Abnormalities of thirst regulation

Relationships between Morning Thirst and Later Hydration Status and Total Water Intake

Purpose: To investigate the effects of thirst on later hydration status, total water intake (TWI-MA), and its potential sex differences. Methods: Twelve men (mean ± standard deviation; age: 21 ± 2 years; mass: 81.0 ± 15.9 kg) and twelve women (age: 22 ± 3 years; mass: 68.8 ± 15.2 kg) visited the laboratory in the morning (first thing in the morning) and afternoon (2:00-4:00 p.m.) for three consecutive days under a free-living condition. At each visit, urine osmolality (UOSM), urine specific gravity (USG), urine color (UCOL), body mass loss (BML), thirst, and plasma osmolality (POSM) were collected and analyzed. The participants recorded their food and fluid intake between the visits to determine TWI-MA. Linear regression was used to predict the effect of morning thirst on the afternoon hydration indices for all the participants, as well as for males and females separately. Results: Higher morning thirst predicted lower UOSM (r2 = 0.056, p = 0.045), USG (r2 = 0.096, p = 0.008), UCOL (r2 = 0.074, p = 0.021), and higher thirst (r2 = 0.074, p = 0.021) in the afternoon. However, morning thirst did not predict afternoon BML, POSM, or TWI-MA (p > 0.05). In males, higher morning thirst predicted lower afternoon UOSM (r2 = 0.130, p = 0.031) and USG (r2 = 0.153, p = 0.018). Additionally, higher morning thirst predicted higher TWI-MA (r2 = 0.154, p = 0.018) in females. Conclusions: Morning thirst had a negligible impact on later hydration status, specifically with afternoon urine indices. Furthermore, higher thirst sensation did not impact BML, POSM, or TWI-MA. However, thirst sensation minimally contributed to drinking behavior in females. Overall, individuals may not rely solely on thirst sensation to manipulate their drinking behavior to optimize their fluid balance during their daily lives due to the complexity of thirst mechanisms.

Limosilactobacillus reuteri promotes the expression and secretion of enteroendocrine- and enterocyte-derived hormones

Observations that intestinal microbes can beneficially impact host physiology have prompted investigations into the therapeutic usage of such microbes in a range of diseases. For example, the human intestinal microbe Limosilactobacillus reuteri strains ATCC PTA 6475 and DSM 17938 are being considered for use for intestinal ailments including colic, infection, and inflammation as well as non-intestinal ailments including osteoporosis, wound healing, and autism spectrum disorder. While many of their beneficial properties are attributed to suppressing inflammatory responses in the gut, we postulated that L. reuteri may also regulate hormones of the gastrointestinal tract to affect physiology within and outside of the gut. To determine if L. reuteri secreted factors impact the secretion of enteric hormones, we treated an engineered jejunal organoid line, NGN3-HIO, which can be induced to be enriched in enteroendocrine cells, with L. reuteri 6475 or 17938 conditioned medium and performed transcriptomics. Our data suggest that these L. reuteri strains affect the transcription of many gut hormones, including vasopressin and luteinizing hormone subunit beta, which have not been previously recognized as being produced in the gut epithelium. Moreover, we find that these hormones appear to be produced in enterocytes, in contrast to canonical gut hormones which are produced in enteroendocrine cells. Finally, we show that L. reuteri conditioned media promotes the secretion of several enteric hormones including serotonin, GIP, PYY, vasopressin, and luteinizing hormone subunit beta. These results support L. reuteri affecting host physiology through intestinal hormone secretion, thereby expanding our understanding of the mechanistic actions of this microbe.

When Thirst Ceases to Exist: A Case Report and Literature Review of Adipsic Diabetes Insipidus Following Coil Embolization of a Ruptured Anterior Communicating Artery Aneurysm

Diabetes insipidus is a condition characterized by inappropriately dilute urine in the setting of serum hyperosmolality. The two predominant subtypes include central (from lack of vasopressin production) and nephrogenic diabetes insipidus (from renal resistance to circulating vasopressin). A common manifestation is the significant pursuant thirst from excessive polyuria. We present a case report and literature review of an infrequent variation of central diabetes insipidus known as adipsic (hypothalamic) diabetes insipidus, characterized by the absence of thirst, secondary to coiling of a ruptured anterior communicating artery aneurysm. Due to the loss of thirst, patients are at a heightened risk for hypernatremia and complications secondary to dehydration. Our patient's course was complicated by recurrent polyuria and hypernatremia, requiring a fixed-dose desmopressin regimen. On follow-up, only partial thirst sensation was restored. We provide a literature review to compare our case report to the scant literature available to broaden the awareness of this infrequent, perilous, manifestation.

Validity of combined hydration self-assessment measurements to estimate a low vs. high urine concentration in a small sample of (tactical) athletes

PURPOSE

Relationships between body weight, urine color (Uc), and thirst level (WUT) have been proposed as a simple and inexpensive self-assessment method to predict dehydration. This study aimed to determine if this method also allowed us to accurately identify a low vs. high urine concentration in (tactical) athletes.

METHODS

A total of n = 19 Army Reserve Officer Training Corps cadets and club sports athletes (22.7 ± 3.8 years old, of which 13 male) were included in the analysis, providing morning body weight, thirst sensation, and Uc for five consecutive days. Each item received a score 0 or 1, resulting in a WUT score ranging from 0 (likely hydrated) to 3 (very likely dehydrated). WUT model and individual item outcomes were then compared with a ≥ 1.020 urine specific gravity (USG) cut-off indicating a high urine concentration, using descriptive comparisons, generalized linear mixed models, and logistic regression (to calculate the area under the curve (AUC)).

RESULTS

WUT score was not significantly predictive of urine concentration, z = 1.59, p = 0.11. The AUC ranged from 0.54 to 0.77 for test days, suggesting a fair AUC on most days. Only Uc was significantly related to urine concentration, z = 2.49, p = 0.01. The accuracy of the WUT model for correctly classifying urine samples with a high concentration was 68% vs. 51% of samples with a low concentration, resulting in an average accuracy of 61%.

CONCLUSION

This study shows that WUT scores were not predictive of urine concentration, and the method did not substantially outperform the accuracy of Uc scoring alone.

Water consumption patterns impact hydration markers in males working in accordance with the National Institute for Occupational Safety and Health recommendations

The impact of water consumption bolus volume and frequency on hydration biomarkers during work in the heat is unknown. In a randomized, crossover fashion, eight males consumed either 500 mL of water every 40 min or 237 mL of water every 20 min during 2 hr of continuous walking at 6.4 kph, 1.0% grade in a 34 °C/30% relative humidity environment, followed by 2 hr of rest. Hydration biomarkers and variables were assessed pre-work, post-work, and after the 2 hr recovery. There were no differences in body mass between trials at any time point (all p > 0.05). Percent change in plasma volume during work was not different when 237 mL of water was repeatedly consumed (-1.6 ± 8.2%) compared to 500 mL of water (-1.3 ± 3.0%, p = 0.92). Plasma osmolality was maintained over time (p = 0.55) with no difference between treatments (p = 0.21). When consuming 500 mL of water repeatedly, urine osmolality was lower at recovery (205 ± 108 mOsmo/L) compared to pre-work (589 ± 95 mOsmo/L, p < 0.01), different from repeatedly consuming 237 mL of water which maintained urine osmolality from pre-work (548 ± 144 mOsmo/L) through recovery (364 ± 261 mOsmo/L, p = 0.14). Free water clearance at recovery was greater with repeated consumption of 500 mL of water (1.2 ± 1.0 mL/min) compared to 237 mL of water (0.4 ± 0.8 mL/min, p = 0.02). Urine volume was not different between treatments post-work (p = 0.62), but greater after 2 hr of recovery when repeatedly consuming 500 mL of water compared to 237 mL (p = 0.01), leading to greater hydration efficiency upon recovery with repeated consumption of 237 mL of water (68 ± 12%) compared to 500 mL (63 ± 14%, p = 0.01). Thirst and total gastrointestinal symptom scores were not different between treatments at any time point (all p > 0.05). Body temperatures and heart rate were not different between treatments at any time point (all p > 0.05). Drinking larger, less frequent water boluses or drinking smaller, more frequent water boluses are both reasonable strategies to promote adequate hydration and limit changes in body mass in males completing heavy-intensity work in the heat.

Age-Associated Abnormalities of Water Homeostasis

Deficits in renal function, thirst, and responses to osmotic and volume stimulation have been repeatedly demonstrated in older populations. The lessons learned over the past six decades serve to emphasize the fragile nature of water balance characteristic of aging. Older individuals are at increased risk for disturbances of water homeostasis due to both intrinsic disease and iatrogenic causes. These disturbances have real-life clinical implications in terms of neurocognitive effects, falls, hospital readmission and need for long-term care, incidence of bone fracture, osteoporosis, and mortality.

Hyponatremia Demystified: Integrating Physiology to Shape Clinical Practice

Hyponatremia is one of the most common problems encountered in clinical practice and one of the least-understood because accurate diagnosis and management require some familiarity with water homeostasis physiology, making the topic seemingly complex. The prevalence of hyponatremia depends on the nature of the population studied and the criteria used to define it. Hyponatremia is associated with poor outcomes including increased mortality and morbidity. The pathogenesis of hypotonic hyponatremia involves the accumulation of electrolyte-free water caused by either increased intake and/or decrease in kidney excretion. Plasma osmolality, urine osmolality, and urine sodium can help to differentiate among the different etiologies. Brain adaptation to plasma hypotonicity consisting of solute extrusion to mitigate further water influx into brain cells best explains the clinical manifestations of hyponatremia. Acute hyponatremia has an onset within 48 hours, commonly resulting in severe symptoms, while chronic hyponatremia develops over 48 hours and usually is pauci-symptomatic. However, the latter increases the risk of osmotic demyelination syndrome if hyponatremia is corrected rapidly; therefore, extreme caution must be exercised when correcting plasma sodium. Management strategies depend on the presence of symptoms and the cause of hyponatremia and are discussed in this review.

Ex vivo LPS-stimulated cytokine production is associated with hydration status in community-dwelling middle-to-older-aged adults

PURPOSE

Suboptimal hydration has been linked to a variety of adverse health outcomes. Few studies have examined the impact of hydration status on immune function, a plausible physiological mechanism underlying these associations. Therefore, we tested how variation in hydration status was associated with circulating pro-inflammatory cytokine levels and ex vivo lipopolysaccharide (LPS)-stimulated pro-inflammatory cytokine production.

METHODS

Blood samples were obtained from a community sample of healthy middle-to-older-aged adults (N = 72). These samples were used to assess serum osmolality, a biomarker of hydration status, and markers of immune function including circulating pro-inflammatory cytokines and stimulated pro-inflammatory cytokine production after 4 and 24 h of incubation with LPS. Multiple linear regressions were used to test the association between serum osmolality (as a continuous variable) and markers of immune function at baseline and after 4 and 24 h adjusting for age, sex, and BMI. These models were re-estimated with serum osmolality dichotomized at the cut-off for dehydration (> 300 mOsm/kg).

RESULTS

While not significantly associated with circulating cytokines (B = - 0.03, p = 0.09), serum osmolality was negatively associated with both 4 h (B = - 0.05, p = 0.048) and 24 h (B = - 0.05, p = 0.03) stimulated cytokine production when controlling for age, sex, and BMI. Similarly, dehydration was associated with significantly lower cytokine production at both 4 h (B = - 0.54, p = 0.02) and 24 h (B = - 0.51, p = 0.02) compared to adequate hydration.

CONCLUSION

These findings suggest that dehydration may be associated with suppressed immune function in generally healthy middle-to-older aged community-dwelling adults. Further longitudinal research is needed to more clearly define the role of hydration in immune function.

Hydration Strategies for Physical Activity and Endurance Events at High (>2500 m) Altitude: A Practical Management Article

A growing number of adventurous athletes are seeking new challenges through endurance events or physical activities held at high altitude (>2500 m). This coincides with a significant increase in the numbers of trekkers who ascend into the world's mountains. Altitude itself influences and complicates the athlete's effective and safe hydration. This article considers the physiology of adaptation to altitude and the effects on hydration at altitude compared with sea level, reviews the "ad libitum versus programmed hydration" controversy in conventional endurance event hydration, examines the evidence for extrapolation of sea level hydration strategies to the high-altitude environment, and synthesizes these disparate factors into a set of practical recommendations for hydration management during high-altitude physical activity. The guidelines will be relevant to participants of physical activity at altitude and health care staff who may care for them in the preparation or performance phases of their adventure.

Autoimmunity Related to Adipsic Hypernatremia and ROHHAD Syndrome

Specific antibody responses to subfornical organs, including Na_x antibody, have been reported in patients with adipsic hypernatremia of unknown etiology who do not have structural lesions in the hypothalamic–pituitary gland. The subfornical organ, also referred to as the window of the brain, is a sensing site that monitors sodium and osmotic pressure levels. On the other hand, ROHHAD syndrome is a rare disease for which the etiology of the hypothalamic disorder is unknown, and there have been some reports in recent years describing its association with autoimmune mechanisms. In addition, abnormal Na levels, including hypernatremia, are likely to occur in this syndrome. When comparing the clinical features of adipsic hypernatremia due to autoimmune mechanisms and ROHHAD syndrome, there are similar hypothalamic–pituitary dysfunction symptoms in addition to abnormal Na levels. Since clinical diagnoses of autoimmunological adipsic hypernatremia and ROHAD syndrome might overlap, we need to understand the essential etiology and carry out precise assessments to accurately diagnose patients and provide effective treatment. In this review, I review the literature on the autoimmune mechanism reported in recent years and describe the findings obtained so far and future directions.

[Adipsic diabetes insipidus after transsphenoidal surgery for suprasellar intraventricular]

Presented case demonstrates a rare diencephalic pathology - adipsic diabetes insipidus (ADI) with severe hypernatremia in a 58-year-old woman after ttranssphenoidal removal of stalk intraventricular craniopharyngioma. ADI was diagnosed because of hypernatremia (150-155 mmol/L), polyuria (up to 4 liters per day) and absence of thirst. Normalization of water-electrolyte balance occurred on the background of desmopressin therapy and sufficient hydration in postoperative period. After release from the hospital, the patient independently stopped desmopressin therapy and did not consume an adequate amount of fluid of the background of polyuria. This led to severe hypernatremia (155-160 mmol/L) and rough mental disorders.Patients with ADI need closely monitoring of medical condition and water-electrolyte parameters, appointment of fixed doses of desmopressin and adequate hydration.

Evidence that variations in pre-training hydration status are related to the frequency of endurance training in elite adolescent sprinters

BACKGROUND

Although the effect of dehydration on performance is widely studied, limited data concerning the levels of risk training types pose to hydration status exists. This study sought to determine: (a) pre-training hydration status in adolescent sprinters relative to non-athletes, (b) changes in hydration markers across a season of adolescent sprinters relative to non-athletes, and (c) if frequency of training type explains unique variance in hydration.

METHODS

Hydration [via pre-training urine osmolality (UOsm) and thirst perception (TP)], daily water intake (TWI) [via 24-h food/fluid diaries] and frequencies of resistance, endurance and sprint training types (via training regime questionnaires) were assessed in 26 sprinters (age: 15.6±1.9 years) and 26 non-athletes (age: 16.0±1.6 years), during 4 mesocycles [general (T1) and specific (T2) preparation; pre-competitive (T3) and peaking (T4) phases], over 26 weeks.

RESULTS

Most athletes (62%-81%) and non-athletes (73%-92%) were underhydrated (UOsm>700 mOsmol/kg) pre-training across the season, despite a low TP. There were significant time (p =.042) and group (p =.006) effects, and a main group by time interaction for UOsm (p =.006) but not TP across the season, after controlling for TWI. Greater UOsm (in mOsmol/kg) were observed during T1 (906.3±250.1) and T2 (934.5±257.0) compared to T3 (852.1±268.8) and T4 (854.2±218.8). There was no significant change across the season for non-athletes. Frequencies of endurance training were positively associated with UOsm and explained unique variances across the season (R2 range from 7%-16%).

CONCLUSIONS

Underhydration is high in the adolescent population. Training type may be related to the variations in hydration throughout a season, which may help to inform hydration practices of sprint athletes.

Adipsic Hypernatremia after Clipping of a Ruptured Aneurysm in the Anterior Communicating Artery: A Case Report

Adipsia is a rare disorder that occurs due to damage to the osmoreceptor and not feeling thirst despite hyperosmolality. Adipsic hypernatremia can occur when there is damage to the anterior communicating artery that supplies blood to osmoreceptors, and the level of arginine vasopressin secretion varies widely. A 37-year-old woman, suffering from severe headache, was consulted to the nephrology department for hypernatremia and polyuria after clipping of a ruptured aneurysm in the anterior communicating artery. Despite her hypernatremic hyperosmolar state, she denied thirst and did not drink spontaneously. She was diagnosed adipsic hypernatremia by evaluating the osmoregulatory and baroregulatory function tests. Because adipsic hypernatremia is caused by not enough drinking water even for hyperosmolality due to the lack of thirst stimulus, the strategies of treatment are that setting the target body weight when serum osmolality is normal and have the patient drink water until patient reach the target body weight. Adipsic hypernatremia should be considered to be a rare complication of subarachnoid hemorrhage associated with an anterior communicating artery aneurysm.

[Analysis of dehydration in older people in a nursing home in Spain: prevalence and associated factors]

Introduction

Background: dehydration in institutionalized elderly people has not been extensively studied. There are not clear data on the Spanish context. Aim: to estimate the prevalence of dehydration and to identify the associated factors in institutionalized older people in a nursing home. Methods: a cross-sectional study was carried out. Dehydration was measured through the colour of urine. For the identification of the associated factors, sociodemographic, clinical, functional, and mental variables were selected. Results: the total sample studied was comprised of 96 individuals with a mean age of 86.6 years (± 7.1), of whom 80.2 % were women. The prevalence of dehydration was 31.3% (95 % CI, 22.0 to 40.6). The factors that were independently associated with dehydration were the presence of sunken eyes (OR = 8.67; p = 0.004), low fluid intake (OR = 3.96; p = 0.041), and both functional (OR = 0.97; p = 0.012) and cognitive (OR = 1.10; p = 0.009) impairment. Conclusions: this study highlights the problem of dehydration in institutionalized older people in Spain. An urine colour table may be used routinely, non-invasively, and cheaply. So, it may well be the best simple method for detecting dehydration in this population. Taking into account that chronic dehydration is most prevalent in elderly people, the identification of associated factors is a key factor for a successful approach.

Fluid Intake Recommendation Considering the Physiological Adaptations of Adults Over 65 Years: A Critical Review

The aim of this critical review was to clarify recommended fluid intake for older people. A literature search of published articles and guidelines on fluid intake recommendations until April 2020 was carried out using PUBMED, Scopus, Cochrane, and Google Scholar. In this review, we focused on people over 65 years old at different care levels. The results show that the mean fluid intake ranges between 311 and 2390 mL/day. However, it is difficult to know whether this corresponds to the real pattern of fluid intake, due to the variability of data collection methods. With respect to the recommendations, most international organizations do not take into consideration the physiology of ageing or the health problems associated with an older population. In conclusions, we recommend to follow the guideline of the European Society for Clinical Nutrition and Metabolism (ESPEN) and the European Food Safety Authority (EFSA). ESPEN is the only guideline which takes into account age. It is also based on EFSA recommendations. This authority takes into consideration all fluids consumed (ranging from food to fluids). If it is known that around 20% of all fluids consumed come from food, the result would effectively be that the EFSA recommends the same as the ESPEN guidelines: 1.6 L/day for females and 2.0 L/day for males. The findings could help raise the awareness of professionals in the sector with respect to the required fluid intake of the elderly and, in this way, contribute to avoiding the consequences of dehydration.

The effect of hydration status on plasma FGF21 concentrations in humans: A subanalysis of a randomised crossover trial

Aim

Fibroblast growth factor 21 (FGF21) has recently been implicated in thirst in rodent models. The mechanisms for this are currently uncertain, and it is unclear whether hydration status can alter FGF21 concentrations, potentially providing an additional mechanism by which hypohydration induces thirst. The aim of this study is therefore to understand whether hydration status can alter circulating FGF21 in humans.

Methods

Using a heat tent and fluid restriction, we induced hypohydration (1.9% body mass loss) in 16 healthy participants (n = 8 men), and compared their glycaemic regulation to a rehydration protocol (heat tent and fluid replacement) in a randomised crossover design.

Results

After the hypohydration procedure, urine specific gravity, urine and serum osmolality, and plasma copeptin (as a marker for arginine vasopressin) increased as expected, with no change after the rehydration protocol. In the fasted state, the median paired difference in plasma FGF21 concentrations from the rehydrated to hypohydrated trial arm was -37 (interquartile range -125, 10) pg∙mL^-1(P = 0.278), with average concentrations being 458 ± 462 pg∙mL^-1 after hypohydration and 467 ± 438 pg∙mL^-1 after rehydration; mean difference -9 ± 173 pg∙mL^-1.

Conclusion

To our knowledge, these are the first causal data in humans investigating hydration and FGF21, demonstrating that an acute bout of hypohydration does not impact fasted plasma FGF21 concentrations. These data may suggest that whilst previous research has found FGF21 administration can induce thirst and drinking behaviours, a physiological state implicated in increased thirst (hypohydration) does not appear to impact plasma FGF21 concentrations in humans.

The Utility of Thirst as a Measure of Hydration Status Following Exercise-Induced Dehydration

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

Markers of systemic inflammation in response to osmotic stimulus in healthy volunteers

Osmotic stimulus or stress results in vasopressin release. Animal and human in vitro studies have shown that inflammatory parameters, such as interleukin-8 (IL-8) and tumor necrosis factor-α (TNF-α), increase in parallel in the central nervous system and bronchial, corneal or intestinal epithelial cell lines in response to osmotic stimulus. Whether osmotic stimulus directly causes a systemic inflammatory response in humans is unknown. We therefore investigated the influence of osmotic stimulus on circulatory markers of systemic inflammation in healthy volunteers. In this prospective cohort study, 44 healthy volunteers underwent a standardized test protocol with an osmotic stimulus leading into the hyperosmotic/hypernatremic range (serum sodium ≥150 mmol/L) by hypertonic saline infusion. Copeptin - a marker indicating vasopressin activity - serum sodium and osmolality, plasma IL-8 and TNF-α were measured at baseline and directly after osmotic stimulus. Median (range) serum sodium increased from 141 mmol/L (136, 147) to 151 mmol/L (145, 154) (P < 0.01), serum osmolality increased from 295 mmol/L (281, 306) to 315 mmol/L (304, 325) (P < 0.01). Median (range) copeptin increased from 4.3 pg/L (1.1, 21.4) to 28.8 pg/L (19.9, 43.4) (P < 0.01). Median (range) IL-8 levels showed a trend to decrease from 0.79 pg/mL (0.37, 1.6) to 0.7 pg/mL (0.4, 1.9) (P < 0.09) and TNF-α levels decreased from 0.53 pg/mL (0.11, 1.1) to 0.45 pg/mL (0.12, 0.97) (P < 0.036). Contrary to data obtained in vitro, circulating proinflammatory cytokines tend to or decrease in human plasma after osmotic stimulus. In this study, osmotic stimulus does not increase circulating markers of systemic inflammation.

Hydration, Arginine Vasopressin, and Glucoregulatory Health in Humans: A Critical Perspective

Glucoregulatory diseases, such as type 2 diabetes are currently a key public health priority. Public health messages have started to include the addition of water in their dietary guidelines. Such guidelines however are not based on causal evidence pertaining to the health effects of increased water intake, but rather more heavily based upon non-causal or mechanistic data. One line of thinking linking fluid intake and health is that hypohydration induces elevated blood concentrations of arginine vasopressin (AVP). Research in the 1970s and 1980s implicated AVP in glucoregulation, supported by observational evidence. This important area of research subsequently appeared to stop until the 21st century during which interest in hypertonic saline infusion studies, animal AVP receptor knockout models, dietary and genetic associations, and human interventions manipulating hydration status have resurged. This narrative review briefly describes and critically evaluates the usefulness of the current AVP-glucoregulatory research. We offer suggestions on how to test the independent glucoregulatory effects of body water changes compared to elevated circulating AVP concentrations, such as investigating hydration manipulations using 3,4-Methylenedioxymethamphetamine. Whilst much research is still needed before making firm conclusions, the current evidence suggests that although AVP may be partially implicated in glucoregulation, more ecologically valid models using human participants suggests this effect might be independent of the hydration status. The key implication of this hypothesis if confirmed in future research is that manipulating the hydration status to reduce circulating AVP concentrations may not be an effective method to improve glucoregulatory health.

Adipsic Diabetes Insipidus-The Challenging Combination of Polyuria and Adipsia: A Case Report and Review of Literature

Adipsic Diabetes Insipidus is a rare hypothalamic disorder characterized by a loss of thirst in response to hypernatraemia accompanied by diabetes insipidus. These occur secondary to a congregation of defects in the homeostatic mechanisms of water balance. A 27-year old Chinese female presented with Adipsic Diabetes Insipidus after cerebral arteriovenous malformation (AVM) surgery. Initial diagnosis and management was extremely challenging. Long term management required a careful interplay between low dose vasopressin analog treatment and fluids. Detailed charts of medication and sodium balance are described in the case presentation. We performed a literature search of similarly reported cases and describe the possible pathogenesis, etiology, clinical presentation, acute and chronic management, and prognosis.

Fluid Intake Habits in Type 1 Diabetes Individuals during Typical Training Bouts

BACKGROUND/AIMS

Hyperglycemia may influence the hydration status in diabetic individuals. During exercise, type 1 diabetes mellitus (T1DM) individuals may be challenged by a higher risk of dehydration due to a combination of fluid losses from sweat and increased urine output via glycosuria. So far, no study has characterised spontaneous fluid intake in T1DM individuals during active trainings.

METHODS

A validated questionnaire was used to assess T1DM participants' diabetes therapy, sports characteristics and fluid intake during training; results were then compared to an age- and sport-matched sample of non-diabetic individuals.

RESULTS

Ninety individuals completed the survey (n = 45 T1DM individuals, n = 45 matched controls). A proportion of T1DM -individuals reported blood glucose levels greater than 10.0 mmol at both the start (28.9%) and end (24.4%) of the exercise. The mean self-reported fluid intake was greater in T1DM (0.60 ± 0.47 L·h-1) compared to that of the control (0.37 ± 0.28 L·h-1, p < 0.05). In spite of drinking fluid volumes in line with international guidelines, 84.4% of those with T1DM reported that they were still feeling thirsty at the end of their training session.

CONCLUSIONS

T1DM individuals self-report spontaneously consuming fluid adequate volumes suggested by sport nutrition guidelines for non-diabetic athletes. Discrepancies in the T1DM subjectively reported feelings of thirst suggest that more education on hydration during exercise is needed for this population to adequately compensate for elevated blood glucose levels. It remains to be established whether fluid volumes suggested for healthy athletes are adequate for maintaining euhydration in T1DM patients due to their altered diuresis.

Dehydration is how you define it: comparison of 318 blood and urine athlete spot checks

Clinical medicine defines dehydration using blood markers that confirm hypertonicity (serum sodium concentration ([Na⁺])>145 mmol/L) and intracellular dehydration. Sports medicine equates dehydration with a concentrated urine as defined by any urine osmolality (UOsm) ≥700 mOsmol/kgH₂O or urine specific gravity (USG) ≥1.020.

The sensitivity of the human thirst response to changes in plasma osmolality: a systematic review

BACKGROUND

Dehydration is highly prevalent and is associated with adverse cardiovascular and renal events. Clinical assessment of dehydration lacks sensitivity. Perhaps a patient's thirst can provide an accurate guide to fluid therapy. This systematic review examines the sensitivity of thirst in responding to changes in plasma osmolality in participants of any age with no condition directly effecting their sense of thirst.

METHODS

Medline and EMBASE were searched up to June 2017. Inclusion criteria were all studies reporting the plasma osmolality threshold for the sensation of thirst.

RESULTS

A total of 12 trials were included that assessed thirst intensity on a visual analogue scale, as a function of plasma osmolality (pOsm), and employed linear regression to define the thirst threshold. This included 167 participants, both healthy controls and those with a range of pathologies, with a mean age of 41 (20-78) years.The value ±95% CI for the pOsm threshold for thirst sensation was found to be 285.23 ± 1.29 mOsm/kg. Above this threshold, thirst intensity as a function of pOsm had a mean ± SEM slope of 0.54 ± 0.07 cm/mOsm/kg. The mean ± 95% CI vasopressin release threshold was very similar to that of thirst, being 284.3 ± 0.71 mOsm/kg.Heterogeneity across studies can be accounted for by subtle variation in experimental protocol and data handling.

CONCLUSION

The thresholds for thirst activation and vasopressin release lie in the middle of the normal range of plasma osmolality. Thirst increases linearly as pOsm rises. Thus, osmotically balanced fluid administered as per a patient's sensation of thirst should result in a plasma osmolality within the normal range. This work received no funding.

Evaluation and review of body fluids saliva, sweat and tear compared to biochemical hydration assessment markers within blood and urine

Evaluating and testing hydration status is increasingly requested by rehabilitation, sport, military and performance-related activities. Besides commonly used biochemical hydration assessment markers within blood and urine, which have their advantages and limitations in collection and evaluating hydration status, there are other potential markers present within saliva, sweat or tear. This literature review focuses on body fluids saliva, sweat and tear compared to blood and urine regarding practicality and hydration status influenced by fluid restriction and/or physical activity. The selected articles included healthy subjects, biochemical hydration assessment markers and a well-described (de)hydration procedure. The included studies (n=16) revealed that the setting and the method of collecting respectively accessing body fluids are particularly important aspects to choose the optimal hydration marker. To obtain a sample of saliva is one of the simplest ways to collect body fluids. During exercise and heat exposures, saliva composition might be an effective index but seems to be highly variable. The collection of sweat is a more extensive and time-consuming technique making it more difficult to evaluate dehydration and to make a statement about the hydration status at a particular time. The collection procedure of tear fluid is easy to access and causes very little discomfort to the subject. Tear osmolarity increases with dehydration in parallel to alterations in plasma osmolality and urine-specific gravity. But at the individual level, its sensitivity has to be further determined.

Electrolytes and Fluid Management in Hemodialysis and Peritoneal Dialysis

The kidney is a complex and vital organ, regulating the electrolyte and fluid status of the human body. As hemodialysis (HD) and peritoneal dialysis (PD) are forms of renal replacement therapy and not an actual kidney, they do not possess the same physiologic regulation of both fluid and electrolytes. Precise regulation of fluid and electrolytes in the HD and PD population remains a constant challenge. In this review, fluid status of both HD and PD will be examined, as well as sodium, potassium, phosphorous, and calcium. Each electrolyte will be analyzed by its physiological significance, the complications that arise when a proper balance cannot be maintained, and methods to correct these imbalances. An overview of the fluid compartments and volume of distribution within the body will be discussed. Ultrafiltration, a modality used in both forms of renal replacement therapy, will be defined, along with its impact on fluid status. Fluid assessment will be addressed, along with proper maintenance of fluid homeostasis. By having an understanding of the pathophysiology behind the fluid and electrolyte abnormalities that occur in end-stage renal disease, one can direct proper management with medications, diet, and alterations in dialysis to provide patients with the most optimal form of renal replacement therapy available.

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.

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

Background: Surprisingly little is known about the physiological and perceptual differences of women who consume different volumes of water each day. The purposes of this investigation were to (a) analyze blood osmolality, arginine vasopressin (AVP), and aldosterone; (b) assess the responses of physiological, thirst, and hydration indices; and (c) compare the responses of individuals with high and low total water intake (TWI; HIGH and LOW, respectively) when consuming similar volumes of water each day and when their habitual total water intake was modified. Methods: In a single-blind controlled experiment, we measured the 24 h total water intake (TWI; water + beverages + food moisture) of 120 young women. Those who consumed the highest (HIGH, 3.2 ± 0.6 L·day⁻¹, mean ± SD) and the lowest (LOW, 1.6 ± 0.5 L·day⁻¹) mean habitual TWI were identified and compared. Outcome variables were measured during two ad libitum baseline days, a four-day intervention of either decreased TWI (HIGH) or increased TWI (LOW), and one ad libitum recovery day. Results: During the four-day intervention, HIGH and LOW experienced differences in thirst (p = 0.002); also, a statistically significant change of AVP occurred (main effect of TWI and day, p < 0.001), with no effect (TWI or day) on aldosterone and serum osmolality. Urine osmolality and volume distinguished HIGH from LOW (p = 0.002) when they consumed similar 24 h TWI.

Diagnosis and treatment of hypernatremia

Hypernatremia is defined as a serum sodium level above 145 mmol/L. It is a frequently encountered electrolyte disturbance in the hospital setting, with an unappreciated high mortality. Understanding hypernatremia requires a comprehension of body fluid compartments, as well as concepts of the preservation of normal body water balance. The human body maintains a normal osmolality between 280 and 295 mOsm/kg via Arginine Vasopressin (AVP), thirst, and the renal response to AVP; dysfunction of all three of these factors can cause hypernatremia. We review new developments in the pathophysiology of hypernatremia, in addition to the differential diagnosis and management of this important electrolyte disorder.

The effect of hypohydration on endothelial function in young healthy adults

PURPOSE

Hypohydration has been suggested as a predisposing factor for several pathologies including cardiovascular diseases (CVD). While CVD are the leading cause of death worldwide, no study has investigated whether acute hypohydration affects endothelial function and cardiovascular function.

METHODS

Ten young, healthy males participated in this crossover study (age: 24.3 ± 2.3 year; weight: 80.8 ± 5.3 kg; BMI: 24.3 ± 0.4 kg m^-2). Each subject completed two measurements of endothelial function by flow-mediated dilation (FMD) in euhydrated and hypohydrated state separated by 24 h. Following baseline assessment of hydration status and FMD, the subjects completed 100 min of low-intensity intermittent walking exercise to achieve hypohydration of -2 % of individual body mass. For the rest of the day, a standardized, low water content diet was provided. The following morning, hydration markers and endothelial function were recorded.

RESULTS

Hypohydration by -1.9 ± 0.1 % of body mass resulted in decreased plasma volume by -3.5 ± 1.8 % and increased plasma osmolality by 9 ± 2 mmol kg^-1 (P < 0.001). FMD as a response to hypohydration decreased by -26.8 ± 3.9 % (P < 0.05).

CONCLUSION

The data suggested that a small degree of hypohydration induced by moderate exercise and fluid restriction significantly impaired endothelial function.

Optimal composition of fluid-replacement beverages

The objective of this article is to provide a review of the fundamental aspects of body fluid balance and the physiological consequences of water imbalances, as well as discuss considerations for the optimal composition of a fluid replacement beverage across a broad range of applications. Early pioneering research involving fluid replacement in persons suffering from diarrheal disease and in military, occupational, and athlete populations incurring exercise- and/or heat-induced sweat losses has provided much of the insight regarding basic principles on beverage palatability, voluntary fluid intake, fluid absorption, and fluid retention. We review this work and also discuss more recent advances in the understanding of fluid replacement as it applies to various populations (military, athletes, occupational, men, women, children, and older adults) and situations (pathophysiological factors, spaceflight, bed rest, long plane flights, heat stress, altitude/cold exposure, and recreational exercise). We discuss how beverage carbohydrate and electrolytes impact fluid replacement. We also discuss nutrients and compounds that are often included in fluid-replacement beverages to augment physiological functions unrelated to hydration, such as the provision of energy. The optimal composition of a fluid-replacement beverage depends upon the source of the fluid loss, whether from sweat, urine, respiration, or diarrhea/vomiting. It is also apparent that the optimal fluid-replacement beverage is one that is customized according to specific physiological needs, environmental conditions, desired benefits, and individual characteristics and taste preferences.

Characterization of the effects of the vasopressin V2 receptor on sweating, fluid balance, and performance during exercise

A regulatory effect of arginine vasopressin (AVP) on sweat water conservation has been hypothesized but not definitively evaluated. AVP-mediated insertion of sweat and salivary gland aquaporin-5 (AQP5) water channels through activation of the vasopressin type 2 receptor (V2R) remains an attractive, yet unexplored, mechanism that could result in a more concentrated sweat with resultant decreased water loss. Ten runners participated in a double-blind randomized control treadmill trial under three separate pharmacological conditions: a placebo, V2R agonist (0.2 mg desmopressin), or V2R antagonist (30 mg tolvaptan). After a familiarization trial, runners ran for 60 min at 60% of peak speed followed by a performance trial to volitional exhaustion. Outcome variables were collected at three exercise time points: baseline, after the steady-state run, and after the performance run. Body weight losses were <2% across all three trials. Significant pharmacological condition effects were noted for urine osmolality [F = 84.98; P < 0.0001] and urine sodium concentration ([Na(+)]) [F = 38.9; P < 0.0001], which verified both pharmacological activation and inhibition of the V2R at the kidney collecting duct. Plasma osmolality and [Na(+)] demonstrated significant exercise (F = 26.0 and F = 11.1; P < 0.0001) and condition (F = 5.1 and F = 3.8; P < 0.05) effects (osmolality and [Na(+)], respectively). No significant exercise or condition effects were noted for either sweat or salivary [Na(+)]. Significant exercise effects were noted for plasma [AVP] (F = 22.3; P < 0.0001), peak core temperature (F = 103.3; P < 0.0001), percent body weight change (F = 6.3; P = 0.02), plasma volume change (F = 21.8; P < 0.0001), and thirst rating (F = 78.2; P < 0.0001). Performance time was not altered between conditions (P = 0.80). In summary, AVP acting at V2R does not appear to regulate water losses from body fluids other than renal excretion during exercise.

Hyperosmolarity drives hypertension and CKD--water and salt revisited

An epidemic of chronic kidney disease (CKD) in Mesoamerica is providing new insights into the mechanisms by which salt and water might drive hypertension and CKD. Increasingly, evidence suggests that recurrent dehydration and salt loss might be a mechanism that causes CKD, and experimental studies suggest a key role for increased plasma osmolarity in activating both intrarenal (polyol-fructokinase) and extrarenal (vasopressin) pathways that drive renal injury. Thus, we propose that water and salt might influence blood pressure and kidney disease through the timing and combination of their intake, which affect plasma osmolarity as well as intrarenal and extrarenal mechanisms of renal injury. The type of fluid intake might also be important, as fluids containing fructose can trigger activation of these pathways. Future studies should investigate the effects of salt, sugar and fluid intake on plasma osmolarity as a potential pathogenetic mechanism in renal injury and high blood pressure.

Diagnosis, evaluation, and treatment of hyponatremia: expert panel recommendations

Hyponatremia is a serious, but often overlooked, electrolyte imbalance that has been independently associated with a wide range of deleterious changes involving many different body systems. Untreated acute hyponatremia can cause substantial morbidity and mortality as a result of osmotically induced cerebral edema, and excessively rapid correction of chronic hyponatremia can cause severe neurologic impairment and death as a result of osmotic demyelination. The diverse etiologies and comorbidities associated with hyponatremia pose substantial challenges in managing this disorder. In 2007, a panel of experts in hyponatremia convened to develop the Hyponatremia Treatment Guidelines 2007: Expert Panel Recommendations that defined strategies for clinicians caring for patients with hyponatremia. In the 6 years since the publication of that document, the field has seen several notable developments, including new evidence on morbidities and complications associated with hyponatremia, the importance of treating mild to moderate hyponatremia, and the efficacy and safety of vasopressin receptor antagonist therapy for hyponatremic patients. Therefore, additional guidance was deemed necessary and a panel of hyponatremia experts (which included all of the original panel members) was convened to update the previous recommendations for optimal current management of this disorder. The updated expert panel recommendations in this document represent recommended approaches for multiple etiologies of hyponatremia that are based on both consensus opinions of experts in hyponatremia and the most recent published data in this field.

Thirst in critically ill patients: from physiology to sensation

Critically ill patients often report distressful episodes of severe thirst, but the complex biochemical, neurohormonal mechanisms that regulate this primal sensation still elude clinicians. The most potent stimuli for thirst are subtle increases in plasma osmolality. These minute changes in osmolality stimulate central osmoreceptors to release vasopressin (also known as antidiuretic hormone). Vasopressin in turn acts on the kidneys to promote the reabsorption of water to correct the increased osmolality. If this compensatory mechanism fails to decrease osmolality, then thirst is triggered to motivate drinking. In contrast, thirst induced by marked volume loss, or hypovolemic thirst, is subject to the tight osmoregulation of the renin-angiotensin aldosterone system and accompanying adrenergic agonists. Understanding the essential role that thirst plays in salt and water regulation can provide clinicians with a better appreciation for the complex physiology that underlies this intense sensation.

Acute kidney injury, sodium disorders, and hypercalcemia in the aging kidney: diagnostic and therapeutic management strategies in emergency medicine

This article summarizes the current literature regarding the structural and functional changes of the aging kidney and describes how these changes make the older patient more susceptible to acute kidney injury and fluid and electrolyte disorders. It discusses the clinical manifestations, evaluation, and management of hyponatremia and shows how the management of hypernatremia in geriatric patients involves addressing the underlying cause and safely correcting the hypernatremia. The current literature regarding evaluation and management of hypercalcemia in older patients is summarized. The management of severe hypercalcemia is discussed in detail. The evaluation and management of acute kidney injury is described.

Thirst and hydration status in everyday life

Water is an essential nutrient for all persons; thus, maintaining a chronic state of optimal hydration is recognized to provide health benefits. Fluid balance is maintained via thirst, a feedback-controlled variable, regulated acutely by central and peripheral mechanisms. However, voluntary drinking is also a behavior influenced by numerous social and psychological cues. Therefore, whether "thirst-guided" drinking maintains optimal hydration status is a multifactorial issue. Thirst perception is typically assessed by subjective ratings using either categorical or visual analog scales; however, which instrument yields greater sensitivity to change in hydration status has not been examined. Ratings of thirst perception do not always yield predictable patterns of voluntary drinking following dehydration; therefore, perceived thirst and ad libitum drinking are not equivalent measures of human thirst. The recommendation "drink to thirst" is frequently given to healthy individuals during daily life. However, factors and conditions (e.g., age, disease) that influence thirst should be recognized and probed further.

Hypernatremia secondary to post-stroke hypodipsia: just add water!

Disorders in water metabolism may occur in stroke patients. When hypernatremia arises in this setting, it is usually secondary to the development of central diabetes insipidus or it is the result of neurologic lesions that prevent patients from having free access to water. Much rarer are the cases of post-stroke hypernatremia caused by hypodipsia secondary to lesions of the thirst center. We report the case of a patient with severe hypernatremia, probably secondary to post-hemorrhagic stroke hypodipsia. The hypernatremia seen in this case was corrected by scheduling the patient's water intake.

Arginine vasopressin, fluid balance and exercise: is exercise-associated hyponatraemia a disorder of arginine vasopressin secretion?

The ability of the human body to regulate plasma osmolality (POsm) within a very narrow and well defined physiological range underscores the vital importance of preserving water and sodium balance at rest and during exercise. The principle endocrine regulator of whole body fluid homeostasis is the posterior pituitary hormone, arginine vasopressin (AVP). Inappropriate AVP secretion may perpetuate either slow or rapid violation of these biological boundaries, thereby promoting pathophysiology, morbidity and occasional mortality. In the resting state, AVP secretion is primarily regulated by changes in POsm (osmotic regulation). The osmotic regulation of AVP secretion during exercise, however, may possibly be enhanced or overridden by many potential non-osmotic factors concurrently stimulated during physical activity, particularly during competition. The prevalence of these highly volatile non-osmotic AVP stimuli during strenuous or prolonged physical activity may reflect a teleological mechanism to promote water conservation during exercise. However, non-osmotic AVP secretion, combined with high fluid availability plus sustained fluid intake (exceeding fluid output), has been hypothesized to lead to an increase in both the incidence and related deaths from exercise-associated hyponatraemia (EAH) in lay and military populations. Inappropriately, high plasma AVP concentrations ([AVP](p)) associated with low blood sodium concentrations facilitate fluid retention and sodium loss, thereby possibly reconciling both the water intoxication and sodium loss theories of hyponatraemia that are currently under debate. Therefore, given the potential for a variety of exercise-induced non-osmotic stimuli for AVP secretion, hydration strategies must be flexible, individualized and open to change during competitive events to prevent the occurrence of rare, but life-threatening, EAH. This review focuses on the potential osmotic and non-osmotic stimuli to AVP secretion that may affect fluid homeostasis during physical activity. Recent laboratory and field data support: (i) stimulatory effects of exercise intensity and duration on [AVP](p); (ii) possible relationships between changes in POsm with changes in both sweat and urinary osmolality; (iii) alterations in the AVP osmoregulatory set-point by sex steroid hormones; (iv) differences in [AVP](p) in trained versus untrained athletes; and (v) potential inter-relationships between AVP and classical (aldosterone, atrial natriuretic peptide) and non-classical (oxytocin, interleukin-6) endocrine mediators. The review concludes with a hypothesis on how sustained fluid intakes beyond the capacity for fluid loss might possibly facilitate the development of hyponatraemia if exercise-induced non-osmotic stimuli override 'normal' osmotic suppression of AVP when hypo-osmolality exists.

A review of drug-induced hypernatraemia

Drug-induced electrolyte abnormalities have been increasingly reported and may be associated with considerable morbidity and/or mortality. In clinical practice, hypernatraemia (serum sodium higher than 145 mmol/L) is usually of multifactorial aetiology and drug therapy not infrequently is disregarded as a contributing factor for increased serum sodium concentration. Strategies to prevent this adverse drug effect involve careful consideration of risk factors and clinical and laboratory evaluation in the course of treatment. Herein, we review evidence-based information via PubMed and EMBASE and the relevant literature implicating pharmacologic treatment as an established cause of hypernatraemia and discuss its incidence and the underlying pathophysiologic mechanisms.

A case for water in the treatment of polycystic kidney disease

Autosomal dominant polycystic disease (ADPKD) is an inherited disorder characterized by the development within renal tubules of innumerable cysts that progressively expand to cause renal insufficiency. Tubule cell proliferation and transepithelial fluid secretion combine to enlarge renal cysts, and 3'-5'-cyclic adenosine monophosphate (cAMP) stimulates that growth. The antidiuretic hormone, arginine vasopressin (AVP), operates continuously in ADPKD patients to stimulate the formation of cAMP, thereby contributing to cyst and kidney enlargement and renal dysfunction. Studies in animal models of ADPKD provide convincing evidence that blocking the action of AVP dramatically ameliorates the disease process. In the current analysis, the authors reason that increasing the amount of solute-free water drunk evenly throughout the day in patients with ADPKD and normal renal function will decrease plasma AVP concentrations and mitigate the action of cAMP on the renal cysts. Potential pitfalls of increasing fluid intake in ADPKD patients are considered, and suggestions for how physicians may prudently implement this therapy are offered.

Disorders of sodium and water balance in hospitalized patients

PURPOSE

To review and discuss the epidemiology, contributing factors, and approach to clinical management of disorders of sodium and water balance in hospitalized patients.

SOURCE

An electronic search of the MEDLINE, Embase, and Cochrane Central Register of Controlled Trials databases and a search of the bibliographies of all relevant studies and review articles for recent reports on hyponatremia and hypernatremia with a focus on critically ill patients.

PRINCIPAL FINDINGS

Disorders of sodium and water balance are exceedingly common in hospitalized patients, particularly those with critical illness and are often iatrogenic. These disorders are broadly categorized as hypo-osmolar or hyper-osmolar, depending on the balance (i.e., excess or deficit) of total body water relative to total body sodium content and are classically recognized as either hyponatremia or hypernatremia. These disorders may represent a surrogate for increased neurohormonal activation, organ dysfunction, worsening severity of illness, or progression of underlying chronic disease. Hyponatremic disorders may be caused by appropriately elevated (volume depletion) or inappropriately elevated (SIADH) arginine vasopressin levels, appropriately suppressed arginine vasopressin levels (kidney dysfunction), or alterations in plasma osmolality (drugs or body cavity irrigation with hypotonic solutions). Hypernatremia is most commonly due to unreplaced hypotonic water depletion (impaired mental status and/or access to free water), but it may also be caused by transient water shift into cells (from convulsive seizures) and iatrogenic sodium loading (from salt intake or administration of hypertonic solutions).

CONCLUSION

In hospitalized patients, hyponatremia and hypernatremia are often iatrogenic and may contribute to serious morbidity and increased risk of death. These disorders require timely recognition and can often be reversed with appropriate intervention and treatment of underlying predisposing factors.

Physiology and pathophysiology of the vasopressinergic system

Arginine vasopressin, a hypothalamic peptide hormone, has multiple physiological functions, including body water regulation, control of blood pressure and effects on body temperature, insulin release, corticotropin release, memory and social behaviour. These functions are achieved via at least three specific G-protein-coupled vasopressin receptors. Development of specific vasopressin receptor antagonists in recent years is helping to elucidate the precise actions of vasopressin at each of these receptor types. The complex signalling and messenger processes which take place after receptor stimulation are now more clearly understood. Vasopressin dysregulation can occur in various disease processes, and a better understanding of the mechanisms underlying physiological synthesis, release and regulation of vasopressin will help in the development of therapies to treat these conditions.