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ObjectiveTo compare hyperthermia and physiological dehydration risk during exercise heat stress between children of different ages and adults and evaluate an existing adult sweat rate calculator in children.Methods68 fit and recreationally active children aged 10–16 years (31 girls), and 24 adults aged 18–40 years (11 females) completed three separate 45 min treadmill walking/running trials at different intensities on different days at 30°C, 40% relative humidity (RH) (WARM) or 40°C, 30% RH (HOT). Exposures were randomised to elicit intensities scaled to (1) fitness, (2) mass and (3) surface area. Core (gastrointestinal (Tgi)) temperature was measured continuously and dehydration determined using body mass changes.ResultsExcept for 60% V̇O2peak in WARM, in which adults exhibited a greater Tgi rise compared with 10–13 years, there was no effect of age on Tgi during exercise (p≥0.176). Physiological rates of dehydration were not affected by age in WARM (p≥0.08) or HOT (p≥0.08). Mean predicted sweat rate error was +0.08 kg/hour (95% CIs: −0.10, +0.25) across WARM and HOT, and 80.5% of variability in sweating was explained by the adult sweat rate calculator.ConclusionsUsing the most comprehensive paediatric exercise heat stress dataset from a single study to date, we show that children aged 10–16 years are at a similar risk of hyperthermia and dehydration as adults during exercise up to 40°C. This supports recent changes to paediatric sport heat policies that were based on limited data. Practitioners can potentially reduce behavioural dehydration risks from inadequate fluid consumption using an existing adult sweat rate calculator for children.

Water accounts for 75% of brain mass. Associations may exist between hydration and cognitive performance. The objective of this study was to investigate the effects of dehydration and rehydration on cognitive performance and mood. In this self-control trial, 12 men were recruited from a medical college in Cangzhou, China. After 12 h of overnight fasting, the participants took baseline tests at 8:00 AM on day 2. First morning urine and blood osmolality were analyzed to determine hydration state. Height, weight, and blood pressure were measured following standardized procedures. A visual analog scale for the subjective sensation of thirst was applied, and a profile of mood states questionnaire was applied. Tests were conducted for cognitive performance, including a test of digit span forward and backward, digit-symbol substitutions, dose-work, and stroop effects. Participants were required not to drink water for 36 h but were given three meals on day 3. On day 4, the same indexes were tested as a baseline test. At 8:30 AM, participants drank 1500 mL of purified water over 15 min. After a 1 h interval, the same measurements were performed. Compared with baseline test results, during the dehydration test, participants had lower scores of vigor (11.9 vs. 8.8, %, p = 0.007) and esteem-related affect (8.2 vs. 5.7, %, p = 0.006), lower total scores of digit span (14.3 vs. 13.3, %, p = 0.004), and higher error rates for dose-work (0.01 vs. 0.16, %, p = 0.005). Compared with the dehydration test scores, rehydration test scores showed that fatigue (4.3 vs. 2.1, %, p = 0.005) and total mood disturbance (TMD) (99.0 vs. 90.2, %, p = 0.008) improved, and scores of forward, backward, and total digit span increased (7.7 vs. 8.6, p = 0.014; 5.7 vs. 1.2, p = 0.019; 13.3 vs. 15.4, p = 0.001). Increases were also noted in correct number of digit symbol substitutions, reading speed, and mental work ability (70.8 vs. 75.4, p < 0.001; 339.3 vs. 486.4, n/min, p < 0.001; 356.1 vs. 450.2, p < 0.001), and reaction time decreased (30.2 vs. 28.7, s, p = 0.002). As a conclusion, dehydration had negative effects on vigor, esteem-related affect, short-term memory, and attention. Rehydration after water supplementation alleviated fatigue and improved TMD, short-term memory, attention, and reaction.

Purpose The effects of low-intensity exercise, heat-induced hypo-hydration and rehydration on maximal strength and the underlying neurophysiological mechanisms are not well understood. Methods To assess this, 12 participants took part in a randomised crossover study, in a prolonged (3 h) submaximal (60 W) cycling protocol under 3 conditions: (i) in 45 °C (achieving ~ 5% body mass reduction), with post-exercise rehydration in 2 h (RHY2), (ii) with rehydration across 24 h (RHY24), and (iii) a euhydrated trial in 25 °C (CON). Dependent variables included maximal voluntary contractions (MVC), maximum motor unit potential (M MAX ), motor evoked potential (MEP RAW ) amplitude and cortical silent period (cSP) duration. Blood–brain-barrier integrity was also assessed by serum Ubiquitin Carboxyl-terminal Hydrolase (UCH-L1) concentrations. All measures were obtained immediately pre, post, post 2 h and 24 h. Results During both dehydration trials, MVC (RHY2: p  < 0.001, RHY24: p  = 0.001) and MEP RAW (RHY2: p  = 0.025, RHY24: p  = 0.045) decreased from pre- to post-exercise. MEP RAW returned to baseline during RHY2 and CON, but not RHY24 ( p  = 0.020). MEP/M MAX ratio decreased across time for all trials ( p  = 0.009) and returned to baseline, except RHY24 ( p  < 0.026). Increased cSP ( p  = 0.011) was observed during CON post-exercise, but not during RHY2 and RHY24. Serum UCH-L1 increased across time for all conditions ( p  < 0.001) but was not significantly different between conditions. Conclusion Our findings demonstrate an increase in corticospinal inhibition after exercise with fluid ingestion, but a decrease in corticospinal excitability after heat-induced hypo-hydration. In addition, low-intensity exercise increases peripheral markers of blood–brain-barrier permeability. Graphical abstract The mechanisms and time-course of change in neuromuscular function after intracellular dehydration and subsequent rehydration, are not well understood. In this present study, twelve healthy participants underwent a control trial (CON) and two experimental trials in heat (45 °C, 45% relative humidity [RH]) to achieve a 5% reduction in body mass via dehydration and low-intensity cycling (60 W), then rehydrated rapidly (RHY2) or progressively (RHY24). Participants underwent various measures of transcranial magnetic stimulation (TMS) and peripheral motor nerve stimulation (MNS) before (PRE), after core temperature (CT) returned to baseline (POST), and after 2 h and 24 h. These measures included: motor evoked potential amplitude (MEP), TMS-evoked cortical silent period (cSP), compound muscle action potential (M MAX ), voluntary activation (VA), potentiated twitch (Qtw,pot), maximal voluntary contractions (MVC) and serum Ubiquitin carboxyl-terminal hydrolase (UCH-L1). The novel finding was a different corticospinal response to low-intensity exercise (i.e., higher corticospinal inhibition when hydrated) and rehydration strategies (i.e., lower corticospinal excitability after gradual rehydration).

Background/Objectives: Acute dehydration, commonly induced through fluid restriction and/or excessive sweating, is a common weight-cutting strategy among combat sport athletes. However, its impact on muscle strength and power remains a concern. The aim of the study was to evaluate the impact of 2% body mass reduction via dehydration on lower-limb strength and power in elite karate athletes. Methods: Fourteen male elite karate athletes completed two conditions: euhydrated (EUH) and dehydrated (DEH) (−2% body mass via 24-h fluid restriction). Performance was assessed using squat jump (SJ) and countermovement jump (CMJ) tests, along with isokinetic knee flexion and extension at 60, 180, and 300°/s. Results: Dehydration significantly reduced squat jump height (37.19 ± 3.69 vs. 39.34 ± 5.08 cm (EUH), p = 0.04), power output (2188.2 ± 307.2 vs. 2351.1 ± 347.2 W (EUH), p = 0.001), and knee extension and flexion strength at 60°/s (p = 0.018). CMJ height and higher-velocity knee flexion/extension were unaffected (p > 0.05). Conclusions: Acute dehydration impairs lower-body maximal force production at low velocities but has no significant effect on high velocity movements. Athletes and coaches should carefully manage hydration strategies when “cutting weight” to avoid any negative performance effects.

PurposeWhilst there is evidence to suggest that hypohydration caused by physical work in the heat increases renal injury, whether this is the case during exercise in temperate conditions remains unknown. This study investigated the effect of manipulating hydration status during high-intensity intermittent running on biomarkers of renal injury.MethodsAfter familiarisation, 14 males (age: 33 ± 7 years; V̇O2peak: 57.1 ± 8.6 ml/kg/min; mean ± SD) completed 2 trials in a randomised cross-over design, each involving 6, 15 min blocks of shuttle running (modified Loughborough Intermittent Shuttle Test protocol) in temperate conditions (22.3 ± 1.0 °C; 47.9 ± 12.9% relative humidity). During exercise, subjects consumed either a volume of water equal to 90% of sweat losses (EU) or 75 mL water (HYP). Body mass, blood and urine samples were taken pre-exercise (baseline/pre), 30 min post-exercise (post) and 24 h post-baseline (24 h).ResultsPost-exercise, body mass loss, serum osmolality and urine osmolality were greater in HYP than EU (P ≤ 0.024). Osmolality-corrected urinary kidney injury molecule-1 (uKIM-1) concentrations were increased post-exercise (P ≤ 0.048), with greater concentrations in HYP than EU (HYP: 2.76 [1.72–4.65] ng/mOsm; EU: 1.94 [1.1–2.54] ng/mOsm; P = 0.003; median [interquartile range]). Osmolality-corrected urinary neutrophil gelatinase-associated lipocalin (uNGAL) concentrations were increased post-exercise (P < 0.001), but there was no trial by time interaction effect (P = 0.073).ConclusionThese results suggest that hypohydration produced by high-intensity intermittent running increases renal injury, compared to when euhydration is maintained, and that the site of this increased renal injury is at the proximal tubules.

Purpose Hypohydration has been suggested as a predisposing factor for several pathologies including cardiovascular diseases (CVD). While CVD are the leading cause of death worldwide, no study has investigated whether acute hypohydration affects endothelial function and cardiovascular function. Methods Ten young, healthy males participated in this crossover study (age: 24.3 ± 2.3 year; weight: 80.8 ± 5.3 kg; BMI: 24.3 ± 0.4 kg m −2 ). Each subject completed two measurements of endothelial function by flow-mediated dilation (FMD) in euhydrated and hypohydrated state separated by 24 h. Following baseline assessment of hydration status and FMD, the subjects completed 100 min of low-intensity intermittent walking exercise to achieve hypohydration of −2 % of individual body mass. For the rest of the day, a standardized, low water content diet was provided. The following morning, hydration markers and endothelial function were recorded. Results Hypohydration by −1.9 ± 0.1 % of body mass resulted in decreased plasma volume by −3.5 ± 1.8 % and increased plasma osmolality by 9 ± 2 mmol kg −1 ( P  < 0.001). FMD as a response to hypohydration decreased by −26.8 ± 3.9 % ( P  < 0.05). Conclusion The data suggested that a small degree of hypohydration induced by moderate exercise and fluid restriction significantly impaired endothelial function.

Purpose 7 days L-citrulline supplementation has been reported to improve blood pressure, V . O 2 kinetics, gastrointestinal (GI) perfusion and endurance cycling performance through increasing arterial blood flow. In situations where blood volume is compromised (e.g., hyperthermia/hypohydration), L-citrulline may improve thermoregulation and exercise performance by redistributing blood flow to aid heat loss and/or muscle function. This study assessed 7 days L-citrulline supplementation on running performance in the heat, whilst mildly hypohydrated. Methods 13 endurance runners (2 female, 31 ± 8 y, V . O 2 peak 60 ± 6 mL/kg/min) participated in a randomised crossover study with 7 days L-citrulline (CIT; 6 g/d) or placebo (maltodextrin powder; PLA) supplementation. Participants completed a 50 min running ‘preload’ at 65% V . O 2 peak (32 °C, 50% relative humidity) to induce hyperthermia and hypohydration before a 3 km running time trial (TT). Body mass and blood samples were collected at baseline, pre-preload, post-preload and post-TT, whilst core and skin temperature, heart rate and perceptual responses were collected periodically throughout. Results TT performance was not different between trials (CIT 865 ± 142 s; PLA 892 ± 154 s; P  = 0.437). Core and skin temperature and heart rate ( P  ≥ 0.270), hydration (sweat rate, plasma volume, osmolality) indices ( P  ≥ 0.216), GI damage ( P  ≥ 0.260) and perceptual responses ( P  ≥ 0.610) were not different between trials during the preload and TT. Conclusions 7 days of L-citrulline supplementation had no effect on 3 km running performance in the heat or any effects on thermoregulation or GI damage in trained runners in a hypohydrated state.

The purpose of this study was to examine the perception of thirst as a marker of hydration status following prolonged exercise in the heat. Twelve men (mean ± SD; age, 23 ± 4 y; body mass, 81.4 ± 9.9 kg; height, 182 ± 9 cm; body fat, 14.3% ± 4.7%) completed two 180 min bouts of exercise on a motorized treadmill in a hot environment (35.2 ± 0.6 °C; RH, 30.0 ± 5.4%), followed by a 60 min recovery period. Participants completed a euhydrated (EUH) and hypohydrated (HYPO) trial. During recovery, participants were randomly assigned to either fluid replacement (EUHFL and HYPOFL; 10 min ad libitum consumption) or no fluid replacement (EUHNF and HYPONF). Thirst was measured using both a nine-point scale and separate visual analog scales. The percent of body mass loss (%BML) was significantly greater immediately post exercise in HYPO (HYPOFL, 3.0% ± 1.2%; HYPONF, 2.6% ± 0.6%) compared to EUH (EUHFL, 0.2% ± 0.7%; EUHNF, 0.6% ± 0.5%) trials (p < 0.001). Following recovery, there were no differences in %BML between HYPOFL and HYPONF (p > 0.05) or between EUHFL and EUHNF (p > 0.05). Beginning at minute 5 during the recovery period, thirst perception was significantly greater in HYPONF than EUHFL, EUHNF, and HYPOFL (p < 0.05). A 10 min, ad libitum consumption of fluid post exercise when hypohydrated (%BML > 2%), negated differences in perception of thirst between euhydrated and hypohydrated trials. These results represent a limitation in the utility of thirst in guiding hydration practices.

The present study evaluated, using a well-controlled dehydration protocol, the effects of 24 h fluid deprivation (FD) on selected mood and physiological parameters. In the present cross-over study, twenty healthy women (age 25 ( se 0·78) years) participated in two randomised sessions: FD-induced dehydration v . a fully hydrated control condition. In the FD period, the last water intake was between 18.00 and 19.00 hours and no beverages were allowed until 18.00 hours on the next day (23–24 h). Water intake was only permitted at fixed periods during the control condition. Physiological parameters in the urine, blood and saliva (osmolality) as well as mood and sensations (headache and thirst) were compared across the experimental conditions. Safety was monitored throughout the study. The FD protocol was effective as indicated by a significant reduction in urine output. No clinical abnormalities of biological parameters or vital signs were observed, although heart rate was increased by FD. Increased urine specific gravity, darker urine colour and increased thirst were early markers of dehydration. Interestingly, dehydration also induced a significant increase in saliva osmolality at the end of the 24 h FD period but plasma osmolality remained unchanged. The significant effects of FD on mood included decreased alertness and increased sleepiness, fatigue and confusion. The most consistent effects of mild dehydration on mood are on sleep/wake parameters. Urine specific gravity appears to be the best physiological measure of hydration status in subjects with a normal level of activity; saliva osmolality is another reliable and non-invasive method for assessing hydration status.

Background Acute diarrhoea is a common cause of illness and death among children in low- to middle-income settings. World Health Organization guidelines for the clinical management of acute watery diarrhoea in children focus on oral rehydration, supplemental zinc and feeding advice. Routine use of antibiotics is not recommended except when diarrhoea is bloody or cholera is suspected. Young children who are undernourished or have a dehydrating diarrhoea are more susceptible to death at 90 days after onset of diarrhoea. Given the mortality risk associated with diarrhoea in children with malnutrition or dehydrating diarrhoea, expanding the use of antibiotics for this subset of children could be an important intervention to reduce diarrhoea-associated mortality and morbidity. We designed the Antibiotics for Childhood Diarrhoea (ABCD) trial to test this intervention. Methods ABCD is a double-blind, randomised trial recruiting 11,500 children aged 2–23 months presenting with acute non-bloody diarrhoea who are dehydrated and/or undernourished (i.e. have a high risk for mortality). Enrolled children in Bangladesh, India, Kenya, Malawi, Mali, Pakistan and Tanzania are randomised (1:1) to oral azithromycin 10 mg/kg or placebo once daily for 3 days and followed-up for 180 days. Primary efficacy endpoints are all-cause mortality during the 180 days post-enrolment and change in linear growth 90 days post-enrolment. Discussion Expanding the treatment of acute watery diarrhoea in high-risk children to include an antibiotic may offer an opportunity to reduce deaths. These benefits may result from direct antimicrobial effects on pathogens or other incompletely understood mechanisms including improved nutrition, alterations in immune responsiveness or improved enteric function. The expansion of indications for antibiotic use raises concerns about the emergence of antimicrobial resistance both within treated children and the communities in which they live. ABCD will monitor antimicrobial resistance. The ABCD trial has important policy implications. If the trial shows significant benefits of azithromycin use, this may provide evidence to support reconsideration of antibiotic indications in the present World Health Organization diarrhoea management guidelines. Conversely, if there is no evidence of benefit, these results will support the current avoidance of antibiotics except in dysentery or cholera, thereby avoiding inappropriate use of antibiotics and reaffirming the current guidelines. Trial registration Clinicaltrials.gov, NCT03130114 . Registered on April 26 2017.