Hormonal and Thirst Modulated Maintenance of Fluid Balance in Young Women with Different Levels of Habitual Fluid Consumption

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.

H2OAthletes study protocol: effects of hydration changes on neuromuscular function in athletes

We aim to understand the effects of hydration changes on athletes' neuromuscular performance, on body water compartments, fat-free mass hydration and hydration biomarkers and to test the effects of the intervention on the response of acute dehydration in the hydration indexes. The H2OAthletes study (clinicaltrials.gov ID: NCT05380089) is a randomised controlled trial in thirty-eight national/international athletes of both sexes with low total water intake (WI) (i.e. < 35·0 ml/kg/d). In the intervention, participants will be randomly assigned to the control (CG, n 19) or experimental group (EG, n 19). During the 4-day intervention, WI will be maintained in the CG and increased in the EG (i.e. > 45·0 ml/kg/d). Exercise-induced dehydration protocols with thermal stress will be performed before and after the intervention. Neuromuscular performance (knee extension/flexion with electromyography and handgrip), hydration indexes (serum, urine and saliva osmolality), body water compartments and water flux (dilution techniques, body composition (four-compartment model) and biochemical parameters (vasopressin and Na) will be evaluated. This trial will provide novel evidence about the effects of hydration changes on neuromuscular function and hydration status in athletes with low WI, providing useful information for athletes and sports-related professionals aiming to improve athletic performance.

Long-term health outcomes associated with hydration status

Body water balance is determined by fundamental homeostatic mechanisms that maintain stable volume, osmolality and the composition of extracellular and intracellular fluids. Water balance is maintained by multiple mechanisms that continuously match water losses through urine, the skin, the gastrointestinal tract and respiration with water gains achieved through drinking, eating and metabolic water production. Hydration status is determined by the state of the water balance. Underhydration occurs when a decrease in body water availability, due to high losses or low gains, stimulates adaptive responses within the water balance network that are aimed at decreasing losses and increasing gains. This stimulation is also accompanied by cardiovascular adjustments. Epidemiological and experimental studies have linked markers of low fluid intake and underhydration - such as increased plasma concentration of vasopressin and sodium, as well as elevated urine osmolality - with an increased risk of new-onset chronic diseases, accelerated aging and premature mortality, suggesting that persistent activation of adaptive responses may be detrimental to long-term health outcomes. The causative nature of these associations is currently being tested in interventional trials. Understanding of the physiological responses to underhydration may help to identify possible mechanisms that underlie potential adverse, long-term effects of underhydration and inform future research to develop preventative and treatment approaches to the optimization of hydration status.

Low daily water intake profile-is it a contributor to disease?

Few previous review articles have focused on the associations between inadequate daily water intake (LOW) or urinary biomarkers of dehydration (UD; low urine volume or high urine osmolality) and multiple diseases. Accordingly, we conducted manual online searches (47 key words) of the PubMed, Embase, and Google Scholar databases with these inclusion criteria: English language, full-text, peer reviewed, no restriction on research design, and three publications minimum. Initially, 3,903 articles were identified based on their titles and abstracts. Evaluations of full length .pdf versions identified 96 studies that were acceptable for inclusion. We concluded that the evidence is insufficient or conflicting for seven disorders or diseases (i.e. suggesting the need for additional clarifying research) and it is lacking for all-cause mortality. Differential characterizations among women and men have been reported in the results of nine studies involving five diseases. Finally, the evidence for associations of LOW or UD is strong for both kidney stones and type 2 diabetes with hyperglycemia. This suggests that great public health value (i.e. reduced disease risk) may result from increased daily water intake-a simple and cost-effective dietary modification.

Athletes with different habitual fluid intakes differ in hydration status but not in body water compartments

Physiological differences have been reported between individuals who have habitual low (LOW) and high (HIGH) water intake (WI). The aims of this study were to explore body water compartments, hydration status, and fat-free mass (FFM) hydration of elite athletes exposed to different habitual WI. A total of 68 athletes (20.6 ± 5.3 years, 23 females) participated in this observational cross-sectional study. Total WI was assessed by seven-day food diaries and through WI, athletes were categorized as HIGH (n = 28, WI≥40.0 mL/kg/d) and LOW (n = 40, WI≤35.0 mL/kg/d). Total body water (TBW) and extracellular water (ECW) were determined by dilution techniques and intracellular water (ICW) as TBW-ECW. Hydration status was assessed by urine-specific gravity (USG) using a refractometer. Fat (FM) and FFM were assessed by dual-energy X-ray absorptiometry (DXA). The FFM hydration was calculated by TBW/FFM. The USG was statistically different between groups for females (LOW: 1.024 ± 0.003; HIGH: 1.015 ± 0.006; p = 0.005) and males (LOW: 1.024 ± 0.002; HIGH: 1.018 ± 0.005; p < 0.001). No differences between groups were detected in body water compartments and FFM hydration in both sexes (p > 0.05). Multiple regression showed that WI remains a predictor of USG regardless of FFM, age, and sex (β = -0.0004, p < 0.01). We concluded that LOW athletes were classified as dehydrated through USG although their water compartments were not different from HIGH athletes. These results suggest that LOW athletes may expectedly maintain the body water compartments' homeostasis through endocrine mechanisms. Interventions should be taken to encourage athletes to have sufficient WI to maintain optimal hydration.

Total water intake guidelines are sufficient for optimal hydration in United States adults

PURPOSE

Recent studies suggest that 24-h urine osmolality (UOsm) for optimal water intake should be maintained < 500 mmol·kg^-1. The purpose of this study was to determine the total water intake (TWI) requirement for healthy adults to maintain optimal hydration as indicated by 24-h urine osmolality < 500 mmol·kg^-1.

METHODS

Twenty-four-hour UOsm was assessed in 49 men and 50 women residing in the United States (age: 41 ± 14 y, body mass index: 26.3 ± 5.2 kg·m^-2). TWI was assessed from 7-day water turnover, using a dilution of deuterium oxide, corrected for metabolic water production. The diagnostic accuracy of TWI to identify UOsm < 500 mmol·kg^-1 was evaluated using receiver operating characteristic (ROC) analysis in men and women separately.

RESULTS

Twenty-four-hour UOsm was 482 ± 229 and 346 ± 182 mmol·kg^-1 and TWI was 3.57 ± 1.10 L·d^-1 and 3.20 ± 1.27 L·d^-1 in men and women, respectively. ROC analysis for TWI detecting 24-h UOsm < 500 mmol·kg^-1 in men yielded an area under the curve (AUC) of 77.4% with sensitivity, specificity, and threshold values of 83.3%, 64.5%, and 3.39 L·d^-1, respectively. The AUC was 82.4% in women with sensitivity, specificity, and threshold values of 85.7%, 72.1%, and 2.61 L·d^-1.

CONCLUSION

Considering threshold values in men and women of 3.4 L·d^-1 and 2.6 L·d^-1, respectively, maintaining TWI in line with National Academy of Medicine guidelines of 3.7 L·d^-1 in men and 2.7 L·d^-1 in women should be sufficient for most individuals in the United States to maintain 24-h UOsm < 500 mmol·kg^-1.

Habitual Total Drinking Fluid Intake Did Not Affect Plasma Hydration Biomarkers among Young Male Athletes in Beijing, China: A Cross-Sectional Study

The purposes of this study were to explore the drinking patterns, and urinary and plasma hydration biomarkers of young adults with different levels of habitual total drinking fluid intake. A cross-sectional study was conducted among 111 young male athletes in Beijing, China. Total drinking fluids and water from food were assessed by a 7-day, 24-h fluid intake questionnaire and the duplicate portion method, respectively. The osmolality and electrolyte concentrations of the 24-h urine and fasting blood samples were tested. Differences in groups LD1 (low drinker), LD2, HD1, and HD2 (high drinker), divided according to the quartiles of total drinking fluids, were compared using one-way ANOVA, Kruskal−Wallis H-tests, and chi-squared tests. A total of 109 subjects completed the study. The HD2 group had greater amounts of TWI (total water intake) and higher and lower contributions of total drinking fluids and water from food to TWI, respectively, than the LD1, LD2, and HD1 groups (p < 0.05), but the amounts of water from food did not differ significantly among the four groups (all p > 0.05). Participants in the HD2 group had higher amounts of water than participants in the LD1, LD2, and HD1 groups (p < 0.05); SSBs were the second top contributor of total drinking fluids, ranging from 24.0% to 31.8%. The percentage of subjects in optimal hydration status increased from 11.8% in the LD1 group to 58.8% in the HD2 group (p < 0.05). The HD2 and HD1 groups had 212−227 higher volumes of urine than the LD1 and LD2 groups (p < 0.05). No significant differences were found in the plasma biomarkers (p > 0.05), with the exception of higher concentrations of K in the HD1 group than in the LD1 group (p < 0.05). Subjects with higher amounts of total drinking fluids had better hydration status than those with lower total drinking fluids, but not better drinking patterns. Habitual total drinking fluids did not affect the plasma biomarkers.

Clinical characteristics and outcomes of hyponatraemia associated with oral water intake in adults: a systematic review

INTRODUCTION

Excessive water intake is rarely associated with life-threatening hyponatraemia. The aim of this study was to determine the clinical characteristics and outcomes of hyponatraemia associated with excess water intake.

METHODS

This review was conducted using Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. All studies (case reports, observational or interventional studies) reporting excess water intake and hyponatraemia in adults (1946-2019) were included.

RESULTS

A total of 2970 articles were identified and 177 were included (88.7% case reports), consisting of 590 patients. The mean age was 46±16 years (95% CI 44 to 48 years), 47% female, 52% had a chronic psychiatric disorder and 31% had no underlying condition. The median volume of water consumed and serum sodium at presentation was 8 L/day (95% CI 8.9 to 12.2 L/day) and 118 mmol/L (95% CI 116 to 118 mmol/L), respectively. The motivator for increased water consumption was psychogenic polydipsia (55%); iatrogenic (13%); exercise (12%); habitual/dipsogenic polydipsia (7%) and other reasons (13%). The clinical features on presentation were severe in 53% (seizures, coma); moderate in 35% (confusion, vomiting, agitation) and mild in 5% (dizziness, lethargy, cognitive deficit) and not reported in 5% of studies. Treatment was supportive in 41% of studies (fluid restriction, treatment of the underlying cause, emergency care), and isotonic and hypertonic saline was used in 18% and 28% of cases, respectively. Treatment-related complications included osmotic demyelination (3%) and rhabdomyolysis (7%), and death occurred in 13% of cases.

CONCLUSION

Water intoxication is associated with significant morbidity and mortality and requires daily intake to substantially exceed population-based recommendations. The limitations of this analysis are the low quality and high risk of bias of the included studies.

PROSPERO REGISTRATION NUMBER

A pre-existing protocol in the international prospective register of systematic reviews was updated to incorporate any new amendments and reregistered at http://www.crd.york.ac.uk/PROSPERO (registration no. CRD42019129809).

Estrogen to Progesterone Ratio and Fluid Regulatory Responses to Varying Degrees and Methods of Dehydration

The purpose of this study was to investigate the relationship between volume regulatory biomarkers and the estrogen to progesterone ratio (E:P) prior to and following varying methods and degrees of dehydration. Ten women (20 ± 1 year, 56.98 ± 7.25 kg, 164 ± 6 cm, 39.59 ± 2.96 mL•kg•min⁻¹) completed four intermittent exercise trials (1.5 h, 33.8 ± 1.3°C, 49.5 ± 4.3% relative humidity). Testing took place in two hydration conditions, dehydrated via 24-h fluid restriction (Dehy, USG > 1.020) and euhydrated (Euhy, USG ≤ 1.020), and in two phases of the menstrual cycle, the late follicular phase (days 10–13) and midluteal phase (days 18–22). Change in body mass (%BMΔ), serum copeptin concentration, and plasma osmolality (P_osm) were assessed before and after both dehydration stimuli (24-h fluid restriction and exercise heat stress). Serum estrogen and progesterone were analyzed pre-exercise only. Estrogen concentration did not differ between phases or hydration conditions. Progesterone was significantly elevated in luteal compared to follicular in both hydration conditions (Dehy—follicular: 1.156 ± 0.31, luteal: 5.190 ± 1.56 ng•mL⁻¹, P < 0.05; Euhy—follicular: 0.915 ± 0.18, luteal: 4.498 ± 1.38 ng·mL⁻¹, P < 0.05). As expected, E:P was significantly greater in the follicular phase compared to luteal in both hydration conditions (Dehy—F:138.94 ± 89.59, L: 64.22 ± 84.55, P < 0.01; Euhy—F:158.13 ± 70.15, L: 50.98 ± 39.69, P < 0.01, [all •10³]). Copeptin concentration was increased following 24-h fluid restriction and exercise heat stress (mean change: 18 ± 9.4, P < 0.01). We observed a possible relationship of lower E:P and higher copeptin concentration following 24-h fluid restriction (r = −0.35, P = 0.054). While these results did not reach the level of statistical significance, these data suggest that the differing E:P ratio may alter fluid volume regulation during low levels of dehydration but have no apparent impact after dehydrating exercise in the heat.

Associations between urinary hydration markers and metabolic dysfunction: a cross-sectional analysis of NHANES data, 2008-2010

PURPOSE

Growing evidence suggests hydration plays a role in metabolic dysfunction, however data in humans are scarce. This study examined the cross-sectional association between hydration and metabolic dysfunction in a representative sample of the US population.

METHODS

Data from 3961 adult NHANES (National Health and Nutrition Examination Survey) participants (49.8% female; age 46.3 ± 0.5 years) were grouped by quartile of urine specific gravity (U_SG, 2007-2008 cohort) or urine osmolality (U_Osm, 2009-2010 cohort) as measures of hydration. Metabolic dysfunction was assessed by glycemic and insulinemic endpoints and by components of the metabolic syndrome. Multivariate-adjusted linear and logistic regression models were used.

RESULTS

Increasing quartiles of U_SG but not U_Osm was associated with higher fasting plasma glucose (FPG), glycated hemoglobin (all P < 0.01), HOMA-IR and elevated insulin (all P < 0.05). Compared with the lowest quartile, those with the highest U_SG but not U_Osm had greater risk of metabolic syndrome (Q4 vs. Q1, OR (99% CI): 1.6 (1.0, 2.7), P = 0.01) and diabetes (Q4 vs. Q1, OR: 1.8 (1.0, 3.4), P < 0.05). Additionally, those with U_SG > 1.013 or U_Osm > 500 mOsm/kg, common cut-off values for optimal hydration based on retrospective analyses of existing data, had less favorable metabolic markers. In a subset of participants free from diabetes mellitus, impaired kidney function, hypertension and diuretic medication, U_SG remained positively associated with FPG (P < 0.01) and elevated FPG (P < 0.05).

CONCLUSION

These analyses provide population-based evidence that U_SG as a proxy for hydration is associated with glucose homeostasis in NHANES 2007-2008. The same association was not significant when U_Osm was used as a proxy for hydration in the 2009-2010 wave.

CLINICAL TRIAL REGISTRY

Not applicable, as this was a reanalysis of existing NHANES data.

A Journey through the Early Evidence Linking Hydration to Metabolic Health

The idea that water intake or hydration may play an intrinsic, independent role in modulating metabolic disease risk is relatively recent. Here, we outline the journey from early experimental works to more recent evidence linking water and hydration to metabolic health. It has been known for decades that individuals with existing metabolic dysfunction experience challenges to body water balance and have elevated arginine vasopressin (AVP), <underline>a key</underline> hormone regulating body fluid homeostasis. Later, intervention studies demonstrated that altering fluid balance in these individuals could worsen their condition, suggesting that hydration played a role in modulating glycemic control. More recently, observational and interventional studies in healthy individuals have implicated the hydration-vasopressin axis in the pathophysiology of metabolic diseases. Individuals with higher AVP (or its surrogate, copeptin) are at higher risk for developing type 2 diabetes and components of the metabolic syndrome, an association that remains even when controlling for known risk factors. Supporting preclinical work also suggests a causal role for AVP in metabolic dysfunction. It is known that individuals who habitually drink less fluids tend to have higher circulating AVP, which may be lowered by increasing water intake. In the short term, water supplementation in habitual low drinkers with high copeptin may reduce fasting glucose or glucagon, generating a proof of concept for the role of water supplementation in reducing incident metabolic disease. A large randomized trial is ongoing to determine whether water supplementation for 1 year in subjects with low water intake can meaningfully reduce fasting glucose, risk of new-onset diabetes, and other cardiometabolic risk factors.

Renal water conservation and the volume kinetics of fluid-induced diuresis: A retrospective analysis of two cohorts of elderly men

Renal water conservation after an overnight fast mirrors the habitual intake of liquid. The hypothesis in the present study was that water conservation influences the diuretic response to infusion of two types of crystalloid fluid. Twenty-three elderly male patients (mean age 72 years) underwent a total of 46 intravenous infusions of 1.0 or 1.5 L of either hypotonic non-electrolyte fluid (glycine 1.5%) or isotonic electrolyte fluid (Ringer's acetate or 0.9% saline). Urine osmolality (used to indicate renal water conservation) and plasma creatinine were measured before the infusions started. A two-volume model was fitted to repeated measurements of the blood haemoglobin concentration and the urinary excretion, using mixed-effects modelling software. Urine osmolality was examined as a potential covariate to the fixed kinetic parameters. The results show that distribution and redistribution of infused fluid occurred twice as fast for the non-electrolyte fluids as for the electrolyte-containing fluids, while the urine flow showed less difference. For both types of fluid, high urine osmolality served as a statistically significant covariate to the rate constant describing urinary excretion. Simulations showed that a high pre-infusion urine osmolality doubled the time required for the kidneys to excrete 50% of a 30-minute infusion. High plasma creatinine independently prolonged the elimination of non-electrolyte fluid. The use of 0.9% saline instead of Ringer's prolonged the excretion of electrolyte-containing fluid. In conclusion, renal water conservation is a determinant of the diuretic response to crystalloid fluid, regardless of whether the fluid contains electrolytes, and it should be considered in fluid balance studies.

The comparison of water intake patterns and hydration biomarkers among young adults with different hydration statuses in Hebei, China

Background

Water is essential for maintaining the functions of human body properly. Studies have shown that the amounts and contributions of fluids were associated with health and hydration status. The objectives of the study was that to explore the differences of water intake pattern and hydration biomarkers among young males and females in different hydration statuses.

Methods

A cross-sectional study was implemented among 159 young adults aged 18–23 years in Hebei, China. The total drinking fluids and water from food were obtained by 7-day 24-h fluid intake questionnaire and duplicate portion method, respectively. The osmolality and electrolyte concentrations of the 24 h urine and plasma were tested. Differences in optimal hydration (OH), middle hydration (MH) and hypohydration (HH) groups, divided by the osmolality of 24 h urine, were compared.

Results

Totally, 156 participants (80 males and 76 females) completed the study. OH group had highest proportions of participants met the recommendations of total water intake (TWI) and total drinking fluids of China (34.5%, 36.2%), while HH group had lowest (7.7%, 0.0%). OH group had higher amounts of TWI, total drinking fluids, water and lower amounts of sugar-sweetened-beverages (SSBs) (P < 0.05). The percentage of total drinking fluids in TWI decreased from 54.1% in OH group to 42.6% in HH group (P < 0.05). OH group had higher and lower contributions of water and SSBs to total drinking fluids (P < 0.05); produced 551–950 mL more, excreted significantly less quantity of solutes of urine (P < 0.05). No significant differences were found in plasma osmolality among the three groups (P > 0.05). Among both males and females, the amounts of TWI and water were higher in OH group than others (P < 0.05). Males had 4.3% lower, 5.4% and 1.1% higher contributions of milk and milk products, SSBs and alcohol to total drinking fluids than females (P < 0.05); males had higher volume of urine than females only in MH group (P < 0.05). There were no significant differences of plasma osmolality between males and females in the same group (P > 0.05).

Conclusions

Young adults with optimal hydration status had better water intake pattern and less concentrated urine. Females maybe have better water intake pattern than males.

Trial registration Chinese clinical trial registry. Name of the registry: Relationship of drinking water and urination. Trial registration number: ChiCTR-ROC-17010320. Date of registration: 01/04/2017. URL of trial registry record: http://www.chictr.org.cn/edit.aspx?pid=17601&htm=4.

Hydration for health hypothesis: a narrative review of supporting evidence

PURPOSE

An increasing body of evidence suggests that excreting a generous volume of diluted urine is associated with short- and long-term beneficial health effects, especially for kidney and metabolic function. However, water intake and hydration remain under-investigated and optimal hydration is poorly and inconsistently defined. This review tests the hypothesis that optimal chronic water intake positively impacts various aspects of health and proposes an evidence-based definition of optimal hydration.

METHODS

Search strategy included PubMed and Google Scholar using relevant keywords for each health outcome, complemented by manual search of article reference lists and the expertise of relevant practitioners for each area studied.

RESULTS

The available literature suggest the effects of increased water intake on health may be direct, due to increased urine flow or urine dilution, or indirect, mediated by a reduction in osmotically -stimulated vasopressin (AVP). Urine flow affects the formation of kidney stones and recurrence of urinary tract infection, while increased circulating AVP is implicated in metabolic disease, chronic kidney disease, and autosomal dominant polycystic kidney disease.

CONCLUSION

In order to ensure optimal hydration, it is proposed that optimal total water intake should approach 2.5 to 3.5 L day^-1 to allow for the daily excretion of 2 to 3 L of dilute (< 500 mOsm kg^-1) urine. Simple urinary markers of hydration such as urine color or void frequency may be used to monitor and adjust intake.

Effects of diet, habitual water intake and increased hydration on body fluid volumes and urinary analysis of renal fluid retention in healthy volunteers

Purpose

To increase our knowledge about the causes and physiological consequences of concentrated urine, the relevance of which in the general population is uncertain.

Methods

Twenty healthy volunteers (mean age 42 years) recorded all intake of food and water for 2 weeks. During the 2nd week, they increased their daily consumption of water by 716 mL (32%). The volunteers delivered a 24-h and a morning urine sample for analysis of osmolality and creatinine during the first 4 days of both weeks, and a sample each time they voided on the other days. The water content of food and liquid was calculated and the body fluid volumes were measured by bioimpedance. Haemodynamic stability was assessed with the passive leg-raising test.

Results

There was a curvilinear correlation between the daily intake of water and biomarkers measured in the 24-h collection of urine (coefficient of determination 0.37–0.70). Habitual low intake of water was associated with larger body fluid volumes. The increased fluid intake during the 2nd week was best reflected in the 24-h collection (−15 and −20% for the osmolality and creatinine, respectively, P < 0.002), while morning urine and body fluid volumes were unchanged. Increased fluid intake improved the haemodynamic stability in volunteers with a low intake of water (< median), but only in those who had minimally concentrated morning urine.

Conclusions

The 24-h collection reflected recent intake of fluid, whereas the morning urine seemed to mirror long-term corrections of the fluid balance. Concentrated urine was associated with larger body fluid volumes.

Effect of Increased Daily Water Intake and Hydration on Health in Japanese Adults

Increased hydration is recommended as healthy habit with several merits. However, supportive data are sparse. To assess the efficacy of increased daily water intake, we tested the effect of water supplementation on biomarkers in blood, urine, and saliva. Twenty-four healthy Japanese men and 31 healthy Japanese women with fasting blood glucose levels ranging from 90–125 mg/dL were included. An open-label, two-arm, randomized controlled trial was conducted for 12 weeks. Two additional 550 mL bottles of water on top of habitual fluid intake were consumed in the intervention group. The subjects drank one bottle of water (550 mL) within 2 h of waking, and one bottle (550 mL) 2 h before bedtime. Subjects increased mean fluid intake from 1.3 L/day to 2.0 L/day, without changes in total energy intake. Total body water rate increased with associated water supplementation. There were no significant changes in fasting blood glucose and arginine vasopressin levels, but systolic blood pressure was significantly decreased in the intervention group. Furthermore, water supplementation increased body temperature, reduced blood urea nitrogen concentration, and suppressed estimated glomerular filtration rate reduction. Additionally, existence of an intestinal microbiome correlated with decreased systolic blood pressure and increased body temperature. Habitual water supplementation after waking up and before bedtime in healthy subjects with slightly elevated fasting blood glucose levels is not effective in lowering these levels. However, it represents a safe and promising intervention with the potential for lowering blood pressure, increasing body temperature, diluting blood waste materials, and protecting kidney function. Thus, increasing daily water intake could provide several health benefits.

Drinking patterns and hydration biomarkers among young adults with different levels of habitual total drinking fluids intake in Baoding, Hebei Province, China: a cross-sectional study

BACKGROUND

The purposes were to investigate the drinking patterns and hydration biomarkers among young adults with different levels of habitual total drinking fluids intake.

METHODS

A cross-sectional study was conducted among 159 young adults aged 18-23 years in Baoding, China. Total drinking fluids and water from food were assessed by 7-day 24-h fluid intake questionnaire and duplicate portion method, respectively. The osmolality and electrolyte concentrations of the 24 h urine and fasting blood samples were tested. Differences in LD1 (low drinker), LD2, LD3 and HD (high drinker) groups, stratified according to the quartiles of total drinking fluids, were compared using one-way ANOVA, Kruskal-Wallis H test and chi-square test.

RESULTS

A total of 156 participants (80 males and 76 females) completed the study. HD group had greater amounts of TWI (Total Water Intake), water from food, higher and lower contributions of total drinking fluids and water from food to TWI, respectively, than LD1, LD2 and LD3 groups (p < 0.05). Participants in HD group had higher amounts of water and water from dishes than participants in LD1, LD2 and LD3 groups (p < 0.05). No significant differences were found in the contributions of different fluids to total drinking fluids within the four groups (p > 0.05). The osmolality of urine was 59-143 mOsm/kg higher in LD1 than that in LD2, LD3 and HD group (p < 0.05). The percentage of participants in optimal hydration status increased from 12.8% in LD1 group to 56.4% in HD group (p < 0.05). HD and LD3 groups had 386~793 higher volumes of urine than that of LD1 and LD2 groups (p < 0.05). Differences were found in the concentrations of electrolytes among the four groups (p < 0.05). No significant differences were found in the plasma biomarkers (p > 0.05), with the exception of higher concentration of Mg in LD3 and HD groups than that in LD1 and LD2 groups (p < 0.05).

CONCLUSIONS

Participants with higher total drinking fluids had better drinking pattern and hydration status. Interventions should be undertaken to advise adults to have adequate total drinking fluids, in order to keep in optimal hydration status.

TRIAL REGISTRATION

The registration number was ChiCTR-ROC-17010320, which was registered on the Chinese clinical trial registry.

Distinguishing Low and High Water Consumers-A Paradigm of Disease Risk

A long-standing body of clinical observations associates low 24-h total water intake (TWI = water + beverages + food moisture) with acute renal disorders such as kidney stones and urinary tract infections. These findings prompted observational studies and experimental interventions comparing habitual low volume (LOW) and high volume (HIGH) drinkers. Investigators have learned that the TWI of LOW and HIGH differ by 1-2 L·d-1, their hematological values (e.g., plasma osmolality, plasma sodium) are similar and lie within the laboratory reference ranges of healthy adults and both groups appear to successfully maintain water-electrolyte homeostasis. However, LOW differs from HIGH in urinary biomarkers (e.g., reduced urine volume and increased osmolality or specific gravity), as well as higher plasma concentrations of arginine vasopressin (AVP) and cortisol. Further, evidence suggests that both a low daily TWI and/or elevated plasma AVP influence the development and progression of metabolic syndrome, diabetes, obesity, chronic kidney disease, hypertension and cardiovascular disease. Based on these studies, we propose a theory of increased disease risk in LOW that involves chronic release of fluid-electrolyte (i.e., AVP) and stress (i.e., cortisol) hormones. This narrative review describes small but important differences between LOW and HIGH, advises future investigations and provides practical dietary recommendations for LOW that are intended to decrease their risk of chronic diseases.

High water intake and low urine osmolality are associated with favorable metabolic profile at a population level: low vasopressin secretion as a possible explanation

Purpose

Elevated plasma concentration of the vasopressin marker copeptin and low water intake are associated with elevated blood glucose and diabetes risk at a population level. Moreover, in individuals with low urine volume and high urine osmolality (u-Osm), water supplementation reduced fasting plasma (fp) copeptin and fp-glucose. In this observational study, we investigated if low total water intake or high u-Osm correlated with high fp-copeptin and components of the metabolic syndrome at the population level.

Methods

In the population-based Malmö Offspring Study (MOS, n = 2599), fp-copeptin and u-Osm from morning urine samples were measured, and diet and total water intake (from beverages and food moisture) was assessed by a 4-day web-based record.

Results

Increasing water intake by tertile was after adjustment for age and sex associated with low fp-triglycerides (p = 0.002) and high fp-HDL (p = 0.004), whereas there was no association with the other investigated metabolic traits (HbA1c, fp-glucose, BMI or waist circumference). Increasing u-Osm by tertile was, after adjustment for age and sex, associated with high fp-glucose (p = 0.007), and borderline significantly associated with high HbA1c (p = 0.053), but no association was observed with fp-HDL, fp-triglycerides, BMI or waist circumference. Fp-copeptin concentration correlated significantly with water intake (r = − 0.13, p < 0.001) and u-Osm (r = 0.27, p < 0.001). High copeptin was associated with all investigated metabolic traits (p < 0.001 for all).

Conclusion

Low concentrations of the vasopressin marker copeptin is linked to high water intake, low u-Osm, and a favorable metabolic profile, suggesting that vasopressin lowering lifestyle interventions, such as increased water intake, may promote metabolic health.

Electronic supplementary material

The online version of this article (10.1007/s00394-020-02202-7) contains supplementary material, which is available to authorized users.

SGLT2 inhibition increases serum copeptin in young adults with type 1 diabetes

AIMS

Copeptin, a surrogate of vasopressin, is elevated in type 1 diabetes (T1D) and predicts kidney disease and cardiovascular mortality. Given the cardiorenal protective effects of SGLT2 inhibition (SGLT2i), our aim was to examine: 1) the relationship between serum copeptin, metabolic, renal and systemic hemodynamic parameters in adults with T1D; and 2) serum copeptin after SGLT2i with empagliflozin.

MATERIALS AND METHODS

In this post-hoc, exploratory analysis, serum copeptin, glomerular filtration rate (GFR_Inulin), effective renal plasma flow (ERPF_PAH), plasma renin angiotensin aldosterone system markers, HbA_1c, 24-hour urine volume and sodium excretion were measured in 40 participants with T1D (24.3±5.1 years) during eu- and hyperglycaemia before and after 8 weeks of 25mg of daily empagliflozin.

RESULTS

Higher baseline copeptin correlated with higher HbA_1c, lower 24-hour urine volume and sodium excretion, after correcting for age, sex, systolic blood pressure, and HbA_1c. Copeptin concentrations increased in response to empagliflozin under euglycaemia (4.1±2.1 to 5.1±2.8pmol/L, P=0.0053) and hyperglycaemia (3.3±1.4 to 5.6±2.8pmol/L, P<0.0001). The rise in copeptin in response to empagliflozin correlated with change in 24-hour urine volume, but was independent of changes in fractional excretion of sodium and haematocrit.

CONCLUSIONS

Elevated serum copeptin was associated with worse glycaemic control and lower diuresis and natriuresis. SGLT2i increased serum copeptin in adults with T1D, and the rise correlated with change in diuresis, but not natriuresis and hemo-concentration. Further work is required to evaluate the clinical implications of elevated copeptin with SGLT2i, including whether it is simply a marker of diuresis or may contribute to cardiorenal disease long-term.

Osmotic stimulation of vasopressin acutely impairs glucose regulation: a counterbalanced, crossover trial

BACKGROUND

Epidemiological studies in humans show increased concentrations of copeptin, a surrogate marker of arginine vasopressin (AVP), to be associated with increased risk for type 2 diabetes.

OBJECTIVES

To examine the acute and independent effect of osmotically stimulated AVP, measured via the surrogate marker copeptin, on glucose regulation in healthy adults.

METHODS

Sixty subjects (30 females) participated in this crossover design study. On 2 trial days, separated by ≥7 d (males) or 1 menstrual cycle (females), subjects were infused for 120 min with either 0.9% NaCl [isotonic (ISO)] or 3.0% NaCl [hypertonic (HYPER)]. Postinfusion, a 240-min oral-glucose-tolerance test (OGTT; 75 g) was administered.

RESULTS

During HYPER, plasma osmolality and copeptin increased (P < 0.05) and remained elevated during the entire 6-h protocol, whereas renin-angiotensin-aldosterone system hormones were within the lower normal physiological range at the beginning of the protocol and declined following infusion. Fasting plasma glucose did not differ between trials (P > 0.05) at baseline and during the 120 min of infusion. During the OGTT the incremental AUC for glucose from postinfusion baseline (positive integer) was greater during HYPER (401.5 ± 190.5 mmol/L·min) compared with the ISO trial (354.0 ± 205.8 mmol/L·min; P < 0.05). The positive integer of the AUC for insulin during OGTT did not differ between trials (HYPER 55,850 ± 36,488 pmol/L·min compared with ISO 57,205 ± 31,119 pmol/L·min). Baseline values of serum glucagon were not different between the 2 trials; however, the AUC of glucagon during the OGTT was also significantly greater in HYPER (19,303 ± 3939 ng/L·min) compared with the ISO trial (18,600 ± 3755 ng/L·min; P < 0.05).

CONCLUSIONS

The present data indicate that acute osmotic stimulation of copeptin induced greater hyperglycemic responses during the oral glucose challenge, possibly due to greater glucagon concentrations.This study was registered at clinicaltrials.gov as NCT02761434.

Thirst and Drinking Paradigms: Evolution from Single Factor Effects to Brainwide Dynamic Networks

The motivation to seek and consume water is an essential component of human fluid–electrolyte homeostasis, optimal function, and health. This review describes the evolution of concepts regarding thirst and drinking behavior, made possible by magnetic resonance imaging, animal models, and novel laboratory techniques. The earliest thirst paradigms focused on single factors such as dry mouth and loss of water from tissues. By the end of the 19th century, physiologists proposed a thirst center in the brain that was verified in animals 60 years later. During the early- and mid-1900s, the influences of gastric distention, neuroendocrine responses, circulatory properties (i.e., blood pressure, volume, concentration), and the distinct effects of intracellular dehydration and extracellular hypovolemia were recognized. The majority of these studies relied on animal models and laboratory methods such as microinjection or lesioning/oblation of specific brain loci. Following a quarter century (1994–2019) of human brain imaging, current research focuses on networks of networks, with thirst and satiety conceived as hemispheric waves of neuronal activations that traverse the brain in milliseconds. Novel technologies such as chemogenetics, optogenetics, and neuropixel microelectrode arrays reveal the dynamic complexity of human thirst, as well as the roles of motivation and learning in drinking behavior.

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.

Adequacy of Daily Fluid Intake Volume Can Be Identified From Urinary Frequency and Perceived Thirst in Healthy Adults

Objective: Achieving and maintaining an optimal level of hydration has significant implications for both acute and chronic health, yet many hydration assessments are not feasible for the general public. Urinary frequency (UF) is a reliable method to self-assess hydration status in healthy individuals, and thirst can provide additional sensory information on adequacy of daily fluid intake volume (DFI). However, threshold values for these indices to detect optimal hydration have not been determined. In this study, we sought to determine threshold values for 24-hour UF and perceived thirst that could accurately distinguish between optimal and suboptimal hydration states.Methods: Thirty-two healthy adults (age 22 ± 3 years, body mass index 24.9 ± 4.1 kg/m²) collected urine over 24 hours on four separate occasions, where UF was recorded as well as thirst at each void using a numbered perceptual scale. Using urine osmolality as the criterion standard, all samples were either classified as representing an optimal (≤500 mOsm·kg^-1) or suboptimal hydration status (>500 mOsm·kg^-1).Results: A 24-hour UF ≤6 was able to detect suboptimal hydration with good accuracy (area under the curve [AUC] 0.815) and a 24-hour average perceived thirst rating > 3 ("a little thirsty") could detect it with reasonable accuracy (AUC 0.725). In addition, a UF ≤4 had a considerably higher positive likelihood ratio to detect suboptimal hydration versus a UF ≤6 (9.03 versus 2.18, respectively).Conclusions: These analyses suggest that individuals with a 24-hour UF ≤6 or perceiving themselves to be, on average, "a little thirsty" throughout the day are likely to be suboptimally hydrated and thus underconsuming an adequate DFI.

Of Mice and Men-The Physiology, Psychology, and Pathology of Overhydration

The detrimental effects of dehydration, to both mental and physical health, are well-described. The potential adverse consequences of overhydration, however, are less understood. The difficulty for most humans to routinely ingest ≥2 liters (L)-or "eight glasses"-of water per day highlights the likely presence of an inhibitory neural circuit which limits the deleterious consequences of overdrinking in mammals but can be consciously overridden in humans. This review summarizes the existing data obtained from both animal (mostly rodent) and human studies regarding the physiology, psychology, and pathology of overhydration. The physiology section will highlight the molecular strength and significance of aquaporin-2 (AQP2) water channel downregulation, in response to chronic anti-diuretic hormone suppression. Absence of the anti-diuretic hormone, arginine vasopressin (AVP), facilitates copious free water urinary excretion (polyuria) in equal volumes to polydipsia to maintain plasma tonicity within normal physiological limits. The psychology section will highlight reasons why humans and rodents may volitionally overdrink, likely in response to anxiety or social isolation whereas polydipsia triggers mesolimbic reward pathways. Lastly, the potential acute (water intoxication) and chronic (urinary bladder distension, ureter dilation and hydronephrosis) pathologies associated with overhydration will be examined largely from the perspective of human case reports and early animal trials.

Water Supplementation Reduces Copeptin and Plasma Glucose in Adults With High Copeptin: The H2O Metabolism Pilot Study

OBJECTIVE

Because elevated copeptin, a marker of vasopressin, is linked to low water intake and high diabetes risk, we tested the effect of water supplementation on copeptin and fasting glucose.

DESIGN, SETTING, AND PARTICIPANTS

Thirty-one healthy adults with high copeptin (>10.7 pmol · L-1 in men and >6.1 pmol·L-1 in women) identified in a population-based survey from 2013 to 2015 and with a current 24-hour urine osmolality of >600 mOsm · kg-1 were included.

INTERVENTION

Addition of 1.5 L water daily on top of habitual fluid intake for 6 weeks.

MAIN OUTCOME MEASURE

Pre- and postintervention fasting plasma copeptin concentrations.

RESULTS

Reported mean water intake increased from 0.43 to 1.35 L · d-1 (P < 0.001), with no other observed changes in diet. Median (interquartile range) urine osmolality was reduced from 879 (705, 996) to 384 (319, 502) mOsm · kg-1 (P < 0.001); urine volume increased from 1.06 (0.90, 1.20) to 2.27 (1.52, 2.67) L · d-1 (P < 0.001); and baseline copeptin decreased from 12.9 (7.4, 21.9) pmol · L-1 to 7.8 (4.6;11.3) pmol · L-1 (P < 0.001). Water supplementation reduced fasting plasma glucose from a mean (SD) of 5.94 (0.44) to 5.74 (0.51) (P = 0.04). The water-associated reduction of both fasting copeptin and glucose concentration in plasma was most pronounced in participants in the top tertile of baseline copeptin.

CONCLUSIONS

Water supplementation in persons with habitually low water consumption and high copeptin levels is effective in lowering copeptin. It appears a safe and promising intervention with the potential of lowering fasting plasma glucose and thus reducing diabetes risk. Further investigations are warranted to support these findings.

Water Intake, Water Balance, and the Elusive Daily Water Requirement

Water is essential for metabolism, substrate transport across membranes, cellular homeostasis, temperature regulation, and circulatory function. Although nutritional and physiological research teams and professional organizations have described the daily total water intakes (TWI, L/24h) and Adequate Intakes (AI) of children, women, and men, there is no widespread consensus regarding the human water requirements of different demographic groups. These requirements remain undefined because of the dynamic complexity inherent in the human water regulatory network, which involves the central nervous system and several organ systems, as well as large inter-individual differences. The present review analyzes published evidence that is relevant to these issues and presents a novel approach to assessing the daily water requirements of individuals in all sex and life-stage groups, as an alternative to AI values based on survey data. This empirical method focuses on the intensity of a specific neuroendocrine response (e.g., plasma arginine vasopressin (AVP) concentration) employed by the brain to regulate total body water volume and concentration. We consider this autonomically-controlled neuroendocrine response to be an inherent hydration biomarker and one means by which the brain maintains good health and optimal function. We also propose that this individualized method defines the elusive state of euhydration (i.e., water balance) and distinguishes it from hypohydration. Using plasma AVP concentration to analyze multiple published data sets that included both men and women, we determined that a mild neuroendocrine defense of body water commences when TWI is ˂1.8 L/24h, that 19⁻71% of adults in various countries consume less than this TWI each day, and consuming less than the 24-h water AI may influence the risk of dysfunctional metabolism and chronic diseases.

Effects of hydration on plasma copeptin, glycemia and gluco-regulatory hormones: a water intervention in humans

Purpose

High plasma copeptin, a marker of vasopressin, predicts diabetes mellitus. We tested if copeptin could be suppressed by increased water intake in healthy individuals, and if a water-induced change in copeptin was accompanied by altered concentrations of glucose, insulin or glucagon.

Methods

Thirty-nine healthy individuals underwent, in random order, 1 week of high water intake (3 L/day on top of habitual intake) and 1 week of normal (habitual) fluid intake (control). Fasting plasma concentrations of copeptin, glucose, insulin and glucagon were compared between the ends of both periods. Furthermore, acute copeptin kinetics were mapped for 4 h after ingestion of 1 L of water.

Results

After acute intake of 1 L water, copeptin was significantly reduced within 30 min, and reached maximum reduction within 90 min with on average 39% reduction (95% confidence interval (95 CI) 34–45) (p < 0.001) and remained low the entire test period (4 h). One week of increased water intake led to a 15% reduction (95 CI 5–25) (p = 0.003) of copeptin compared to control week. The greatest reduction occurred among subjects with habitually high copeptin and concentrated urine (“water-responders”). Water-responders had significant water-induced reduction of glucagon, but glucose and insulin were unaffected.

Conclusions

Both acute and 1 week extra water intake potently reduced copeptin concentration. In those with the greatest decline (water-responders), who are typically low drinkers with high baseline copeptin, water induced a reduction in fasting glucagon. Long-term trials assessing the effect of water on glucometabolic traits should focus on low-water drinkers with high copeptin concentration.

Electronic supplementary material

The online version of this article (10.1007/s00394-017-1595-8) contains supplementary material, which is available to authorized users.

Two Liters a Day Keep the Doctor Away? Considerations on the Pathophysiology of Suboptimal Fluid Intake in the Common Population

Suboptimal fluid intake may require enhanced release of antidiuretic hormone (ADH) or vasopressin for the maintenance of adequate hydration. Enhanced copeptin levels (reflecting enhanced vasopressin levels) in 25% of the common population are associated with enhanced risk of metabolic syndrome with abdominal obesity, type 2 diabetes, hypertension, coronary artery disease, heart failure, vascular dementia, cognitive impairment, microalbuminuria, chronic kidney disease, inflammatory bowel disease, cancer, and premature mortality. Vasopressin stimulates the release of glucocorticoids which in turn up-regulate the serum- and glucocorticoid-inducible kinase 1 (SGK1). Moreover, dehydration upregulates the transcription factor NFAT5, which in turn stimulates SGK1 expression. SGK1 is activated by insulin, growth factors and oxidative stress via phosphatidylinositide-3-kinase, 3-phosphoinositide-dependent kinase PDK1 and mTOR. SGK1 is a powerful stimulator of Na+/K+-ATPase, carriers (e.g. the Na+,K+,2Cl- cotransporter NKCC, the NaCl cotransporter NCC, the Na+/H+ exchanger NHE3, and the Na+ coupled glucose transporter SGLT1), and ion channels (e.g. the epithelial Na+ channel ENaC, the Ca2+ release activated Ca2+ channel Orai1 with its stimulator STIM1, and diverse K+ channels). SGK1 further participates in the regulation of the transcription factors nuclear factor kappa-B NFκB, p53, cAMP responsive element binding protein (CREB), activator protein-1, and forkhead transcription factor FKHR-L1 (FOXO3a). Enhanced SGK1 activity fosters the development of hypertension, obesity, diabetes, thrombosis, stroke, inflammation including inflammatory bowel disease and autoimmune disease, cardiac fibrosis, proteinuria, renal failure as well as tumor growth. The present brief review makes the case that suboptimal fluid intake in the common population may enhance vasopressin and glucocorticoid levels thus up-regulating SGK1 expression and favouring the development of SGK1 related pathologies.

Reduced water intake deteriorates glucose regulation in patients with type 2 diabetes

Epidemiological research has demonstrated that low daily total water intake is associated with increased diagnosis of hyperglycemia. Possible mechanisms for this increase include hormones related to the hypothalamic pituitary axis as well as the renin-angiotensin-aldosterone system (RAAS). Therefore, the hypothesis of the present study was that acute low water intake would result in differential hormonal profiles and thus impaired blood glucose regulation during an oral glucose tolerance test (OGTT) in people with type 2 diabetes mellitus (T2DM). Nine men (53 ± 9 years, 30.0 ± 4.3 m∙kg^-2, 32% ± 6% body fat) diagnosed with T2DM completed OGTTs in euhydrated (EUH) and hypohydrated (HYP) states in counterbalanced order. Water restriction led to hypohydration of -1.6% of body weight, with elevated plasma (EUH: 288 ± 4, HYP: 298 ± 6 mOsm·kg^-1; P < .05) and urine (EUH: 512 ± 185, HYP: 994 ± 415 mOsm·kg^-1; P < .05) osmolality. There was a significant main effect of condition for serum glucose (at time 0 minute 9.5 ± 4.2 vs 10.4 ± 4.4 mmol∙L^-1 and at time 120 minutes 19.1 ± 4.8 vs 21.0 ± 4.1 mmol∙L^-1 for EUH and HYP, respectively; P < .001) but not insulin (mean difference between EUH and HYP -12.1 ± 44.9 pmol∙L^-1, P = .390). An interaction between time and condition was observed for cortisol: decrease from minute 0 to 120 in EUH (-85.3 ± 82.1 nmol∙L^-1) vs HYP (-25.0 ± 43.0 nmol∙L^-1; P = .017). No differences between conditions were found within RAAS-related hormones. Therefore, we can conclude that 3 days of low total water intake in people with T2DM acutely impairs blood glucose response during an OGTT via cortisol but not RAAS-mediated glucose regulation.

Water intake and urinary hydration biomarkers in children

BACKGROUND/OBJECTIVES

The aims of the study were as follows: (1) examine fluid intake and urinary hydration markers of children in Greece, (2) determine the calculated relative risk of hypohydration in children who did not meet the recommendations for daily water intake provided by the Institute of Medicine and the European Food Safety Authority compared with those who did and (3) analyze the efficacy of the recommendations as a method to achieve euhydration in children.

SUBJECTS/METHODS

One hundred and fifty Greek boys and girls (age 9-13) recorded their fluid intake for 2 consecutive days. A 24-h urine collection was obtained during the second day. Fluid intake records were analyzed for total water intake from fluids (TWI-F), and urine samples were analyzed for osmolality, color, specific gravity and volume. Urine osmolality ⩾800 mmol/kg H₂O was defined as hypohydration.

RESULTS

Water intake from fluids was 1729 (1555-1905) and 1550 (1406-1686) ml/d for boys and girls, respectively. Prevalence of hypohydration was 33% (44% of boys, 23% of girls). Children who failed to meet TWI-F recommendations demonstrated a risk of hypohydration that was 1.99-2.12 times higher than those who met recommendations (P⩽0.01). Boys between 9 and 13 years displayed urine osmolality of 777 (725-830) mmol/kg, and urine specific gravity of 1.021 (1.019-1.022), which was higher than those in girls between 9-13 years (P⩽0.015), and >27% were classified as hypohydrated despite meeting water intake recommendations.

CONCLUSIONS

Failure to meet TWI-F guidelines increased calculated relative risk of hypohydration in children. Boys between 9 and 13 years are at greater hazard regardless of meeting guidelines and may require greater water intake to avoid elevated urine concentration and ensure adequate hydration.

Why do team-sport athletes drink fluid in excess when exercising in cool conditions?

This study assessed the potential physiological and perceptual drivers of fluid intake and thirst sensation during intermittent exercise. Ten male rugby players (17 ± 1 years, stature: 179.1 ± 4.2 cm, body mass (BM): 81.9 ± 8.1 kg) participated in six 6-min small-sided games, interspersed with 2 min rest, where fluid intake was ad libitum during rest periods. Pre- and postmeasurements of BM, subjective ratings (thirst, thermal comfort, thermal sensation, mouth dryness), plasma osmolality (POsm), serum sodium concentration (S[Na⁺]), haematocrit and haemoglobin (to calculate plasma volume change; PV) were taken. Fluid intake was measured during rest periods. BM change was -0.17 ± 0.59% and fluid intake was 0.88 ± 0.38 L. Pre- to post-POsm decreased (-3.1 ± 2.3 mOsm·kg^-1; p = 0.002) and S[Na⁺] remained similar (-0.3 ± 0.7 mmol·L^-1, p = 0.193). ΔPV was 5.84 ± 3.65%. Fluid intake displayed a relationship with pre-POsm (r = -0.640, p = 0.046), prethermal comfort (r = 0.651; p = -0.041), ΔS[Na⁺] (r = 0.816, p = 0.004), and ΔPV (r = 0.740; p = 0.014). ΔThirst sensation displayed a relationship with premouth dryness (r = 0.861, p = 0.006) and Δmouth dryness (r = 0.878, p = 0.004). Yet a weak positive relationship between Δthirst sensation and fluid intake was observed (r = 0.085, p = 0.841). These data observed in an ambient temperature of 13.6 ± 0.9 °C, suggest team-sport athletes drink in excess of fluid homeostasis requirements and thirst sensation in cool conditions; however, this was not influenced by thermal discomfort.