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Hypohydration, defined as a state of low body water, increases thirst sensations, arginine vasopressin release, and elicits renin–angiotensin–aldosterone system activation to replenish intra- and extra-cellular fluid stores. Hypohydration impairs mental and physical performance, but new evidence suggests hypohydration may also have deleterious effects on cardiovascular health. This is alarming because cardiovascular disease is the leading cause of death in the United States. Observational studies have linked habitual low water intake with increased future risk for adverse cardiovascular events. While it is currently unclear how chronic reductions in water intake may predispose individuals to greater future risk for adverse cardiovascular events, there is evidence that acute hypohydration impairs vascular function and blood pressure (BP) regulation. Specifically, acute hypohydration may reduce endothelial function, increase sympathetic nervous system activity, and worsen orthostatic tolerance. Therefore, the purpose of this review is to present the currently available evidence linking acute hypohydration with altered vascular function and BP regulation.

Exercise-associated dehydration is a common problem, especially at sporting events. Although recommendations have been made to drink a certain volume per kilogram body mass lost after exercise, no clear guidance about the type of rehydration beverage is available. We conducted a systematic review to assess the effectiveness of carbohydrate-electrolyte (CE) solutions as a rehydration solution for exercise-associated dehydration. MEDLINE (via the PubMed interface), Embase, and the Cochrane Library databases were searched up until June 1, 2022. Controlled trials involving adults and children were included when dehydration was the result of physical exercise and when drinking carbohydrate-electrolyte solutions, of any percentage carbohydrate, was compared with drinking water. All languages were included if an English abstract was available. Data on study design, study population, interventions, outcome measures, and study limitations were extracted from each included article. Certainty was assessed using Grading of Recommendations Assessment, Development and Evaluation. Of 3485 articles screened, 19 articles in which authors assessed CE solutions (0%-9% carbohydrate) compared with water were included. Although variability was present among the identified studies, drinking 0% to 3.9% and especially 4% to 9% CE solution may be effective for rehydration. A potential beneficial effect of drinking CE drinks compared with water was observed for many of the reviewed outcomes. Commercial CE drinks (ideally 4%-9% CE drinks or alternatively 0%-3.9% CE drinks) could be suggested for rehydration in individuals with exercise-associated dehydration when whole foods are not available.

Background Despite a substantial body of research, no clear best practice guidelines exist for the assessment of hydration in athletes. Body water is stored in and shifted between different sites throughout the body complicating hydration assessment. This review seeks to highlight the unique strengths and limitations of various hydration assessment methods described in the literature as well as providing best practice guidelines. Main body There is a plethora of methods that range in validity and reliability, including complicated and invasive methods (i.e. neutron activation analysis and stable isotope dilution), to moderately invasive blood, urine and salivary variables, progressing to non-invasive metrics such as tear osmolality, body mass, bioimpedance analysis, and sensation of thirst. Any single assessment of hydration status is problematic. Instead, the recommended approach is to use a combination, which have complementary strengths, which increase accuracy and validity. If methods such as salivary variables, urine colour, vital signs and sensation of thirst are utilised in isolation, great care must be taken due to their lack of sensitivity, reliability and/or accuracy. Detailed assessments such as neutron activation and stable isotope dilution analysis are highly accurate but expensive, with significant time delays due to data analysis providing little potential for immediate action. While alternative variables such as hormonal and electrolyte concentration, bioimpedance and tear osmolality require further research to determine their validity and reliability before inclusion into any test battery. Conclusion To improve best practice additional comprehensive research is required to further the scientific understanding of evaluating hydration status.

The ACSM recommends drinking to avoid loss of body mass >2% during exercise to avert compromised performance. Our study aimed to assess the level of dehydration in elite runners following a city marathon in a tropical environment. Prospective cohort design. Twelve elite runners (6 males, 6 females; age 24–41 y) had body mass measured to the nearest 0.01kg in their race attire immediately before and after the 2017 Standard Chartered Singapore Marathon 2017. Body mass change was corrected for respiratory water loss, gas exchange, and sweat retained in clothing, and expressed as % of pre-race mass (i.e. % dehydration). Data are expressed as means±SD (range). Dry bulb temperature and humidity were 27.9±0.1°C (27.4–28.3°C) and 79±2% (73–82%). Finish time was 155±10min (143−172min). Male runners finishing positions ranged from 2–12 out of 7627 finishers, whilst female runners placed 1–8 out of 1754 finishers. Body mass change (loss) and % dehydration for all runners were 2.5±0.5kg (1.8–3.5kg) and 4.6±0.9% (3.6–6.8%). Male runners experienced body mass loss of 2.8±0.5kg and 4.9±1.2% while females experienced body mass loss of 2.1±0.2kg and 4.3±0.6%. Despite experiencing dehydration (4.6% body mass loss) two-fold higher than current fluid replacement guidelines recommend (≤2%), elite male and female runners performed successfully and without medical complication in a hot weather marathon.

PurposeAlthough low water intake has been associated with adverse health outcomes, available literature indicated that the majority of children do not meet the water intake guidelines and they are underhydrated based on elevated hydration biomarkers. This review examined the water intake habits and hydration status in children from 32 observational studies (n = 36813).MethodsPubMed, Web of Science, and CINAHL were used to identify relevant articles. Total water/fluid intake from 25 countries was compared with water intake recommendations and underhydration (urine osmolality greater than 800 mmol kg−1) was assessed. Risk of bias was assessed using customized categories following the review guideline for observational studies.ResultsFrom 32 studies, only 11 studies reported both water intake and hydration status. 12 out of 24 studies reported mean/median water/fluid intake below the guidelines, while 4 out of 13 studies that assessed hydration status indicated underhydration based on urine osmolality (greater than 800 mmol kg−1). Among the 19 countries that reported comparison of water/fluid intake with guidelines, 60 ± 24% of children (range 10–98%) failed to meet them.ConclusionThese findings suggest that children are not consuming enough water to be adequately hydrated.

Hypohydration is linked to increased risk of a variety of diseases and can be life-threatening, especially in elderly populations. Dehydration induces cellular damage partially through the production of reactive oxygen species (ROS) in cells, tissues and organs. Hydrogen molecules are known to convert ROS to harmless water. Therefore, theoretically hydrogen-rich water (HW) might eliminate dehydration-induced ROS and reverse its harmful effects in cells. In this in vitro study, we demonstrated that air-drying for 5 min could induce ROS generation in both nucleus and cytoplasm of human keratinocytes HaCaT as quantified by CellROX® Green/Orange reagents (Thermo Fisher Scientific, Waltham, Massachusetts, U.S.), respectively. Conversely, when the air-drying time was increased to 10 and 20 min, HaCaT cells lost the ability to produce ROS. Scanning electron microscopic (SEM) images showed that 10 min air-drying could induce severe membrane damage in HaCaT cells. PrestoBlue assay showed that, when HaCaT cells were air-dried for 20 min, cell viability was decreased to 27.6% of the control cells 48 h later. However, once HaCaT cells were pretreated with HW-prepared media, dehydration-induced intracellular ROS, cell membrane damage and cell death were significantly reduced as compared with double distilled water (DDW) under the same conditions. In conclusion, our data suggested that HW can decrease dehydration-induced harmful effects in human cells partially through its antioxidant capacity.

The debate is centred on the level of dehydration that can be incurred prior to its effects influencing physiological responses and performance.2 Whole-body sweat rate during exercise ranges from 0.5 to 2.0 L hour−1 with some athletes (~2%) sweating substantially more (>3.0 L hour−1).3 Gradual reductions in body mass of 2%–5% can occur if body water losses are not replenished, resulting in marked decrements in plasma (≥10%) and blood (≥6%) volume.4 Such levels of dehydration lead to a state of hyperosmotic hypovolaemia that is proportional to the decrement in total body water.5 Hyperosmolality during exercise reduces sweat rate, and thus evaporative heat loss, for any given core temperature.6 As a result, the magnitude of hyperthermia experienced during endurance exercise under heat stress is exacerbated. Progressive dehydration and hyperthermia also intensify tachycardia and compromise the ability to maintain cardiac output, leading to a reduction in systemic, locomotor muscle and skin blood flow.7 Decrements in endurance capacity generally emerge when body mass losses surpass 2%–3% in association with thermal, cardiovascular and perceptual strain.6 7 To optimise performance, it is recommended to drink during exercise to prevent excessive dehydration (>2% body mass) and electrolyte imbalances.8 9 A dehydration-induced body mass loss of ~2% in a 70 kg individual is associated with a 2%–3% increase in plasma osmolality (~6 mmol kg−1), which is the approximate osmotic threshold that triggers renal water conservation and acquisition (ie, thirst).10 As such, it has also been suggested that athletes need only drink to thirst to maximise performance.11 Given that the sensation of thirst may only be perceived when ~1.5 L of body water has been lost, coupled with thirst being alleviated before complete rehydration is achieved,12 drinking to thirst may lead to involuntary dehydration. The choice of drinking to thirst or adopting a planned hydration strategy should be considered based on exercise intensity, duration and climatic conditions, with the aim to minimise body mass loss within the constraints of the sport (rules and regulations), rather than fully replace the deficit. Podium placements are often determined within the final moments of competition, so having an individualised hydration strategy is likely to optimise high-intensity exercise performance in the heat by reducing the detrimental effects of dehydration in the latter stages of an event.

Children with chronic kidney disease (CKD) can have an inherent vulnerability to dehydration. Younger children are unable to freely access water, and CKD aetiology and stage can associate with reduced kidney concentrating capacity, which can also impact risk. This article aims to review the risk factors and consequences of mild dehydration and underhydration in CKD, with a particular focus on evidence for risk of CKD progression. We discuss that assessment of dehydration in the CKD population is more challenging than in the healthy population, thus complicating the definition of adequate hydration and clinical research in this field. We review pathophysiologic studies that suggest mild dehydration and underhydration may cause hyperfiltration injury and impact renal function, with arginine vasopressin as a key mediator. Randomised controlled trials in adults have not shown an impact of improved hydration in CKD outcomes, but more vulnerable populations with baseline low fluid intake or poor kidney concentrating capacity need to be studied. There is little published data on the frequency of dehydration, and risk of complications, acute or chronic, in children with CKD. Despite conflicting evidence and the need for more research, we propose that paediatric CKD management should routinely include an assessment of individual dehydration risk along with a treatment plan, and we provide a framework that could be used in outpatient settings. Graphical Abstract A higher resolution version of the Graphical abstract is available as Supplementary information