underwent self-directed summer training (HAz). Following post-HAz trials, participants underwent 5 days of HA sessions, which involved exercising to induce hyperthermia (38.50°C-39.75°C) for 60 minutes in the heat (Tamb, 39.1 ± 0.5°C; %RH, 51.8% ± 2.6%; WBGT, 33.4 ± 0.8°C) over an eight-day stretch. RESULTS: RER was significantly higher post-HAz (mean ± standard error; 0.85 ± 0.01) compared to baseline (0.81 ± 0.01, p2 difference between baseline (38.9 ± 1.2 ml·kg−1·min−1), post-HAz (37.5 ± 1.3 ml·kg−1·min−1, p= 0.062), or post-HA (38.5 ± 1.3 ml·kg−1·min−1, p=0.668). CONCLUSION: Self-directed HAz may impact athlete substrate utilization when performing submaximal exercise in a heated environment. Additionally, RER decreases as time of submaximal exercise in the heat increases. Other factors such as, athlete fitness level and exercise intensity, should also be considered when drawing conclusions regarding RER

Sex Differences in The Accuracy of WUT (Weight, Urine Color, Thirst) Diagrams Assessing Hydration Status

The WUT (Weight, Urine Color, Thirst) Venn diagram is a practical method to assess hydration status using percent body mass loss (%BML), urine color (UCOL), and thirst perception (TP). However, sex differences and the accuracy of WUT diagrams between males and females has not yet been investigated. PURPOSE: To observe sex differences in the accuracy of WUT diagrams assessing hydration status. METHODS: 8 males [M] (age: 21 ± 3; mass: 76.3 ± 15.6 kg) and 5 females [F] (age: 22 ± 2; mass: 60.5 ± 13.6) visited the laboratory twice a day (morning (7:00am-9:00am) and afternoon (2:00pm-4:00pm)) for six days as free-living for the first three consecutive days and euhydrated (urine specific gravity (USG) \u3c 1.020) for the last three consecutive days. During each visit, TP, body mass (BM), USG, UOSM, UCOL, and plasma osmolality (POSM) were collected. Values of USG \u3e1.020, UOSM \u3e700, and POSM \u3e290 indicated dehydration status. TP \u3e5, UCOL \u3e5, and %BML \u3e1% values were used as dehydration thresholds for WUT scores. Total WUT score (0-3) was determined by the total amount of respective dehydration markers identified. One-way ANOVA was used to analyze differences in POSM, UOSM, and USG between the different WUT scores for both sexes. Receiver operating characteristics analysis was used to calculate sensitivity (SENS) and specificity (SPEC) identifying dehydration or euhydration with WUT scores. RESULTS: For POSM, WUT3 (M: 291 ± 5; F: 286 ± 0 mOsmol), WUT2 (289 ± 6; 286 ± 7), WUT1 (286 ± 5; 286 ± 6), and WUT0 (289 ± 5; 285 ± 7) were not different between sexes (p \u3e .05). For USG, WUT3 (1.022 ± .004; 1.020 ± .000), WUT2 (1.019 ± .008; 1.020 ± .007), WUT1 (1.015 ± .006; 1.010 ± .005), and WUT0 (1.010 ± .006; 1.008 ± .006) were not different between sexes (p \u3e .05). For UOSM, WUT3 (819 ± 147; 744 ± .000 mOsmol), WUT2 (679 ± 244; 788 ± 261), WUT1 (521 ± 266; 461 ± 212), and WUT0 (383 ± 212; 322 ± 203) were not different between sexes (p \u3e .05). For POSM, WUT2SPEC was higher in M (WUT2Mspec, .860) than F (WUT2Fsepc, .786) while WUT3, WUT1, and WUT0 were similar between sexes (WUT3Mspec, .965; WUT3Fspec, .976; WUT1Mspec, .526; WUT1Fspec, .380). For USG, WUT2SENS was higher in F (WUT2Fsens, .889) than M (WUT2Msens, .571) while WUT3, WUT1, and WUT0 were similar between sexes (WUT3Msens, .238; WUT3Fsens, .111; WUT1Msens, .905; WUT1Fsens, .889). For UOSM, SPEC and SENS were similar between sexes for each WUT score. CONCLUSION: There are no sex differences in POSM, USG, and UOSM between WUT0-WUT3. However, based on SPEC and SENS, WUT3 and WUT0 can accurately detect hydration status in both sexes. WUT2 might be used to detect hydration status only for females