The effect of active hypohydration on cognitive function: A systematic review and meta-analysis

Ad libitum dehydration is associated with poorer performance on a sustained attention task but not other measures of cognitive performance among middle-to-older aged community-dwelling adults: A short-term longitudinal study

OBJECTIVE

Hydration status and water intake are critical to physiological health. Despite a popular narrative that dehydration impairs cognitive performance, results are mixed in the literature. Therefore, we tested how hydration status was associated with cognitive performance in an ad libitum state over the course of 3 months.

METHODS

Data come from a short-term longitudinal study among middle-to-older aged US adults (n = 78) measured three times (207 observations). All participants were scheduled for 8:00 a.m. visits for the baseline, two-week, and 3-month examinations where they completed surveys, neuropsychological tests to measure cognitive performance, anthropometrics, and a blood draw for biomarker analysis. Serum osmolality (Sosm) was measured as a biomarker of hydration status using osmometry. Four cognitive performance tasks were assessed, including inhibition, working memory, cognitive flexibility, and sustained attention.

RESULTS

Panel random effects linear regressions demonstrate that there was an inverse association between dehydration and sustained attention, whereas there were no significant relationships between dehydration and inhibition, working memory, and cognitive flexibility. Adults who were dehydrated (defined as Sosm >300 mOsm/kg) performed substantially worse (B = 0.65 z-score; SE = 0.28; p = .020) on the sustained attention task than those who were not dehydrated adjusting for time fixed effects, age, body mass index, sex, and educational attainment.

CONCLUSION

This short-term longitudinal study found that dehydration was only associated with poorer performance on a cognitive performance task that required sustained attention. Maintaining adequate hydration may be increasingly important for middle-to-older aged adults to ensure proper cognitive function, particularly as water needs increase in future climatic scenarios.

Effect of Various Hydration Strategies on Work Intensity and Selected Physiological Indices in Young Male Athletes during Prolonged Physical Exercise at High Ambient Temperatures

Background: In high temperatures, adequate hydration is vital for sustained physical exercise. This study explores the effect of three hydration strategies on physiological indices and work intensity. Methods: The research involved 12 healthy males who engaged in three test series, each separated by a one-week interval. During the trials, participants underwent a 120 min cycling session in a thermal climate chamber (temperature: 31 ± 2 °C, humidity: 60 ± 3%, air movement: <1 m/s). Measurements of rectal temperature (Tre) and heart rate (HR), and assessment of subjective workload perception, and thermal comfort were made both before and during the exercise. The computation of the physical strain index (PSI) relied on Tre and HR values. Three hydration strategies (isotonic drink, water, and no hydration) were administered before, during, and after the exercise. Results: Regardless of the hydration strategy, the participants' mean body mass decreased as a result of the exercise. Statistically significant differences in HR were observed between the no-hydration and water groups (p < 0.036). The mean PSI values significantly varied between hydration strategies, with the no hydration group exhibiting a higher PSI compared to the isotonic drink or water groups (p < 0.001). Conclusions: All hydration strategies contribute to thermoregulatory processes and mitigate the rise in internal body temperature during sustained physical exercise in elevated ambient temperatures.

The Relationship between Beverages Consumption and Cognitive Impairment in Middle-Aged and Elderly Chinese Population

Some evidence shows that beverage consumption has an impact on cognitive performance. This is a follow-up study of dietary habits and cognitive function in the Chinese middle-aged and elderly population. The objective of this study was to explore the relationship between beverage consumption and cognitive impairment. The source and grouping of the participants can be seen in the previous article, "Study of Diet Habits and Cognitive Function in the Chinese Middle-Aged and Elderly Population: The Association between Folic Acid, B Vitamins, Vitamin D, Coenzyme Q10 Supplementation and Cognitive Ability". Among 892 participants, one-third (296) completed both Amyloid beta(Aβ)-PET and plasma biomarkers. The results showed that the consumption of beverages (green tea, coffee, pure milk) was a protective factor for cognitive impairment, daily water consumption <1500 mL (especially <500 mL) was a risk factor for cognitive impairment, and the above correlated with baseline cognitive status. The relationship of green tea, coffee, and pure milk consumption with cognitive impairment was related to gender. We also found that among the participants with Aβ deposition, the consumption of pure milk and green tea was associated with low levels of p-Tau-181. In conclusion, the relationship between beverage consumption and cognitive impairment in Chinese middle-aged and elderly adults may be related to baseline cognitive status, gender, and Aβ deposition.

Effects of Different Rapid Weight Loss Strategies and Percentages on Performance-Related Parameters in Combat Sports: An Updated Systematic Review

Most combat sports (CS) are structured in weight categories, and it is very common to carry out body weight adjustment strategies in order to compete in lower weight categories. For this reason, different rapid weight loss (RWL) strategies are usually performed to pass the pre-competition weigh-in test, and then a replenishment of fluids and carbohydrate-rich foods is conducted in an attempt to recover the weight and avoid a performance loss. In this context, no clear references have been found on whether these types of strategies have negative effects, impairing the athlete's combat and/or physical performance. For this reason, the aim of this study was to review the scientific literature on the effect of rapid weight reduction strategies on the performance of CS athletes. A literature search was performed through four different databases (PubMed, SPORTDiscus, Web of Science and ScienceDirect). Four inclusion criteria were established as follows: (1) the subjects had to be competitors in the CS and carry out RWL strategies; (2) at least two measurement points, that is, normal conditions and dehydration condition; (3) measurements in a real competition or simulating the same conditions; (4) original research articles written in English or Spanish and available in full text. Finally, a total of 16 articles were finally included in this research. All subjects (n = 184) were athletes from combat disciplines, with a minimum of 3-4 years of practice, as well as with certain experience in RWL. Six of the studies reported that an RWL strategy of around 5% of body weight loss did not affect performance parameters. However, the other ten studies with RWL between 3 and 6% or even higher reported negative effects or impairments on different parameters related to performance and/or athlete's psychophysiology, such as perceived fatigue, mood states, strength and power production, as well as changes in hormonal, blood and urine parameters, body composition, or the kinematics of the technical gesture. Although there is still no clear answer to the issue approached in this research, in general terms, it seems that in order to guarantee an acceptable athletic performance of the competitor, the weight loss should not exceed 3% to ≤5% of body weight together with ≥24 h for adequate (or at least partial) recovery and rehydration processes. In addition, it is highly recommended to lose weight progressively over several weeks, especially focusing on competitions lasting several days, as well as multiple rounds or qualifying stages.

Comparative proteomic changes in rabbit vocal folds undergoing systemic dehydration and systemic rehydration

BACKGROUND

A considerable body of clinical evidence suggests that systemic dehydration can negatively affect voice production, leading to the common recommendation to rehydrate. Evidence for the corrective benefits of rehydration, however, is limited with mixed conclusions, and biological data on the underlying tissue changes with rehydration is lacking. In this study, we used a rabbit model (n = 24) of acute (5 days) water restriction-induced systemic dehydration with subsequent rehydration (3 days) to explore the protein-level changes underlying the molecular transition from euhydration to dehydration and following rehydration using LC-MS/MS protein quantification in the vocal folds. We show that 5-day water restriction led to an average 4.3% decrease in body weight with relative increases in anion gap, Cl-, creatinine, Na+, and relative decreases in BUN, iCa2+, K+, and tCO2 compared to control (euhydrated) animals. A total of 309 differentially regulated (p < 0.05) proteins were identified between the Control and Dehydration groups. We observed a noteworthy similarity between the Dehydration and Rehydration groups, both well differentiated from the Control group, highlighting the distinct timelines of resolution of the clinical symptoms of systemic dehydration and the underlying molecular changes.

SIGNIFICANCE

Voice disorders are a ubiquitous problem with considerable economic and psychological impact. Maintenance of proper hydration is commonly prescribed as a general vocal hygiene practice. There is evidence that dehydration negatively impacts phonation, but our understanding of the state of vocal folds in the context of systemic dehydration are limited, particular from a molecular perspective. Further, ours is a novel molecular study of the short-term impact of rehydration on the tissue. Given the relatively minimal difference in vocal fold proteomic profiles between the Dehydration and Rehydration groups, our data demonstrate a complex physiological response to acute systemic dehydration, and highlight the importance of considering persistent underlying molecular pathology despite the rapid resolution of clinical measures. This study sets a foundation for future research to confirm the nature of potential beneficial outcomes of clinical recommendations related to hydration.

Hypohydration alters pre-frontal cortex haemodynamics, but does not impair motor learning

It is unknown how hypohydration influences fine motor performance training and motor learning. Here, 30 participants (aged 19–46 years) were randomly assigned to a hypohydration (HYPO) or control (CON) group (both n = 15). Moderate hypohydration (~ 2.4% loss in body mass) was produced in HYPO via active dehydration before a 46 min fluid restricted rest period was undertaken. The conclusion of rest coincided with when CON attended the facilities. Both groups undertook a discrete sequence production task consisting of 6 training blocks, and returned ~ 300 min later to complete a delayed retention and transfer test while euhydrated. Bilateral pre-frontal cortex (PFC) haemodynamics were assessed using functional near-infrared spectroscopy throughout training and delayed learning assessments. Response time improved across training (P < 0.01) and was similar between the groups (both P = 0.22). Analysis of training PFC haemodynamics revealed a significant group by block interaction for oxygenated (O₂Hb; P < 0.01), but not deoxygenated haemoglobin (P = 0.77). In training block 1, bilateral O₂Hb was higher in HYPO (P = 0.02), while bilateral O₂Hb increased in CON between blocks 2–3 and 5–6 (both P ≤ 0.03). During the delayed retention and transfer test, no group differences or interactions were found in response time, response error, or PFC haemodynamics (all P ≥ 0.27). Moderate hypohydration does increase PFC activation during motor skill learning, however, this appears to be transient and of little consequence to training or delayed retention or transfer performance.

Effects of heat strain on cognitive function among a sample of miners

Heat stress is associated with workplace injuries, likely through a combination of fatigue, reduced cognitive function, and thermal discomfort. The purpose of this study was to evaluate four cognitive tasks for sensitivity to heat stress. Eight participants performed treadmill exercise followed by assessments of serial reaction time (RT), Stroop effect, verbal delayed memory, and continuous performance working memory in an environmental chamber. A control (21.1 °C) trial, and "Hot 1" and "Hot 2" (both 37.8 °C) trials were run sequentially on two separate days to evaluate the four cognitive tasks. Heat strain (comparing Hot 1 and Hot 2 with the control trial) resulted in impairments in the serial RT test response and Stroop accuracy. Delayed memory was impacted only in the Hot 2 trial compared with the control trial. Given the demonstrated impact of heat on cognitive processes relevant to workers' real-world functioning in the workplace, understanding how to assess and monitor vigilant attention in the workplace is essential.

Impact of dehydration on perceived exertion during endurance exercise: A systematic review with meta-analysis

Background

Understanding the impact of stressors on the rating of perceived exertion (RPE) is relevant from a performance and exercise adherence/participation standpoint. Athletes and recreationally active individuals dehydrate during exercise. No attempt has been made to systematically determine the impact of exercise-induced dehydration (EID) on RPE.

Objective

The present meta-analysis aimed to determine the effect of EID on RPE during endurance exercise and examine the moderating effect of potential confounders.

Data analyses

Performed on raw RPE values using random-effects models weighted mean effect summaries and meta-regressions with robust standard errors, and with a practical meaningful effect set at 1 point difference between euhydration (EUH) and EID. Only controlled crossover studies measuring RPE with a Borg scale in healthy adults performing ≥30 min of continuous endurance exercise while dehydrating or drinking to maintain EUH were included.

Results

Sixteen studies were included, representing 147 individuals. Mean body mass loss with EUH was 0.5 ± 0.4%, compared to 2.3 ± 0.5% with EID (range 1.7-3.1%). Within an EID of 0.5-3% body mass, a maximum difference in RPE of 0.81 points (95% CI: 0.36-1.27) was observed between conditions. A meta-regression revealed that RPE increases by 0.21 points for each 1% increase in EID (95% CI: 0.12-0.31). Humidity, ambient temperature and aerobic capacity did not alter the relationship between EID and RPE.

Conclusion

Therefore, the effect of EID on RPE is unlikely to be practically meaningful until a body mass loss of at least 3%.

Effects of Weight Cutting on Exercise Performance in Combat Athletes: A Meta-Analysis

Weight cutting in combat sports is a prevalent practice whereby athletes voluntarily dehydrate themselves via various methods to induce rapid weight loss (RWL) to qualify for a lower weight category than that of their usual training body weight. The intention behind this practice is to regain the lost body mass and compete at a heavier mass than permitted by the designated weight category. The purpose of this study was to quantitatively synthesize the available evidence examining the effects of weight cutting on exercise performance in combat-sport athletes. Following a systematic search of the literature, meta-analyses were performed to compare maximal strength, maximal power, anaerobic capacity, and/or repeated high-intensity-effort performance before rapid weight loss (pre-RWL), immediately following RWL (post-RWL), and 3 to 36 hours after RWL following recovery and rapid weight gain (post-RWG). Overall, exercise performance was unchanged between pre-RWL and post-RWG (g = 0.22; 95% CI, -0.18 to 0.62). Between pre-RWL and post-RWL analyses revealed small reductions in maximal strength and repeated high-intensity-effort performance (g = -0.29; 95% CI, -0.54 to -0.03 and g = -0.37; 95% CI, -0.59 to -0.16, respectively; both P ≤ .03). Qualitative analysis indicates that maximal strength and power remained comparable between post-RWL and post-RWG. These data suggest that weight cutting in combat-sport athletes does not alter short-duration, repeated high-intensity-effort performance; however, there is evidence to suggest that select exercise performance outcomes may decline as a product of RWL. It remains unclear whether these are restored by RWG.

Cognitive Performance Before and Following Habituation to Exercise-Induced Hypohydration of 2 and 4% Body Mass in Physically Active Individuals

We investigated the effect of repeated exposures to hypohydration upon cognitive performance. In a randomized crossover design, ten physically active adults completed two 4-week training blocks, one where they maintained euhydration (EUH) and the other where they were water-restricted (DEH) during walking/running at 55% V.O2max, 40 °C. Three sessions per week were performed: (1) 1 h of exercise, (2) exercise until 2% or (3) 4% of body mass has been lost or replaced. Limited to the first and fourth training week, a 12 min walking/running time-trial was completed following the 2 and 4% exercise bouts. Trail making, the Wisconsin card sort, the Stop signal task, Simple visual reaction time and Corsi block-tapping tests were performed immediately following the time-trials. Body mass loss was maintained < 1% with EUH and reached 2.7 and 4.7% with DEH following the time-trials. Except for a lower percentage of correct responses (% accuracy) during the Wisconsin card sort test (p < 0.05) with DEH compared to EUH, no statistically significant decline in cognitive performance was induced by low and moderate levels of hypohydration. Compared to week 1, no statistical differences in cognitive responses were observed after repeated exposures to hypohydration (all p > 0.05). From a practical perspective, the gains in cognitive performance following training to DEH were mostly unclear, but under certain circumstances, were greater than when EUH was maintained. Based on the battery of cognitive tests used in the current study, we conclude that whether physically active individuals are habituated or not to its effect, exercise-induced hypohydration of 2 and 4% has, in general, no or unclear impact on cognitive performance immediately following exercise. These results encourage further research in this area.

Modified Stroop Task Performance When Wearing Protective Clothing in the Heat: An Evaluation of the Maximum Adaptability Model

INTRODUCTION

This exploratory study investigated whether performance in a behavioural inhibition task followed the shape proposed by the Maximum Adaptability Model during progressive exertional heat stress-that is, an initial improvement in cognitive performance is followed by a plateau, and subsequent decline once body temperature continues to rise unabated.

METHODS

Seventeen adult males walked on a treadmill at 4 km•h^-1 (1% grade) for up to 120 min, in three protective clothing ensembles, across three simulated environments. The simulated environments were equivalent to wet bulb globe temperatures 21, 30 and 37°C. Cognitive function was assessed using a modified colour-word Stroop Task, with performance expressed as inverse efficiency scores in the simple (congruent) and more complex (incongruent) task conditions. The Stroop Task was completed before a trial, at termination, and every 30 min during walking, and core body temperature was continuously measured. Data were modelled using Bayesian penalised regression, with core body temperature included as a non-linear term (i.e., second degree polynomial).

RESULTS

We did not find any evidence that core body temperature had an effect on congruent or incongruent inverse efficiency scores, and no evidence that the relationship between these variables followed the shaped described by the Maximum Adaptability Model. There was, however, evidence that higher pre-exercise serum osmolality values were associated with slower congruent (β = 9.19) and incongruent (β = 8.67) inverse efficiency scores. The posterior probability that these effects were greater than zero was 0.971 and 0.952, respectively.

CONCLUSIONS

In young, fit men, performance in the behavioural inhibition task was unaffected by increases in body temperature up to 39°C and did not follow the shape proposed by the Maximum Adaptability Model. A secondary finding of the study was that pre-exercise hydration status affected performance in the inhibition task. Future studies are needed to confirm this result.

Effects of Water Restriction and Supplementation on Cognitive Performances and Mood among Young Adults in Baoding, China: A Randomized Controlled Trial (RCT)

The brain is approximately 75% water. Therefore, insufficient water intake may affect the cognitive performance of humans. The present study aimed to investigate the effects of water restriction and supplementation on cognitive performances and mood, and the optimum amount of water to alleviate the detrimental effects of dehydration, among young adults. A randomized controlled trial was conducted with 76 young, healthy adults aged 18–23 years old from Baoding, China. After fasting overnight for 12 h, at 8:00 a.m. of day 2, the osmolality of the first morning urine and blood, cognitive performance, and mood were measured as a baseline test. After water restriction for 24 h, at 8:00 a.m. of day 3, the same indexes were measured as a dehydration test. Participants were randomly assigned into four groups: water supplementation group (WS group) 1, 2, or 3 (given 1000, 500, or 200 mL purified water), and the no water supplementation group (NW group). Furthermore, participants were instructed to drink all the water within 10 min. Ninety minutes later, the same measurements were performed as a rehydration test. Compared with the baseline test, participants were all in dehydration and their scores on the portrait memory test, vigor, and self-esteem decreased (34 vs. 27, p < 0.001; 11.8 vs. 9.2, p < 0.001; 7.8 vs. 6.4, p < 0.001). Fatigue and TMD (total mood disturbance) increased (3.6 vs. 4.8, p = 0.004; 95.7 vs. 101.8, p < 0.001) in the dehydration test. Significant interactions between time and volume were found in hydration status, fatigue, vigor, TMD, symbol search test, and operation span test (F = 6.302, p = 0.001; F = 3.118, p = 0.029; F = 2.849, p = 0.043; F = 2.859, p = 0.043; F = 3.463, p = 0.021) when comparing the rehydration and dehydration test. Furthermore, the hydration status was better in WS group 1 compared to WS group 2; the fatigue and TMD scores decreased, and the symbol search test and operation span test scores increased, only in WS group 1 and WS group 2 (p < 0.05). There was no significant difference between them (p > 0.05). Dehydration impaired episodic memory and mood. Water supplementation improved processing speed, working memory, and mood, and 1000 mL was the optimum volume.

Proposed framework for forecasting heat-effects on motor-cognitive performance in the Summer Olympics

Heat strain impairs performance across a broad spectrum of sport disciplines. The impeding effects of hyperthermia and dehydration are often ascribed to compromised cardiovascular and muscular functioning, but expert performance also depends on appropriately tuned sensory, motor and cognitive processes. Considering that hyperthermia has implications for central nervous system (CNS) function and fatigue, it is highly relevant to analyze how heat stress forecasted for the upcoming Olympics may influence athletes. This paper proposes and demonstrates the use of a framework combining expected weather conditions with a heat strain and motor-cognitive model to analyze the impact of heat and associated factors on discipline- and scenario-specific performances during the Tokyo 2021 games. We pinpoint that hyperthermia-induced central fatigue may affect prolonged performances and analyze how hyperthermia may impair complex motor-cognitive performance, especially when accompanied by either moderate dehydration or exposure to severe solar radiation. Interestingly, several short explosive performances may benefit from faster cross-bridge contraction velocities at higher muscle temperatures in sport disciplines with little or no negative heat-effect on CNS fatigue or motor-cognitive performance. In the analyses of scenarios and Olympic sport disciplines, we consider thermal impacts on “motor-cognitive factors” such as decision-making, maximal and fine motor-activation as well as the influence on central fatigue and pacing. From this platform, we also provide perspectives on how athletes and coaches can identify risks for their event and potentially mitigate negative motor-cognitive effects for and optimize performance in the environmental settings projected.

Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies

A rise in body core temperature and loss of body water via sweating are natural consequences of prolonged exercise in the heat. This review provides a comprehensive and integrative overview of how the human body responds to exercise under heat stress and the countermeasures that can be adopted to enhance aerobic performance under such environmental conditions. The fundamental concepts and physiological processes associated with thermoregulation and fluid balance are initially described, followed by a summary of methods to determine thermal strain and hydration status. An outline is provided on how exercise-heat stress disrupts these homeostatic processes, leading to hyperthermia, hypohydration, sodium disturbances, and in some cases exertional heat illness. The impact of heat stress on human performance is also examined, including the underlying physiological mechanisms that mediate the impairment of exercise performance. Similarly, the influence of hydration status on performance in the heat and how systemic and peripheral hemodynamic adjustments contribute to fatigue development is elucidated. This review also discusses strategies to mitigate the effects of hyperthermia and hypohydration on exercise performance in the heat by examining the benefits of heat acclimation, cooling strategies, and hyperhydration. Finally, contemporary controversies are summarized and future research directions are provided.

Does Hydration Status Influence Executive Function? A Systematic Review

BACKGROUND

Cognitive function, including executive function (EF)-related capacities (eg, working memory, inhibitory and attentional control), has been linked to adherence to healthy lifestyle behaviors. Dehydration is associated with impaired cognitive function, whereas improvements in hydration status may improve inhibitory and attentional performance. No systematic reviews have examined the effects of both dehydration and euhydration on EF.

OBJECTIVE

The objectives of this systematic review are to examine studies that have investigated the spectrum of hydration status and EF in adults, and to identify future research needs.

DESIGN

The review was conducted according to the 2015 Preferred Reporting Items for Systematic Reviews and Meta-Analyses Protocols (PRISMA-P) guidelines. The database search was initially conducted on May 12, 2019 and then updated on April 26, 2020. Databases searched included PubMed, Medline, Psyc Info, SCOPUS, Proquest, and ISI Web of Science. Data extraction included the following: method used to assess de/hydration status, study design, participant characteristics, EF tasks and domain, and results. Article quality ratings were performed on included studies using the Academy of Nutrition and Dietetics Quality Rating Checklist.

PARTICIPANTS/SETTING

Studies done with healthy or diseased adults, aged older than 18 years, in any setting, were included. Studies of individuals with disease states that impact fluid balance or require fluid restrictions as treatments were excluded.

MAIN OUTCOME MEASURE

All EF-related outcomes were included, such as working memory, inhibitory control, task switching, and attention.

RESULTS

Four thousand eight hundred thirty-three articles were screened using title/abstracts. Seventy-one full-text articles were assessed for eligibility; 33 were included (26 included investigations of dehydration; 27 included investigations of rehydration/euhydration) with 3,636 participants across all studies. Little consistency was found across outcomes. Roughly half of the available studies suggested unclear or neutral EF effects, and half suggested effects on EF domains, particularly working memory, inhibitory control, and attention. Studies including a euhydration condition were slightly more likely to demonstrate improvements to EF capacities.

CONCLUSIONS

Overall, there is a strong need for consistent methodological approaches and a greater number of long-term (ie, >3 days) studies of dehydration and euhydration and EF.

Thirst perception exacerbates objective mental fatigue

Thirst is represented within the anterior cingulate and insular cortices, and may share some common neuroanatomical structures that are implicated with the regulation of mental fatigue. This novel study investigated whether thirst might modulate the subjective, behavioural, or neurophysiological representations of mental fatigue. In a crossover design, thirst was monitored in 15 males during 60 min of cycling in normothermic conditions. Participants either consumed water to the dictates of their thirst (sated), or fluid was withheld and replaced with periodic salt water mouth rinses (thirst). Following either satiety or thirst, a 60 min modified Stroop task was completed to evoke mental fatigue. Prefrontal cortex (PFC) haemodynamics were monitored throughout the prolonged task, and subjective perceptions of fatigue were reported through a visual analogue scale. Behavioural performance was quantified as the total number of Stroop task iterations completed in the mentally fatiguing task, and by collating response time and accuracy into the inverse efficiency score (IES) for each 5 min interval throughout the task. During thirst, fewer iterations were completed and poorer IES performance was evident toward the latter portion of the mentally fatiguing task. Compensatory elevations in PFC oxyhaemoglobin were produced in each condition, however, differed temporally, and were premature during thirst. A diminished capacity to sustain cognitive performance is likely the product of an inability to preserve the distribution of resources within the prefrontal cortex, due to heightened activation about thirst regulatory centres. These data provide novel insight into the relationship between thirst and mental fatigue, and suggest that drinking to the dictates of thirst may be a pertinent strategy to sustain prolonged cognitive performance.

[The importance of water consumption in health and disease prevention: the current situation]

Introduction

Water is an essential nutrient for life and the most abundant component in the human body. However, its dietary recommendations or clinical management guidelines do not receive as much attention as they deserve. In addition, there are some obstacles to establishing optimal values, both for the amount of water the body must contain and for water ingestion. Water intake and elimination depend on unsteady factors that are difficult to measure and, at the same time, compensated by the body's ability to regulate homeostasis. Since scientific evidence is lacking for establishing recommendations, "adequate intakes" (to maintain an adequate hydration state) have been estimated using data on water intake from groups of healthy people. The European Food Safety Authority (EFSA) also considers desirable the use of urine osmolarity to estimate the adequacy of water intake in adults. Clinical studies have generally shown the benefits of adequate hydration and the damage caused by water imbalance, whether quantitative (dehydration and overhydration) or qualitative (extracellular and intracellular water). Unfortunately, these studies are few and often have poor cross-sectional, case-control, or prospective designs, and use small samples or indirect methods to assess hydration status. This article presents up-to-date information on subjects such as: 1) compliance with water consumption recommendations and suggestions for improvement; 2) techniques available to measure hydration status and their clinical applications; 3) effects of hydration/dehydration on physical or cognitive activities and chronic diseases; and 4) existing Spanish regulations on the quality and salubrity of water.

Sustainable solutions to mitigate occupational heat strain - an umbrella review of physiological effects and global health perspectives

BACKGROUND

Climate change is set to exacerbate occupational heat strain, the combined effect of environmental and internal heat stress on the body, threatening human health and wellbeing. Therefore, identifying effective, affordable, feasible and sustainable solutions to mitigate the negative effects on worker health and productivity, is an increasingly urgent need.

OBJECTIVES

To systematically identify and evaluate methods that mitigate occupational heat strain in order to provide scientific-based guidance for practitioners.

METHODS

An umbrella review was conducted in biomedical databases employing the following eligibility criteria: 1) ambient temperatures > 28 °C or hypohydrated participants, 2) healthy adults, 3) reported psychophysiological (thermal comfort, heart rate or core temperature) and/or performance (physical or cognitive) outcomes, 4) written in English, and 5) published before November 6, 2019. A second search for original research articles was performed to identify interventions of relevance but lacking systematic reviews. All identified interventions were independently evaluated by all co-authors on four point scales for effectiveness, cost, feasibility and environmental impact.

RESULTS

Following screening, 36 systematic reviews fulfilled the inclusion criteria. The most effective solutions at mitigating occupational heat strain were wearing specialized cooling garments, (physiological) heat acclimation, improving aerobic fitness, cold water immersion, and applying ventilation. Although air-conditioning and cooling garments in ideal settings provide best scores for effectiveness, the limited applicability in certain industrial settings, high economic cost and high environmental impact are drawbacks for these solutions. However, (physiological) acclimatization, planned breaks, shading and optimized clothing properties are attractive alternative solutions when economic and ecological sustainability aspects are included in the overall evaluation.

DISCUSSION

Choosing the most effective solution or combinations of methods to mitigate occupational heat strain will be scenario-specific. However, this paper provides a framework for integrating effectiveness, cost, feasibility (indoors and outdoor) and ecologic sustainability to provide occupational health and safety professionals with evidence-based guidelines.

The Effect of Acute Hypohydration on Indicators of Glycemic Regulation, Appetite, Metabolism and Stress: A Systematic Review and Meta-Analysis

Evidence synthesizing the effects of acute body water losses on various markers of glycemic regulation, appetite, metabolism, and stress is lacking. Thus, the purpose of this review was to summarize the response of various hormonal changes involved in these physiologic functions to dehydration. A comprehensive literature search for peer-reviewed research in the databases PubMed, Scopus, CINAHL, and SportDiscus was conducted. Studies were included if they contained samples of adults (>18 years) and experimentally induced dehydration as measured by acute body mass loss. Twenty-one articles were eligible for inclusion. Findings suggested cortisol is significantly elevated with hypohydration (standard mean difference [SMD] = 1.12, 95% CI [0.583, 1.67], p < 0.0001). Testosterone was significantly lower in studies where hypohydration was accompanied by caloric restriction (SMD= -1.04, 95% CI [-1.93, -0.14], p = 0.02), however, there were no changes in testosterone in studies examining hypohydration alone (SMD = -0.17, 95% CI [-0.51 0.16], p = 0.30). Insulin and ghrelin were unaffected by acute total body water losses. Acute hypohydration increases markers of catabolism but has a negligible effect on markers of glycemic regulation, appetite, anabolism and stress. Given the brevity of existing research, further research is needed to determine the impact of hydration on glucagon, leptin, peptide YY and the subsequent outcomes relevant to both health and performance.

Biobehavioral variation in human water needs: How adaptations, early life environments, and the life course affect body water homeostasis

Body water homeostasis is critical for optimal physiological and cognitive function for humans. The majority of research has illustrated the negative biological consequences of failing to meet water needs. The human body has several mechanisms for detecting, regulating, and correcting body water deficits and excesses. However, variation exists in total water intake and how people meet those water needs as well as thirst thresholds and how well people tolerate water restriction. An evolutionary and developmental framework provides an underexplored perspective into human water needs by examining how adaptations, early life experiences and environments, as well as life course changes in health states and behaviors may shape these critical factors in body water homeostasis. This article first reviews biological and behavioral adaptations to water scarcity among animals and humans. It then examines human variation in water intake in a mostly water secure environment through the analysis of National Health and Nutrition Examination Survey dietary data and the link between water intake patterns and hydration biomarkers. Next, it reviews existing evidence of how maternal water restriction in utero and during lactation shape vasopressin release, thirst thresholds, drinking patterns, and body water homeostasis for the infant. Early life water restriction appears to have implications for hydration status, body size, and cardiovascular health. Finally, it examines how life course changes in health states and behaviors, including obesity, sleep, and parasitic infection, affect body water homeostasis. This article poses new questions about the plasticity and shaping of human water needs, thirst, and hydration behaviors.

Heat-induced hypervolemia: Does the mode of acclimation matter and what are the implications for performance at Tokyo 2020?

Tokyo 2020 will likely be the most heat stressful Olympics to date, so preparation to mitigate the effects of humid heat will be essential for performance in several of the 33 sports. One key consideration is heat acclimation (HA); the repeated exposure to heat to elicit physiological and psychophysical adaptations that improve tolerance and exercise performance in the heat. Heat can be imposed in various ways, including exercise in the heat, hot water immersion, or passive exposure to hot air (e.g., sauna). The physical requirements of each sport will determine the impact that the heat has on performance, and the adaptations required from HA to mitigate these effects. This review focuses on one key adaptation, plasma volume expansion (PVE), and how the mode of HA may affect the kinetics of adaptation. PVE constitutes a primary HA-mediated adaptation and contributes to functional adaptations (e.g., lower heart rate and increased heat loss capacity), which may be particularly important in athletes of "sub-elite" cardiorespiratory fitness (e.g., team sports), alongside athletes of prolonged endurance events. This review: i) highlights the ability of exercise in the heat, hot-water immersion, and passive hot air to expand PV, providing the first quantitative assessment of the efficacy of different heating modes; ii) discusses how this may apply to athletes at Tokyo 2020; and iii) provides recommendations regarding the protocol of HA and the prospect for achieving PVE (and the related outcomes).