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PURPOSE:: This study examined the impact of 10 d of exercise-heat acclimation on sweat mineral concentrations. METHODS:: Eight male subjects walked on a treadmill at 3.5 mph, 4% grade for 100 continuous minutes or until rectal temperature reached 39.5 degrees C on 10 consecutive days in an environmental chamber set at 45 degrees C, 20% relative humidity. Arm sweat samples were collected during the first 30 min of exercise-heat stress on days 1 and 10 using a polyethylene arm glove. RESULTS:: Final core temperature and HR values were significantly lower (P < 0.05) on day 10 versus day 1. Whole-body sweating rates increased by approximately 6% (P = 0.12). Sweat sodium concentration on day 10 (36.22 +/- 7.22 mM) was significantly lower than day 1 (54.49 +/- 16.18 mM) (P < 0.05). Sweat mineral concentrations of calcium ( 29%), copper ( 50%), and magnesium ( 43%) were also significantly lower on day 10 versus day 1 of heat acclimation (P < 0.05). A trend for lower sweat iron ( 75%; P = 0.07) and zinc ( 23%; P = 0.10) concentrations were observed from day 1 to day 10. The estimated hourly sweat mineral losses (arm concentration x whole-body sweat rate) were reduced for calcium ( 27%), copper ( 46%), and magnesium ( 42%) (P < 0.05), but not iron (75%) or zinc ( 16%) (P > 0.05), from day 1 to day 10. CONCLUSION:: Exercise-heat acclimation conserves arm sweat mineral concentrations and possibly whole-body sweat losses of calcium, copper, and magnesium, and may reduce sweat iron and zinc concentrations. [ABSTRACT FROM AUTHOR]

PURPOSE: The purpose of this study was to 1) determine the effect of hypohydration (HYPO) on thirst sensations during moderate exercise in the cold and 2) determine a possible mechanism for a cold-induced decline in thirst. METHODS: In the first phase of the study, eight males walked on four occasions, in T-shirts, shorts, and shoes, at 50% VO2max, for 60 min in either a 4 degrees C (cold) or 27 degrees C (temperate) environment in a state of HYPO or euhydration (EU). In the second phase, nine males in states of EU or HYPO randomly performed four trials consisting of 30 min standing at 27 degrees C, followed by 30 min of standing and 30 min of treadmill exercise at 50% of VO2max, in either 4 degrees C or 27 degrees C air. RESULTS: In phase 1, thirst sensations were lower throughout 60 min of exercise (P < 0.05) in both HYPO and EU conditions during the cold trials. In phase 2, despite elevated plasma osmolality (P < 0.05), perception of thirst and plasma arginine vasopressin [AVP] similarly decreased (P < 0.05) after 30-min standing rest and 30-min exercise in the HYPO-cold trial, compared with the HYPO-temperate, EU-cold, and EU-temperate trials. CONCLUSIONS: When either euhydrated or hypohydrated, cold exposure attenuated thirst by up to 40% at rest and during moderate-intensity exercise. The attenuated thirst when hypohydrated during cold exposure was associated with lower plasma [AVP] despite an elevated plasma osmolality. This decline in thirst and AVP in the cold may be the result of peripheral vasoconstriction, mediating an increase in central blood volume and stimulation of central volume receptors. [ABSTRACT FROM AUTHOR]

Objective: The primary aim of this study was to assess the efficacy of the weight, urine, and thirst (WUT) framework in predicting dehydration after a body water manipulation protocol, while concurrently determining the individual and interactive contributions of the model components., Methods: The total study sample was 93 participants (female, n = 47), recruited from two institutions. Phase 1 involved collecting daily hydration measures from free-living participants (study 1, 58 participants for 3 d; study 2, 35 participants for 7 d). Phase 2 entailed a 2-h passive heating protocol, where participants from study 2 were randomly assigned to one of three groups that manipulated total body water over 24 h using passive heating and fluid restriction. During each phase, participants provided urine samples, underwent body mass measurements, and completed questionnaires pertaining to thirst perception. Morning and 24-h urine samples were assessed for color, osmolality, and specific gravity. Differences between intervention groups, based on the probability of hydration status, were examined (ANOVA), and ridge regression analysis assessed the relative importance of variables within the WUT model., Results: The study revealed significant differences among the intervention groups for predicted probability of dehydration, as determined by changes in body mass ( P = 0.001), urine color ( P = 0.044), and thirst perception ( P < 0.001). Binomial ridge regression indicated that changes in body mass (58%) and thirst perception (26%) were the most influential predictors of dehydration., Conclusions: These data support use of an enhanced version of the WUT model, underscoring the significance of changes in body mass and thirst perception in the assessment of hydration status., (Copyright © 2024 by the American College of Sports Medicine.)

Purpose: Rapid gastric emptying and intestinal absorption of beverages is essential for rapid rehydration, and certain amino acids (AA) may augment fluid delivery. Three sugar-free beverages, containing differing AA concentrations (AA + PZ), were assessed for fluid absorption kinetics against commercial sugar-free (PZ, GZ) and carbohydrate-containing (GTQ) beverages., Methods: Healthy individuals (n = 15-17 per study) completed three randomised trials. Three beverages (550-600 mL) were ingested in each study (Study 1: AA + PZ [17.51 g/L AA], PZ, GZ; Study 2: AA + PZ [6.96 g/L AA], PZ, GZ; Study 3: AA + PZ [3.48 g/L AA], PZ, GTQ), containing 3.000 g deuterium oxide (D 2 O). Blood samples were collected pre-, 2-min, 5-min, and every 5-min until 60-min post-ingestion to quantify maximal D 2 O enrichment (Cmax), time Cmax occurred (Tmax) and area under the curve (AUC)., Results: Study 1: AUC (AA + PZ: 15,184 ± 3532 δ‰ vs. VSMOW; PZ: 17,328 ± 3153 δ‰ vs. VSMOW; GZ: 17,749 ± 4204 δ‰ vs. VSMOW; P ≤ 0.006) and Tmax (P ≤ 0.005) were lower for AA + PZ vs. PZ/GZ. Study 2: D 2 O enrichment characteristics were not different amongst beverages (P ≥ 0.338). Study 3: Cmax (AA + PZ: 440 ± 94 δ‰ vs. VSMOW; PZ: 429 ± 83 δ‰ vs. VSMOW; GTQ: 398 ± 81 δ‰ vs. VSMOW) was greater (P = 0.046) for AA + PZ than GTQ, with no other differences (P ≥ 0.106)., Conclusion: The addition of small amounts of AA (3.48 g/L) to a sugar-free beverage increased fluid delivery to the circulation compared to a carbohydrate-based beverage, but greater amounts (17.51 g/L) delayed delivery., (© 2024. The Author(s).)

Background: Amino-acid based medical foods have shown promise in alleviating symptoms of drug induced gastrointestinal side effects; particularly, diarrhea-predominant symptoms. Irritable bowel syndrome (IBS) is a gastrointestinal disorder that affects up to 9% of people globally, with diarrhea predominant IBS (IBS-D) being the most prevalent subtype. Further trials are needed to explore potential added benefits when integrated into standard care for IBS-D., Aim: To assess the effectiveness of an amino acid-based medical food as an adjunct to standard of care for adults with IBS-D., Methods: This is a pragmatic, real world, open label, single arm study comparing a 2-week baseline assessment to a 2-week intervention period. One hundred adults, aged 18 to 65 years, with IBS-D, according to Rome IV criteria, were enrolled after completing a 2-week baseline assessment period and received a 2-week supply of an amino acid based medical food which was consumed at home twice daily on top of their standard of care. The primary outcome was an assessment of tolerability after 2-weeks of consumption, while secondary outcomes included changes in stool consistency (Bristol Stool Form Scale), severity of abdominal pain & discomfort, symptoms of urgency, Global Improvement Survey (GIS), and the IBS severity scoring system (IBS-SSS)., Results: The test product was well-tolerated as each participant successfully completed the full 14-day trial, and there were no instances of dropouts or discontinuation of the study product reported. Forty percent of participants achieved a 50% or more reduction in the number of days with type 6-7 bowel movements (IBS-D stool consistency responders). Fifty-three percent of participants achieved a clinically meaningful reduction of 30% in mean weekly pain scores, and 55% experienced the same for mean weekly discomfort scores (IBS-D pain and discomfort responders). Participants experienced a mean -109.4 (95% confidence interval: -130.1, -88.8) point reduction on the IBS-SSS and 52% experienced a minimally clinically important difference of > 95 points. An IBS-SSS category shift from severe to moderate or mild occurred in 69% of participants. For functional symptoms, 76% of participants reported symptom relief on the GIS., Conclusion: The amino acid-based medical food was well-tolerated, when added to the standard of care, and demonstrated improvements in both overall IBS symptom severity and IBS-D symptoms within just 2 wk., Competing Interests: Conflict-of-interest statement: All authors are employees of Entrinsic Bioscience LLC (EBS) and EBS was the research sponsor. Patent 174821-011205/PCT (PCT/US11/53265) is issued to EBS Holdings, and patent 174821-012001/PCT (PCT/US13/25294) is issued to EBS Holding., (©The Author(s) 2023. Published by Baishideng Publishing Group Inc. All rights reserved.)

Introduction: Recent success in predicting individual sweat losses from air temperature and energy expenditure measurements suggests a potential for forecasting individual sweat losses for future combinations of environment and exercise. The purpose of this study is to determine the plausibility of accurately forecasting exercise sweat losses from meteorological air temperature forecasts and individual running energy expenditure forecasts. The potential impact on plasma sodium is also estimated when setting drinking rates equal to forecast sweat losses., Materials and Methods: Individual exercise sweat losses (equated to water needs) and energy expended while running were measured in 33 participants along with air temperature and compared with forecasts of the same. Forecast inputs were used in a web app to forecast exercise sweat losses for comparison with observed values. The bias between forecast and observed exercise sweat losses was used to calculate the potential drinking impact on plasma sodium., Results: The concordance correlation coefficient between forecast and observed values was 0.95, 0.96, and 0.91 for air temperature, energy expenditure, and exercise sweat losses, respectively, indicating excellent agreement and no significant differences observed via t -test. Perfect matching of water intake to sweat losses would lower plasma sodium concentrations from 140 to 138 mmol/L; calculations using the 95% limits of agreement for bias showed that drinking according to forecast exercise sweat losses would alter plasma sodium concentrations from 140 to between 136 and 141 mmol/L., Conclusions: The outcomes support the strong potential for accurately forecasting exercise sweat losses from commonly available meteorological air temperature forecasts and energy expenditure from forecast running distance., Competing Interests: SC is the owner of Sports Science Synergy, LLC and the creator of the patented H2Q™ algorithm (U.S. Patent No. 11,219,410 B2) used in this study. RK was employed by entrinsic bioscience, LLC. The remaining author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (© 2023 Cheuvront, Sollanek and Kenefick.)

Introduction: The importance of providing wet bulb globe temperature (WBGT) heat stress flag category measurements in real time is well recognized by road race directors, and it is bound to become even more important with the rise of extreme weather and pandemic outbreaks. The purpose of this paper is to describe the WBGT index and its components measured during the 125th running of the Boston Marathon on October 11, 2021, for qualitative comparison to measurements made similarly on its traditional April date, 2014 to 2019., Methods: Monitoring occurred at the 7 km, 18 km, and 32 km marks of the race in the towns of Ashland, Natick-Wellesley, and Newton. The outdoor WBGT index was calculated from direct hourly measurements of the dry bulb, black globe, and natural wet bulb temperatures from 0900 to 1500 h., Results: The WBGT index was not different among towns; thus, the average hourly values for the 3 towns were compared to historical data averaged identically. Although the black globe temperature fluctuated considerably in response to changing cloud cover, on average, partly cloudy skies kept the solar load comparable to what has been observed in April. Dry bulb and wet bulb temperatures were higher than those on most April dates shown, which resulted in a yellow (or amber) flag day for most of the race., Conclusions: The historic October Boston Marathon was among the warmest in recent history. Like the impact of COVID-19 on the 2021 Boston Marathon, future climate challenges around outdoor activities could necessitate rescheduling; they underscore the importance of real-time WBGT index measures., (Copyright © 2023 Wilderness Medical Society. Published by Elsevier Inc. All rights reserved.)

Introduction: This study compared the efficacy of three commercial oral rehydration solutions (ORS) for restoring fluid and electrolyte balance, after exercise-induced dehydration., Method: Healthy, active participants ( N  = 20; ♀ = 3; age ∼27 y, V ˙ O 2 peak ∼52 ml/kg/min) completed three randomised, counterbalanced trials whereby intermittent exercise in the heat (∼36°C, ∼50% humidity) induced ∼2.5% dehydration. Subsequently, participants rehydrated (125% fluid loss in four equal aliquots at 0, 1, 2, 3 h) with a glucose-based (G-ORS), sugar-free (Z-ORS) or amino acid-based sugar-free (AA-ORS) ORS of varying electrolyte composition. Urine output was measured hourly and capillary blood samples collected pre-exercise, 0, 2 and 5 h post-exercise. Sodium, potassium, and chloride concentrations in urine, sweat, and blood were determined., Results: Net fluid balance peaked at 4 h and was greater in AA-ORS (141 ± 155 ml) and G-ORS (101 ± 195 ml) than Z-ORS (-47 ± 208 ml; P  ≤ 0.010). Only AA-ORS achieved positive sodium and chloride balance post-exercise, which were greater for AA-ORS than G-ORS and Z-ORS ( P  ≤ 0.006), as well as for G-ORS than Z-ORS ( P  ≤ 0.007) from 1 to 5 h., Conclusion: when provided in a volume equivalent to 125% of exercise-induced fluid loss, AA-ORS produced comparable/superior fluid balance and superior sodium/chloride balance responses to popular glucose-based and sugar-free ORS., Competing Interests: LJ has current funding from Entrinsic Beverage Company LLC, Entrinsic Bioscience, LLC, Herbalife Europe Ltd, Bridge Farm Nurseries and Decathlon SA, and has previously received funding from PepsiCo Inc., Volac International and British Summer Fruits; has performed consultancy for PepsiCo Inc. and Lucozade, Ribena Suntory, and has received conference fees from PepsiCo Inc. and Danone Nutricia. In all cases, monies have been paid to LJ's institution and not to LJ. The funders were involved in the study design, but were not involved in the collection or analyses of data; or the decision to publish the results. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (© 2023 Peden, Funnell, Reynolds, Kenefick, Cheuvront, Mears and James.)

This study sought to (a) estimate how the duration of running influences sweat losses and contributes to the daily fluid requirement, and (b) empirically estimate the drinking rates required to prevent significant dehydration (≥2% body weight as body water). Individual sweating data and running duration were obtained from male (n = 83) and female (n = 36) runners (n = 146 total observations) performing under highly heterogeneous conditions and over a range of exercise durations (33-280 min). Running <60 min/day increased daily fluid needs by a factor of 1.3, whereas running >60 min/day increased the daily fluid need by a factor of 1.9-2.3. Running <60 min/day generally required no fluid intake to prevent significant dehydration before run completion (31/35 runners). In contrast, running >60 min/day required more than 50% replacement of sweating rates to prevent the same (65/111 runners). Overall sweat losses ranged from ∼0.2 to ∼5.0 L/day, whereas the drinking rates required to prevent significant dehydration ranged from 0 to 1.4 L/hr. The characterization of sweat losses, sweat rate, and required drinking among runners in this study indicate wide individual variability that warrants personalized hydration practices, particularly when running is prolonged (>60 min) and performance is important. This study may serve as a useful guidepost for sports dietitians when planning and communicating fluid needs to athletes, as well as complement guidance related to both personalized programmed and thirst-driven drinking strategies.

Background: A historical turning point occurred in the treatment of diarrhea when it was discovered that glucose could enhance intestinal sodium and water absorption. Adding glucose to salt water (oral rehydration solution, ORS) more efficiently replaced intestinal water and salt losses., Aim: Provide a novel hypothesis to explain why mainstream use of ORS has been strongly recommended, but weakly adopted., Methods: Traditional (absorptive) and novel (secretory) physiological functions of glucose in an ORS were reviewed., Results: Small amounts of glucose can stimulate both intestinal absorption and secretion. Glucose can exacerbate a net secretory state and may aggravate pathogen-induced diarrhea, particularly for pathogens that affect glucose transport., Conclusion: A hypothesis is made to explain why glucose-based ORS does not appreciably reduce diarrheal stool volume and why modern food science initiatives should focus on ORS formulations that replace water and electrolytes while also reducing stool volume and duration of diarrhea.

Purpose: This study aimed to compare the wet bulb globe temperature (WBGT) index and other environmental parameters between early and late Boston Marathon race start times from 1995 to 2016., Methods: Environmental data from 1995 to 2016 (excluding 1996) were used to compare two identical time frames using the 0900-1300 h start versus the 1100-1500 h start. This included the WBGT, dry bulb (Tdb), black globe (Tbg), wet bulb (Twb), solar radiation, relative humidity, and air water vapor pressure. To make comparisons between start times, the difference in the area under the curve (AUC) for each environmental variable was compared within each year with a Wilcoxon signed rank test with a Holm-Bonferroni correction., Results: AUC exposures for WBGT (P = 0.027), Twb (P = 0.031), Tdb (P = 0.027), Tbg (P = 0.055), and solar radiation (P = 0.004) were reduced with an earlier start, whereas those for relative humidity and air water vapor pressure were not. Overall, an earlier race start time by 2 h (0900 vs 1100 h) reduced the odds of experiencing a higher flag category 1.42 times (β = 0.1744, P = 0.032)., Conclusions: The 2007 decision to make the Boston Marathon start time earlier by 2 h has reduced by ~1.4 times the odds that runners will be exposed to environmental conditions associated with exertional heat illness., (Copyright © 2021 by the American College of Sports Medicine.)

It is well appreciated that a loss of body water (dehydration) can impair endurance performance and that the effect is magnified by environmental heat stress. A majority of professional sports medicine and nutrition organizations recommend drinking during exercise to replace sweat losses and prevent dehydration, while also avoiding frank over-hydration. Knowledge of sweating rate, which is highest in the heat for any given metabolic rate, is therefore considered key to developing a sound drinking strategy. Exercise duration and the provision of liquid fuel interacts with required drink volumes in important ways that are infrequently discussed but are of utmost practical concern. This review details some challenges related to the optimized coupling of fluid and fuel needs during prolonged exercise in the heat and the need for personalization., (Copyright © 2021. Published by Elsevier Ltd.)

Purpose: This study aimed to characterize HTT specificity and to determine any effect of HA on the outcome., Methods: Thirteen unacclimatized, healthy men (V˙O2peak, 43.0 ± 4.8 mL·kg-1⋅min-1) with no previous history of heat illness completed 8 d of HA using the HTT protocol (40°C/40% RH; 120 min; 5 km·h-1 and 2% grade). Heart rate (HR) and core temperature (Tcore) recorded every 5 min during exercise and at the end of 120 min (terminal value) were compared between days 1 and 8. Test specificity (given no previous history of heat illness, the probability of being heat tolerant) was calculated on days 1 and 8., Results: There was a significant reduction in HR and Tcore between days 1 and 8, indicating successful HA. All volunteers successfully completed 120 min of walking on all days. HTT specificity ranged between 54% and 85% on day 1 and between 77% and 92% on day 8, depending on the HTT criteria used., Conclusion: Young healthy men without any previous heat illness experienced a 15% to 46% false-positive fail rate for the HTT without HA. After HA, the false-positive fail rate decreased to between 8% and 13%. Outcomes of the HTT are significantly affected by the criteria used and by HA status. The use of HTT for RTA decisions should be done with the recognition of these effects., (Copyright © 2020 by the American College of Sports Medicine.)

Toussaint, Claudia M., Robert W. Kenefick, Frank A. Petrassi, Stephen R. Muza, and Nisha Charkoudian. Altitude, acute mountain sickness, and acetazolamide: recommendations for rapid ascent. High Alt Med Biol . 22:5-13, 2021. Background: Sea level natives ascending rapidly to altitudes above 1,500 m often develop acute mountain sickness (AMS), including nausea, headaches, fatigue, and lightheadedness. Acetazolamide (AZ), a carbonic anhydrase inhibitor, is a commonly used medication for the prevention and treatment of AMS. However, there is continued debate about appropriate dosing, particularly when considering rapid and physically demanding ascents to elevations above 3,500 m by emergency medical and military personnel. Aims: Our goal in the present analysis was to evaluate and synthesize the current literature regarding the use of AZ to determine the most effective dosing for prophylaxis and treatment of AMS for rapid ascents to elevations >3,500 m. These circumstances are specifically relevant to military and emergency medical personnel who often need to ascend rapidly and perform physically demanding tasks upon arrival at altitude. Methods: We conducted a literature search from April 2018 to February 2020 using PubMed, Google Scholar, and Web of Science to identify randomized controlled trials that compared AZ with placebo or other treatment with the primary endpoint of AMS incidence and severity. We included only research articles/studies that focused on evaluation of AZ use during rapid ascent. Results: Four doses of AZ (125, 250, 500, and 750 mg daily) were identified as efficacious in decreasing the incidence and/or severity of AMS during rapid ascents, with evidence of enhanced effectiveness with higher doses. Conclusions: For military, emergency medical, or other activities involving rapid ascent to altitudes >3,500 m, doses 500-750 mg/day within 24 hours of altitude exposure appear to be the most effective for minimizing symptoms of AMS.

Purpose: It is often assumed that a person with a higher mean skin temperature (Tsk) will sweat more during exercise. However, it has not yet been demonstrated whether Tsk describes any individual variability in whole-body sweat rate (WBSR) independently of the evaporative requirement for heat balance (Ereq)., Methods: One hundred forty bouts of 2-h treadmill walking completed by a pool of 21 participants (23 ± 4 yr, 174 ± 8 cm, 76 ± 11 kg, 1.9 ± 0.2 m) under up to nine conditions were analyzed. Trials employed varying rates of metabolic heat production (Hprod; 197-813 W), and environmental conditions (15°C, 20°C, 25°C, 30°C; all 50% relative humidity), yielding a wide range of Ereq (86-684 W) and Tsk values (26.9°C-34.4°C)., Results: The individual variation observed in WBSR was best described using Ereq (in watts; R = 0.784) as a sole descriptor, relative to Ereq (in watts per meter squared; R = 0.735), Hprod (in watts; R = 0.639), Hprod (in watts per meter squared; R = 0.584), ambient air temperature (Ta) (R = 0.263), and Tsk (R = 0.077; all, P < 0.001). A multiple stepwise linear regression included only Ereq (in watts; adjusted R = 0.784), with Tsk not significantly correlating with the residual variance (P = 0.285), independently of Ereq (in watts). Hprod (in watts) had similar predictive strength to Ereq (in watts) at a fixed air temperature, explaining only 5.2% at 30°C, 4.9% at 25°C, 2.7% at 20°C, and 0.5% at 15°C (all, P < 0.001) less variance in WBSR compared with Ereq. However, when data from all ambient temperatures were pooled, Hprod alone was a markedly worse predictor of WBSR than Ereq (R = 0.639 vs 0.784; P < 0.001)., Conclusions: Ereq (in watts) explained approximately four-fifths of the individual variation in WBSR over a range of ambient temperatures and exercise intensities, whereas Tsk did not explain any residual variance independently of Ereq.

Selecting the most appropriate performance test is critical in detecting the effect of an intervention. In this investigation we 1 ) used time-trial (TT) performance data to estimate sample size requirements for test selection and 2 ) demonstrated the differences in statistical power between a repeated-measures ANOVA (RM-ANOVA) and analysis of covariance (ANCOVA) for detecting an effect in parallel group design. A retrospective analysis of six altitude studies was completed, totaling 105 volunteers. We quantified the test-retest reliability [i.e., intraclass correlation coefficient (ICC) and standard error of measurement (SEM)] and then calculated the standardized effect size for a 5-20% change in TT performance. With these outcomes, a power analysis was performed and required sample sizes were compared among performance tests. Relative to TT duration, the 11.2-km run had the lowest between-subject variance, and thus greatest statistical power (i.e., required smallest sample size) to detect a given percent change in performance. However, the 3.2-km run was the most reliable test (ICC: 0.89, SEM: 81 s) and thus better suited to detect the smallest absolute (i.e., seconds) change in performance. When TT durations were similar, a running modality (11.2-km run; ICC: 0.83, SEM: 422 s) was far more reliable than cycling (720-kJ cycle; ICC: 0.77, SEM: 480 s). In all scenarios, the ANCOVA provided greater statistical power than the RM-ANOVA. Our results suggest that running tests (3.2 km and 11.2 km) using ANCOVA analysis provide the greatest likelihood of detecting a significant change in performance response to an intervention, particularly in populations unaccustomed to cycling. NEW & NOTEWORTHY This is the first investigation to utilize time-trial (TT) data from previous studies in simulations to estimate statistical power. We developed an easy-to-use decision aid detailing the required sample size needed to detect a given change in TT performance for the purpose of test selection. Furthermore, our detailed methods can be applied to any scenario in which there is an impact of a stressor and the desire to detect a treatment effect.

In order to inform the Athlete Biological Passport (ABP), this study determined whether the elevation in hemoglobin (Hb) following intracellular or extracellular dehydration would trigger an atypical passport finding (ATPF). Seven male and three female volunteers (age: 23 ± 4 y; height: 170 ± 8 cm; body mass: 78 ± 12 kg) were carefully euhydrated (EUH) to determine baseline Hb levels. Volunteers then completed both an exercise-induced sweating dehydration (SW) protocol and a diuretic-induced dehydration (DI) protocol. Dehydration was assessed via body mass changes and Hb was measured via a bench-top automated hematology analyzer. Using the ABP module, the expected baseline range for each individual was determined using EUH trials, and the impact of each dehydration protocol was then assessed in comparison with these thresholds. Volunteers lost on average 3.1% and 3.7% body mass in the SW and DI trials, respectively. While only one subject exceeded the upper threshold following DI dehydration, six additional subjects demonstrated highly unusual ABP profiles; this was not the case for SW. Sweating is not a feasible explanation for elevated Hb during ABP testing; however, recent illness such as secretory diarrhea, which is mimicked by diuretic administration, may be capable of producing elevated Hb in athletes' biological passports., (© 2020 John Wiley & Sons, Ltd.)

Exercise-heat acclimation (EHA) induces adaptations that improve tolerance to heat exposure. Whether adaptations from EHA can also alter responses to hypobaric hypoxia (HH) conditions remains unclear. This study assessed whether EHA can alter time-trial performance and/or incidence of acute mountain sickness (AMS) during HH exposure. Thirteen sea-level (SL) resident men [SL peak oxygen consumption (V̇o 2peak ) 3.19 ± 0.43 L/min] completed steady-state exercise, followed by a 15-min cycle time trial and assessment of AMS before (HH1; 3,500 m) and after (HH2) an 8-day EHA protocol [120 min; 5 km/h; 2% incline; 40°C and 40% relative humidity (RH)]. EHA induced lower heart rate (HR) and core temperature and plasma volume expansion. Time-trial performance was not different between HH1 and HH2 after 2 h (106.3 ± 23.8 vs. 101.4 ± 23.0 kJ, P = 0.71) or 24 h (107.3 ± 23.4 vs. 106.3 ± 20.8 kJ, P > 0.9). From HH1 to HH2, HR and oxygen saturation, at the end of steady-state exercise and time-trial tests at 2 h and 24 h, were not different ( P > 0.05). Three of 13 volunteers developed AMS during HH1 but not during HH2, whereas a fourth volunteer only developed AMS during HH2. Heat shock protein 70 was not different from HH1 to HH2 at SL [1.9 ± 0.7 vs. 1.8 ± 0.6 normalized integrated intensities (NII), P = 0.97] or after 23 h (1.8 ± 0.4 vs. 1.7 ± 0.5 NII, P = 0.78) at HH. Our results indicate that this EHA protocol had little to no effect-neither beneficial nor detrimental-on exercise performance in HH. EHA may reduce AMS in those who initially developed AMS; however, studies at higher elevations, having higher incidence rates, are needed to confirm our findings.

Environmental factors, including high temperature and humidity, can influence dermal absorption of chemicals. Soldiers can be dermally exposed to permethrin while wearing permethrin-treated uniforms. This study aimed at examining the effects of high temperature and a combined high temperature and humid environment on permethrin absorption compared with ambient conditions when wearing a permethrin-treated uniform. Twenty-seven male enlisted soldiers wore study-issued permethrin-treated army uniforms for 33 consecutive hours in three different environments: 1) simulated high temperature (35°C, 40% relative humidity [rh]) ( n = 10), 2) simulated high temperature and humidity (30°C, 70% rh) ( n = 10), and 3) ambient conditions (13°C, 60% rh) ( n = 7). Spot urine samples, collected at 21 scheduled time points before, during, and after wearing the study uniforms, were analyzed for permethrin exposure biomarkers (3-phenoxybenzoic acid, cis - and trans -3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropane-1-carboxylic acid) and creatinine. Biomarker concentrations were 60-90% higher in the heat and combined heat/humidity groups ( P < 0.001-0.022) than the ambient group. Also, the average daily permethrin dose, calculated 12 hours after removing the treated uniforms, was significantly higher in the heat ( P = 0.01) and the heat/humidity ( P = 0.03) groups than the ambient group. There were no significant differences in biomarker concentrations or computed average daily dose between the heat and the heat/humidity groups. Both hot and combined hot and humid environmental conditions significantly increased permethrin absorption in soldiers wearing permethrin-treated uniforms.

Coffman, KE, Luippold, AJ, Salgado, RM, Heavens, KR, Caruso, EM, Fulco, CS, and Kenefick, RW. Aerobic exercise performance during load carriage and acute altitude exposure. J Strength Cond Res 34(4): 946-951, 2020-This study quantified the impact of combined load carriage and acute altitude exposure on 5-km running time-trial (TT) performance and self-selected pacing strategy. Furthermore, this study developed a velocity prediction tool (nomogram) for similar aerobic exercise tasks performed under various combinations of altitude and load stress. Nine volunteers (6M/3F, age: 24 ± 7 years, height: 171 ± 6 cm, body mass: 72 ± 7 kg, and V[Combining Dot Above]O2peak: 50.5 ± 5.2 ml·min·kg) completed a randomized, repeated-measures design protocol. Volunteers performed 3 familiarization (FAM) trials at sea level (SL; 250 m) with no-load carriage. Experimental testing included 3 self-paced, blinded 5-km running TT on a treadmill while carrying a 30% body mass external load at SL, moderate altitude (MA; 2000 m), and high altitude (HA; 3000 m). At SL, load carriage resulted in a 36% decrement in 5-km exercise performance in comparison with FAM trials (43 ± 7 vs. 32 ± 3 minutes; p < 0.001). Time required to complete the 5-km distance while carrying an external load was increased by 11% when performed at HA vs. SL (48 ± 7 vs. 43 ± 7 minutes; p = 0.001). TT pace was not different among experimental conditions (load carriage at SL, MA, and HA) until after 1 km of the running distance had been completed. Heart rate was not different among experimental conditions throughout the entire TT (170 ± 17 b·min). These data quantify the anticipated reduction in aerobic exercise performance under various combinations of acute altitude exposure and load carriage conditions. The self-paced running TT approach used presently allowed for development of an altitude-load nomogram for use in recreational, occupational, or military settings.

Background: The aim of this study was to characterize the time-resolved progression of clinical laboratory disturbances days-following an exertional heat stroke (EHS). Currently, normalization of organ injury clinical biomarker values is the primary indicator of EHS recovery. However, an archetypical biochemical recovery profile following EHS has not been established., Methods: We performed a retrospective analysis of EHS patient records in US military personnel from 2008-2014 using the Military Health System Data Repository (MDR). We focused on commonly reported clinical laboratory analytes measured on the day of injury and all proceeding follow-up visits., Results: Over the prescribed period, there were 2,529 EHS episodes treated at 250 unique treatment locations. Laboratory results, including a standardized set of blood, serum and urine assays, were analyzed from 0-340 days following the initial injury. Indicators of acute kidney injury, including serum electrolyte disturbances and abnormal urinalysis findings, were most prevalent on the day of the injury but normalized within 24-48hours (creatinine, blood urea nitrogen, and blood and protein in urine). Muscle damage and liver function-associated markers peaked 0-4 days after injury and persisted outside their respective reference ranges for 2-16 days (alanine aminotransferase, aspartate aminotransferase, creatine phosphokinase, myoglobin, prothrombin time)., Conclusion: Biochemical recovery from EHS spans a 16-day time course, and markers of end-organ damage exhibit distinct patterns over this period. This analysis underscores the prognostic value of each clinical laboratory analyte and will assist in evaluating EHS patient presentation, injury severity and physiological recovery., Competing Interests: The authors have declared that no competing interests exist.

Purpose: There is growing interest in the measurement of plasma levels of arginine vasopressin (AVP) for the assessment of mild dehydration. However, the principles of biological variation have not been applied to the study of AVP and understanding biological variation of AVP may provide insights regarding measurement thresholds. The purpose of this investigation was to determine the biological variation of AVP in healthy euhydrated individuals to understand the potential for establishing both static and/or change thresholds of importance., Methods: We studied 29 healthy volunteers (24 men and 5 women) while controlling for hydration and pre-analytical factors. All subjects completed between 2-8 trials where biological variation was determined using widely published methods. We determined the intraindividual, interindividual, and analytical coefficients of variation (CV I , CV G , and CV A , respectively) and subsequently the index of individuality and heterogeneity (II and IH, respectively)., Results: AVP did not reach the IH threshold required to be considered useful in the dynamic assessment of physiological deviations from normal. AVP levels approached the II threshold required to be considered useful in the static assessment of physiological deviations from normal., Conclusions: This analysis demonstrates that AVP assessment is unlikely to yield useful information about hydration status.

Acetazolamide (AZ) is a medication commonly used to prevent acute mountain sickness (AMS) during rapid ascent to high altitude. However, it is unclear whether AZ use impairs exercise performance; previous literature regarding this topic is equivocal. The purpose of this study was to evaluate the impact of AZ on time-trial (TT) performance during a 30-h exposure to hypobaric hypoxia equivalent to 3,500-m altitude. Ten men [sea-level peak oxygen consumption (VO 2 peak): 50.8 ± 6.5 mL·kg -1 ·min -1 ; body fat %: 20.6 ± 5.2%] completed 2 30-h exposures at 3,500 m. In a crossover study design, subjects were given 500 mg/day of either AZ or a placebo. Exercise testing was completed 2 h and 24 h after ascent and consisted of 15-min steady-state treadmill walking at 40%-45% sea-level VO 2 peak, followed by a 2-mile self-paced treadmill TT. AMS was assessed after ~12 h and 22 h at 3,500 m. The incidence of AMS decreased from 40% with placebo to 0% with AZ. Oxygen saturation was higher ( P < 0.05) in AZ versus placebo trials at the end of the TT after 2 h (85 ± 3% vs. 79 ± 3%) and 24 h (86 ± 3% vs. 81 ± 4%). There was no difference in time to complete 2 miles between AZ and PL after 2 h (20.7 ± 3.2 vs. 22.7 ± 5.0 min, P > 0.05) or 24 h (21.5 ± 3.4 vs. 21.1 ± 2.9 min, P > 0.05) of exposure to altitude. Our results suggest that AZ (500 mg/day) does not negatively impact endurance exercise performance at 3,500 m. NEW & NOTEWORTHY To our knowledge, this is the first study to examine the impact of acetazolamide (500 mg/day) versus placebo on self-paced, peak-effort exercise performance using a short-duration exercise test in a hypobaric hypoxic environment with a repeated-measures design. In the present study, acetazolamide did not impact exercise performance after 2-h or 24-h exposure to 3,500-m simulated altitude.

Background: The ergogenic effects of supplemental carbohydrate on aerobic exercise performance at high altitude (HA) may be modulated by acclimatization status. Longitudinal evaluation of potential performance benefits of carbohydrate supplementation in the same volunteers before and after acclimatization to HA have not been reported., Purpose: This study examined how consuming carbohydrate affected 2-mile time trial performance in lowlanders at HA (4300 m) before and after acclimatization., Methods: Fourteen unacclimatized men performed 80 min of metabolically-matched (~ 1.7 L/min) treadmill walking at sea level (SL), after ~ 5 h of acute HA exposure, and after 22 days of HA acclimatization and concomitant 40% energy deficit (chronic HA). Before, and every 20 min during walking, participants consumed either carbohydrate (CHO, n = 8; 65.25 g fructose + 79.75 g glucose, 1.8 g carbohydrate/min) or flavor-matched placebo (PLA, n = 6) beverages. A self-paced 2-mile treadmill time trial was performed immediately after completing the 80-min walk., Results: There were no differences (P > 0.05) in time trial duration between CHO and PLA at SL, acute HA, or chronic HA. Time trial duration was longer (P < 0.05) at acute HA (mean ± SD; 27.3 ± 6.3 min) compared to chronic HA (23.6 ± 4.5 min) and SL (17.6 ± 3.6 min); however, time trial duration at chronic HA was still longer than SL (P < 0.05)., Conclusion: These data suggest that carbohydrate supplementation does not enhance aerobic exercise performance in lowlanders acutely exposed or acclimatized to HA., Trial Registration: NCT, NCT02731066, Registered March 292,016.

Exposure to hot environments augments cutaneous vasodilation and sweating during exercise compared with these responses in cooler environments. The effects of hypobaric hypoxia on these responses are less clear, as are the effects of heat and simulated altitude combined. We evaluated the individual and potential additive effects of environmental heat and hypobaric hypoxia on skin blood flow and sweating responses during exercise. Thirteen volunteers (11 M, 2 F; age 25.3 ± 6.1 yr; height 177 ± 9 cm; weight 81.2 ± 16.8 kg) completed 30 min of steady-state (SS) exercise on a cycle ergometer at 50% V̇o 2peak during four separate conditions: 1 ) sea level thermoneutral (SLTN; 250 m, 20°C, 30-50% RH), 2 ) sea level hot (SLH; 250 m, 35°C, 30% RH), 3 ) simulated altitude thermoneutral (ATN; 3,000 m, 20°C, 30-50% RH), and 4 ) simulated altitude hot (AH; 3,000 m, 35°C, 30% RH). Skin blood flow and local sweating rate (LSR) were recorded on the ventral forearm. During exercise, SS cutaneous vascular conductance in AH (63 ± 31% peak) and SLH (52 ± 19% peak) were significantly higher than both SLTN (20 ± 9% peak, P < 0.001) and ATN (25 ± 12% peak, P < 0.05) but were not different from each other ( P > 0.05). SS LSR was similarly increased in the hot environments but unaffected by simulated altitude. We propose that multiple antagonistic mechanisms during exposure to 3,000-m simulated altitude result in no net effect on skin blood flow or sweating responses during exercise in thermoneutral or hot environments.

Purpose: Resting measures of ventilation and gas exchange are impacted by a variety of physiological stressors, such as those resulting from a research intervention or an extreme environment. However, the biological variation of these parameters, an important statistical consideration for identifying a meaningful physiological change, has not been quantified., Methods: We performed a retrospective analysis of 21 studies completed by the U.S. Army Research Institute of Environmental Medicine (USARIEM) from 1985 to present, totaling 411 healthy volunteers. First, we determined the intraindividual, interindividual, and analytic coefficients of variation (CV I , CV G , and CV A , respectively) and subsequently the index of individuality and heterogeneity (II and IH, respectively). Second, when deemed appropriate via these outcomes, we defined the accompanying static and dynamic thresholds, beyond which a significant deviation from normal is indicated., Results: End-tidal partial pressure of oxygen (P ET O 2 ) and the respiratory exchange ratio (RER) approached the II threshold required to be considered useful in the static assessment of physiological deviations from normal. P ET O 2 and peripheral oxygen saturation (SpO 2 ) approached the IH threshold required to be considered useful in the dynamic assessment of physiological deviations from normal., Conclusions: This analysis identifies RER and P ET O 2 as parameters that might be most useful when aiming to identify a meaningful ventilatory change following a research intervention or stressor. Alternatively, other parameters of ventilation and gas exchange, such as P ET CO 2 and V E , may be less useful for observing an anticipated physiological change.

Introduction: Precipitating factors that contribute to the severity of exertional heat stroke (EHS) are unclear. The purpose of this study was to determine the effect of prior illness (PI) on EHS severity., Methods: We performed a retrospective clinical record review of 179 documented cases of EHS at the Marine Corps Base in Quantico, Virginia., Results: Approximately 30% of EHS cases had a medically documented PI. Anthropometrics (height, weight, body mass index) and commonly associated risk factors for EHS (age, number of days in training, wet bulb globe temperature, sleep patterns) did not differ between PI and no illness (NI) groups. PI patients presented with higher maximal rectal core temperatures (40.6 ± 1.0°C vs. 40.3 ± 1.2°C; P = 0.0419), and elevated pulse rates (118.1 ± 16.7 bpm vs. 110.5 ± 24.2 bpm; P = 0.0397). At the point of care, biomarker values were similar between PI and NI groups, with the exception of a trend toward elevated monocytes in those with PI (7.9 ± 2.9% vs 6.7± 2.7%; P = 0.0521). Rate and duration of cooling were similar between PI and NI patients., Conclusion: This study indicates that PI has a minimal effect on the patient presentation, severity and treatment outcome of EHS. The results of this study have important implications for military, civilian, and occupational populations who are at risk for EHS., Competing Interests: The authors have declared that no competing interests exist.

Heat and hypobaric hypoxia independently compromise exercise performance; however, their combined impact on exercise performance has yet to be quantified. This study examined the effects of heat, hypobaric hypoxia, and the combination of these environments on self-paced cycling time trial (TT) performance. Twelve subjects [2 female, 10 male; sea level (SL) peak oxygen consumption (V̇o 2peak ), 41.5 ± 4.4 mL·kg -1 ·min -1 , mean ± SD] completed 30 min of steady-state cycling exercise (50% SL V̇o 2peak ), followed by a 15-min self-paced TT in four environmental conditions: SL thermoneutral [SLTN; 250 m, 20°C, 30-50% relative humidity (rh)], SL hot (SLH; 250 m, 35°C, 30% rh), hypobaric hypoxia thermoneutral (HTN; 3,000 m, 20°C, 30-50% rh), and hypobaric hypoxia hot (HH; 3,000 m, 35°C, 30% rh). Performance was assessed by the total work (kJ) completed. TT performance was lower ( P < 0.05) in SLH, HTN, and HH relative to SLTN (-15.4 ± 9.7, -24.1 ± 16.2, and -33.1 ± 13.4 kJ, respectively). Additionally, the total work completed in HTN and HH was lower ( P < 0.05) than that in SLH. In SLH, HTN, and HH, work rate was reduced versus SLTN ( P < 0.05) within the first 3 min of exercise and was consistent for the remainder of the bout. No differences ( P > 0.05) existed for heart rate or Ratings of Perceived Exertion at the end of exercise among conditions. The decrease in self-paced TT performance in the heat and/or hypobaric hypoxia conditions compared with SLTN conditions resulted from a nearly immediate reduction in work rate that may have been regulated by environmentally induced changes in physiological strain and perception of effort in response to TT exercise. NEW & NOTEWORTHY This is the first known study to examine the combined effects of heat and hypobaric hypoxia on short-duration self-paced cycling time trial performance. Regardless of environmental condition, subjects utilized an even work rate for the entire duration of the time trial. The presence of both environmental stressors led to a greater performance impairment than heat or hypobaric hypoxia alone, and the performance decrement stemmed from an early reduction of work rate.

Oral rehydration solutions (ORS) are specifically formulated with an osmolality to optimize fluid absorption. However, it is unclear how many ORS products comply with current World Health Organization (WHO) osmolality guidelines and the osmotic shelf-life stability is not known. Therefore, the purpose of this investigation was to examine the within and between ORS product osmolality variation in both pre-mixed and reconstituted powders. Additionally, the osmotic stability was examined over time. The osmolality of five different pre-mixed solutions and six powdered ORS products were measured. Pre-mixed solutions were stored at room temperatures and elevated temperatures (31 °C) for two months to examine osmotic shelf stability. Results demonstrated that only one pre-mixed ORS product was in compliance with the current guidelines both before and after the prolonged storage. Five of the six powdered ORS products were in compliance with minimal inter-packet variation observed within the given formulations. This investigation demonstrates that many commercially available pre-mixed ORS products do not currently adhere to the WHO recommended osmolality guidelines. Additionally, due to the presence of particular sugars and possibly other ingredients, the shelf-life stability of osmolality for certain ORS products may be questioned. These findings should be carefully considered in the design of future ORS products.

Background: The beverage hydration index (BHI) is a composite measure of fluid balance after consuming a test beverage relative to water. BHI is a relatively new measure that has been explored in young, but not yet older, adults., Objective: The aim of this study was to investigate potential differences in BHI between euhydrated younger and older adults after drinking 4 different commercial beverages. We hypothesized that 1) older subjects would remain in positive fluid balance longer than young subjects after ingestion of each test beverage due to decreased urinary excretion rates, 2) glucose (glu)- and amino acid (AA)-based hydration beverages with sodium would have a BHI greater than water in both groups, and 3) the traditional 2-h postingestion BHI may be inappropriate for older adults., Methods: On 5 separate visits, 12 young (23 ± 3 yr, 7 M/5F) and 12 older (67 ± 6 yr, 5 M/7F) subjects consumed 1 L of distilled water, G-20 (6% CHO, 20 mmol/L Na+), G-45 (2.5% CHO, 45 mmol/L Na+), AA-30 (5 AAs, 30 mmol/L Na+), or AA-60 (8 AAs, 60 mmol/L Na+) over 30 min. Blood and urine samples were collected before ingestion and at 0, 60, 120, 180, and 240 min postingestion with additional venous blood sampling at 5, 10, 15, and 30 min postingestion., Results: In young subjects, BHI increased with increasing beverage Na+ concentration, and AA-60 had the highest BHI (AA-60 = 1.24 ± 0.10 compared with water = 1.00, P = 0.01). For older subjects, BHI was highest in AA-30 (AA-30; 1.20 ± 0.13 compared with water, P < 0.01) and was still in flux beyond 2 h in AA-60 (P < 0.05)., Conclusions: Beverage Na+ content progressively increased BHI in young adults independent of glucose or AA content. For older adults, the AA-30 beverage had the highest BHI. A 4-h BHI may be more appropriate for older adults due to attenuated urine excretion rates. This trial was registered at clinicaltrials.gov as NCT03559101., (Copyright © American Society for Nutrition 2019.)

Aging is associated with altered water, electrolyte, and glucose handling. Alternative beverages to those containing carbohydrate (CHO) should be considered for older adults. We hypothesized that reduced sodium (CNa + ) and/or water (CH 2 O) clearance would underlie greater beverage retention in older compared with young adults, secondary to reduced glomerular filtration rate (GFR). We further hypothesized that amino acid (AA)- and CHO-based beverages would promote retention better than water. Over five visits, 12 young (23 ± 3 yr; 7 men, 5 women) and 12 older (67 ± 6 yr; 5 men, 7 women) subjects consumed 1 liter of distilled water or beverages with 6% CHO, 0.46 g/l Na + [Gatorade (GAT)]; 2.5% CHO, 0.74 g/l Na + [Pedialyte (PED)]; 5 AA, 1.04 g/l Na + [enterade (ENT)-5]; or 8 AA, 1.38 g/l Na + (ENT-8) over 30 min. Blood and urine were collected every hour for 4 h after ingestion; retention, CH 2 O, and CNa + were calculated at 2 and 4 h. Additional calculations adjusted CH 2 O and CNa + for estimated GFR (eGFR). Water yielded the lowest retention in both groups ( P ≤ 0.02). Retention was higher in older vs. young adults except for ENT-8 at 4 h ( P = 0.73). CH 2 O was higher for older vs. young adults for GAT at 2 h ( P < 0.01) and GAT and PED at 4 h ( P < 0.01) after ingestion but was otherwise similar between groups. CNa + was lower in older vs. young adults except for ENT-8 ( P ≥ 0.19). Adjusting for eGFR resulted in higher CH 2 O for all beverages in older vs. young adults ( P < 0.05) but did not influence CNa + . Older adults may better retain beverages with less Na + than young adults because of reduced CNa + . AA- and CHO-based electrolyte-rich beverages may similarly promote beverage retention. NEW & NOTEWORTHY Commercially available amino acid (AA)-containing beverages may provide an alternative to traditional carbohydrate (CHO)-containing beverages, particularly for older adults with attenuated water, electrolyte, and glucose handling. We compared beverage retention and free water and sodium clearance between young and older adults after ingestion of water, two CHO-based beverages, and two AA-based beverages. Our data suggest that older adults better retain beverages with less sodium compared with young adults and that AA-based and CHO-based electrolyte-containing beverages similarly promote retention.

Purpose: To determine the efficacy residing for 2 d at various altitudes while sedentary (S) or active (A; ~90 min hiking 2 d) on exercise performance at 4300 m., Methods: Sea-level (SL) resident men (n = 45) and women (n = 21) (mean ± SD; 23 ± 5 yr; 173 ± 9 cm; 73 ± 12 kg; V˙O2peak = 49 ± 7 mL·kg·min) were randomly assigned to a residence group and, S or A within each group: 2500 m (n = 11S, 8A), 3000 m (n = 6S, 12A), 3500 m (n = 6S, 8A), or 4300 m (n = 7S, 8A). Exercise assessments occurred at SL and 4300 m after 2-d residence and consisted of 20 min of steady-state (SS) treadmill walking (45% ± 3% SL V˙O2peak) and a 5-mile, self-paced running time trial (TT). Arterial oxygen saturation (SpO2) and HR were recorded throughout exercise. Resting SpO2 was recorded at SL, at 4 and 46 h of residence, and at 4300 m before exercise assessment. To determine if 2-d altitude residence improved 4300 m TT performance, results were compared with estimated performances using a validated prediction model., Results: For all groups, resting SpO2 was reduced (P < 0.01) after 4 h of residence relative to SL inversely to the elevation and did not improve after 46 h. Resting SpO2 (~83%) did not differ among groups at 4300 m. Although SL and 4300 m SS exercise SpO2 (97% ± 2% to 74% ± 4%), HR (123 ± 10 bpm to 140 ± 12 bpm) and TT duration (51 ± 9 to 73 ± 16 min) were different (P < 0.01), responses at 4300 m were similar among all groups, as was actual and predicted 4300 m TT performances (74 ± 12 min)., Conclusions: Residing for 2 d at 2500 to 4300 m, with or without daily activity, did not improve resting SpO2, SS exercise responses, or TT performance at 4300 m.

Background: Small sample volumes may artificially elevate plasma osmolality (Posm) measured by freezing point depression. The purpose of this study was to compare two widely different sample volumes of measured Posm (mmol/kg) to each other, and to calculated osmolarity (mmol/L), across a physiological Posm range (~50 mmol/kg)., Methods: Posm was measured using freezing point depression and osmolarity calculated from measures of sodium, glucose, and blood urea nitrogen. The influence of sample volume was investigated by comparing 20 and 250 μL Posm samples (n = 126 pairs). Thirty-two volunteers were tested multiple times while EUH (n = 115) or DEH (n = 11) by -4.0% body mass. Protinol™ (240, 280, and 320 mmol/kg) and Clinitrol™ (290 mmol/kg) reference solutions were compared similarly (n = 282 pairs)., Results: The 20 μL samples of plasma showed a 7 mmol/kg positive bias compared to 250 μL samples and displayed a nearly constant proportional error across the range tested (slope = 0.929). Calculated osmolarity was lower than 20 μL Posm by the same negative bias (-6.9 mmol/kg) but not different from 250 μL Posm (0.1 mmol/kg). The differences between 20 and 250 μL samples of Protinol™ were significantly higher than Clinitrol™., Conclusions: These results demonstrate that Posm measured by freezing point depression will be ~7 mmol/kg higher when using 20 μL vs 250 μL sample volumes. Approximately half of this effect may be due to plasma proteins. Posm sample volume should be carefully considered when calculating the osmole gap or assessing hydration status., (© 2018 The Authors Journal of Clinical Laboratory Analysis Published by Wiley Periodicals, Inc.)

Exercise or work in hot environments increases susceptibility to exertional heat illnesses such as exertional heat stroke (EHS). EHS occurs when body heat gain exceeds body heat dissipation, resulting in rapid body heat storage and potentially life-threatening consequences. EHS poses a dangerous threat for athletes, agriculture workers, and military personnel, as they are often exposed to hot environmental conditions that restrict body heat loss or contribute to body heat gain. Currently, there is limited guidance on return to activity (RTA) after an episode of EHS. While examining biomarkers in the blood is thought to be beneficial for determining RTA, they are not sensitive or specific enough to be a final determining factor as organ damage may persist despite blood biomarkers returning to baseline levels. As such, additional assessment tests to more accurately determine RTA are desired. One method used for determining RTA is the heat tolerance test (HTT, 120 minutes treadmill walking; 40°C, 40% relative humidity). Unfortunately, the HTT provides even less information about EHS recovery since it offers no test sensitivity or specificity even after years of implementation. We provide an overview of the HTT and the controversy of this test with respect to assessment criteria, applicability to tasks involving high metabolic workloads, and the lack of follow-up analyses to determine its accuracy for determining recovery in order to diminish the likelihood of a second EHS occurrence.

Objective: To determine whether 2 days of staging at 2500-3500 m, combined with either high or low physical activity, reduces acute mountain sickness (AMS) during subsequent ascent to 4300 m., Methods: Three independent groups of unacclimatized men and women were staged for 2 days at either 2500 m (n = 18), 3000 m (n = 16), or 3500 m (n = 15) before ascending and living for 2 days at 4300 m and compared with a control group that directly ascended to 4300 m (n = 12). All individuals departed to the staging altitudes or 4300 m after spending one night at 2000 m during which they breathed supplemental oxygen to simulate sea level conditions. Half in each group participated in ∼3 hours of daily physical activity while half were sedentary. Women accounted for ∼25% of each group. AMS incidence was assessed using the Environmental Symptoms Questionnaire. AMS was classified as mild (≥0.7 and <1.5), moderate (≥1.5 and <2.6), and severe (≥2.6)., Results: While staging, the incidence of AMS was lower (p < 0.001) in the 2500 m (0%), 3000 m (13%), and 3500 m (40%) staged groups than the direct ascent control group (83%). After ascent to 4300 m, the incidence of AMS was lower in the 3000 m (43%) and 3500 m (40%) groups than the 2500 m group (67%) and direct ascent control (83%). Neither activity level nor sex influenced the incidence of AMS during further ascent to 4300 m., Conclusions: Two days of staging at either 3000 or 3500 m, with or without physical activity, reduced AMS during subsequent ascent to 4300 m but staging at 3000 m may be recommended because of less incidence of AMS.

Background: The efficacy of different commercial beverage compositions for meeting oral rehydration therapy (ORT) goals in the treatment of acute dehydration in healthy humans has not been systematically tested. The objective of the study was to compare fluid retention, plasma volume (PV), and interstitial fluid (ISF) volume restoration when using 1 popular glucose-based and 1 novel amino acid-based (AA) commercial ORT beverage following experimental hypertonic or isotonic dehydration., Methods: Twenty-six healthy adults (21 males, 5 females) underwent either a controlled bout of hypertonic (n = 13) or isotonic (n = 13) dehydration (3%-4% body mass) via eccrine or renal body water and electrolyte losses induced using exercise-heat stress (EHS) or Lasix administration (LAS), respectively. Rehydration was achieved over 90 minutes by matching fluid intake to water losses (1:1) using a sports drink (SP) or AA commercial ORT beverage. Fluid retention (water and electrolytes), PV, and ISF volume changes were tracked for 180 minutes., Results: AA produced significantly (P <0.05) greater fluid retention (75% vs 57%), ISF volume restoration, and tended (P = 0.06) to produce greater PV restoration in trial EHS. In trial LAS, neither beverage exceeded 65% retention, but AA replaced electrolytes and preserved ISF volume better than SP (P <0.05)., Conclusion: The results of this study demonstrate superior rehydration when using AA compared with SP for both hypertonic and isotonic dehydration., (© 2018 American Society for Parenteral and Enteral Nutrition.)

Background: Fluid intake during military training is prescribed based on the interactions among work rates, environmental conditions, and uniform configurations. The efficacy of this guidance has not been empirically assessed in over a decade. To determine the acceptability of the fluid intake guidance, sweat losses were measured in a variety of conditions with modern uniform/body armor configurations and were then compared to prescribed fluid intakes for each condition (workload, environment, clothing)., Methods: Whole body sweat losses of 324 Soldiers and 14 model simulations were measured under a variety of work intensities ((Watts) easy, moderate, hard), work durations (2-25 h), environmental conditions (White-Black flag), and uniform configurations (including Army Combat Uniform and body armor). Whole body sweat losses were then calculated relative to 4 h drinking guidance and in accordance with TB MED 507 recommended work/rest ratios. The differences between the prescribed fluid intake and sweat loss were calculated and expressed as a percent loss or gain of body weight. Values within a threshold of ±2% body water flux (BWF) were deemed an acceptable conservative starting point for performance and health concerns., Findings: Values within ±2% BWF numbered 309/338; 25 of 338 observations exceeded the +2% BWF while 4 of 338 observations exceeded the -2% BWF. When total fluid restriction was simulated, all experienced body weight loss with 151 of 338 observations exceeding the -2% BWF., Discussion: When calculated using actual measured sweating rates from the laboratory and model simulations, current fluid intake guidance appears to predict with 91.4% accuracy the volume of fluid required to maintain a proper euhydrated state (±2%) during 4 h of exercise. Simulations of total fluid restriction support the necessity for fluid intake guidance so that the Warfighter's performance does not degrade. It is recommended that the current military fluid intake guidance focuses on methods for accurately tracking fluid intakes., (Published by Oxford University Press on behalf of Association of Military Surgeons of the United States 2018.)

This study investigated how high-altitude (HA, 4300 m) acclimatization affected exogenous glucose oxidation during aerobic exercise. Sea-level (SL) residents ( n = 14 men) performed 80-min, metabolically matched exercise ( V ˙ O 2 ∼ 1.7 L/min) at SL and at HA < 5 h after arrival (acute HA, AHA) and following 22-d of HA acclimatization (chronic HA, CHA). During HA acclimatization, participants sustained a controlled negative energy balance (-40%) to simulate the "real world" conditions that lowlanders typically experience during HA sojourns. During exercise, participants consumed carbohydrate (CHO, n = 8, 65.25 g fructose + 79.75 g glucose, 1.8 g carbohydrate/min) or placebo (PLA, n = 6). Total carbohydrate oxidation was determined by indirect calorimetry and exogenous glucose oxidation by tracer technique with 13 C. Participants lost ( P ≤ 0.05, mean ± SD) 7.9 ± 1.9 kg body mass during the HA acclimatization and energy deficit period. In CHO, total exogenous glucose oxidized during the final 40 min of exercise was lower ( P < 0.01) at AHA (7.4 ± 3.7 g) than SL (15.3 ± 2.2 g) and CHA (12.4 ± 2.3 g), but there were no differences between SL and CHA. Blood glucose and insulin increased ( P ≤ 0.05) during the first 20 min of exercise in CHO, but not PLA. In CHO, glucose declined to pre-exercise concentrations as exercise continued at SL, but remained elevated ( P ≤ 0.05) throughout exercise at AHA and CHA. Insulin increased during exercise in CHO, but the increase was greater ( P ≤ 0.05) at AHA than at SL and CHA, which did not differ. Thus, while acute hypoxia suppressed exogenous glucose oxidation during steady-state aerobic exercise, that hypoxic suppression is alleviated following altitude acclimatization and concomitant negative energy balance.

Carlson, LA, Lawrence, MA, and Kenefick, RW. Hydration status and thermoregulatory responses in drivers during competitive racing. J Strength Cond Res 32(7): 2061-2065, 2018-Stock car drivers are exposed to high ambient temperatures, further complicated by the fact that they are encapsulated in protective clothing; however, the hydration status of these drivers has not been determined. This study quantified the degree of fluid losses during a competitive event in hot conditions. Nine male stock car drivers (29.6 ± 9.4 years, 177.8 ± 3.0 cm, 81.5 ± 18.5 kg) were studied during a Pro Series Division NASCAR race. Sweat rate (SR) and dehydration was determined through nude body weights (BWs). Prerace BW was 81.5 ± 18.5 kg and decreased to 81.1 ± 18.5 kg after race (p = 0.001). Body weight loss after race was 0.77 ± 0.3% and mean SR was 0.63 ± 0.4 L·h. Intestinal core temperature increased from 38.0 ± 0.4 to 38.5 ± 0.4° C after race (p = 0.001). Skin temperature increased from 35.8 ± 0.8 to 36.9 ± 0.8° C after race (p = 0.001), whereas the core-to-skin temperature gradient narrowed from 2.2 ± 0.9 to 1.6 ± 0.9° C, before race to after race (p = 0.001). Heart rates after race were 89 ± 0.0% of the drivers' age-predicted maximum heart rate (HR). Fluid losses during competitive racing can be significant. Without a fluid replacement strategy, fluid losses may exceed 3% of BW and could negatively impact driving performance in longer races.

Background: The beverage hydration index (BHI) assesses the hydration potential of any consumable fluid relative to water. The BHI is a relatively new metric, and the impact of body mass, sex, and reproducibility has yet to be investigated., Objectives: To assess the independent impact of body mass and sex on BHI using beverages not previously assessed, including an amino acid-based oral rehydration solution (AA-ORS), a glucose-containing ORS (G-ORS), and a sports drink (SpD), compared with water (control). The reproducibility of the results was examined using statistical modeling (bootstrap analysis)., Design: Using a repeated-measures design, 40 euhydrated and fasted subjects (17 male, 23 female; urine specific gravity <1.025) were studied on 4 separate occasions. During each trial, subjects ingested 1 L of each beverage, and urine output was measured immediately postingestion and at 1-h intervals for the next 4 h. The BHI was calculated as a ratio of each individual's cumulative urine output after drinking 1 L of water over their cumulative urine output after drinking each of the test beverages., Results: The calculated mean ± SD BHIs of the beverages were as follows: water (1.0 ± 0.0), AA-ORS (1.15 ± 0.28), G-ORS (1.21 ± 0.28), and SpD (1.09 ± 0.26). The BHI for both AA-ORS and G-ORS was greater than that for water (P < 0.05). Despite overall differences in body mass, neither body mass nor sex independently affected BHI. Based upon statistical modeling, our results demonstrate excellent reproducibility of outcomes and external validity., Conclusions: Our results suggest that the BHI may be used and interpreted with confidence independently of body mass or sex. Furthermore, a novel carbohydrate-free AA-ORS and a traditional commercially available G-ORS were superior to water in optimizing hydration, whereas SpD was not. This trial was registered at clinicaltrials.gov as NCT03262597.

In humans, thirst tends to be alleviated before complete rehydration is achieved. When sweating rates are high and ad libitum fluid consumption is not sufficient to replace sweat losses, a cumulative loss in body water results. Body mass losses of 2% or greater take time to accumulate. Dehydration of ≥  2% body mass is associated with impaired thermoregulatory function, elevated cardiovascular strain and, in many conditions (e.g., warmer, longer, more intense), impaired aerobic exercise performance. Circumstances where planned drinking is optimal include longer duration activities of >  90 min, particularly in the heat; higher-intensity exercise with high sweat rates; exercise where performance is a concern; and when carbohydrate intake of 1 g/min is desired. Individuals with high sweat rates and/or those concerned with exercise performance should determine sweat rates under conditions (exercise intensity, pace) and environments similar to that anticipated when competing and tailor drinking to prevent body mass losses >  2%. Circumstances where drinking to thirst may be sufficient include short duration exercise of <  1 h to 90 min; exercise in cooler conditions; and lower-intensity exercise. It is recommended to never drink so much that weight is gained.