Examining the significance of arginine vasopressin release to elucidate the often multifactorial etiology of hypotonic hyponatremia: A novel criterion.

Clinical hyponatremia guidelines, protocols and flowcharts are a convenient means for clinicians to quickly establish an etiological diagnosis for hyponatremia, and facilitate its often complex analysis. Unfortunately, they often erroneously attribute multifactorial hyponatremia to a single cause, which is potentially dangerous. In this manuscript, a novel criterion is proposed to quickly determine the physiological relevance of non-osmotic arginine vasopressin (AVP) release, and to add nuance to hyponatremia analysis. While analyzing hypotonic hyponatremia, it is imperative to not only verify whether or not a certain degree of inappropriate AVP release is present, but also to ascertain whether it-in itself-could sufficiently explain the observed hyponatremia, as these two are not always synonymous. Using well-known concepts from renal physiology to combine the electrolyte-free water balance and solute-free water balance, a novel physiological criterion is derived mathematically to easily distinguish three common hyponatremia scenarios, and to further elucidate the underlying etiology. The derived criterion can hopefully facilitate the clinician's and physiologist's interpretation of plasma and urine parameters in a patient presenting with hyponatremia, and warn against the important clinical pitfall of attributing hyponatremia too readily to a single cause.

Analysis of Fluid Balance and Urine Values in Elite Soccer Players: Impact of Different Environments, Playing Positions, Sexes, and Competitive Levels.

Exercise can disrupt the fluid balance, hindering performance and athlete health. Limited data exist on fluid balance responses in varying climates, sexes, and ages. This study aimed to measure and compare fluid balance and urine values among elite soccer players during training at high and low temperatures, examining the differences between sexes, playing positions, and competitive levels within men's soccer. During the 2022-2023 competitive season, a descriptive observational study was conducted on 87 soccer players from an elite Spanish soccer team. The study found that none of the groups exceeded weight loss values of 1.5% of their body mass. Additionally, the soccer players studied experienced higher weight loss, fluid intake, and a higher sweat rate (SR) during summer training compared to winter training. During the summer, male U23-21 soccer players exhibited higher levels of weight loss, fluid intake, and a higher SR compared to female soccer players or the U19-17 male category. No significant differences were found between playing positions. In conclusion, differences in the fluid balance were observed based on the climatic conditions, competitive level, and sex.

[Features of water-electrolyte balance in persons of the older age group].

Age-related changes have a great influence on the regulation of water and electrolyte homeostasis in the body, which is regulated by a complex interaction of environmental factors, drinking behavior, the secretion of a number of hormones and hormone-like substances, as well as the innervation and functional state of the kidneys. It is well known that the changes that are part of physiological aging underlie fluid and electrolyte imbalances, exacerbated by the presence of age-related diseases, medications, or a number of external factors such as malnutrition, fluid intake, and the presence of dementia. This review considers literature data on the effect of normal aging on the development of pathology of the water-sodium balance, including dehydration of senile patients, hyponatremia, hypernatremia, changes in the secretion of antidiuretic hormone and the activity of elements of the renin-angiotensin-aldosterone system.

Sweat Characteristics and Fluid Balance Responses During Two Heat Training Camps in Elite Female Field Hockey Players.

We examined the sweat characteristics and fluid balance of elite female field hockey players during two heat training camps. Fourteen elite female field hockey players from the Australian national squad participated in two heat training camps held ∼6 months apart, following winter- (Camp 1) and summer-based training (Camp 2). Daily waking body mass (BM) and urine specific gravity (USG) were collected, along with several markers of sweat and fluid balance across two matches per camp. There was a 19% mean reduction in estimated whole-body sweat sodium concentration from Camp 1 (45.8 ± 6.5 mmol/L) to Camp 2 (37.0 ± 5.0 mmol/L; p < .001). Waking urine specific gravity ≥ 1.020 was observed in 31% of samples, with no significant differences in mean urine specific gravity or BM between camps (p > .05), but with substantial interindividual variation. Intramatch sweat rates were high (1.2-1.8 L/hr), with greater BM losses in Camp 1 (p = .030), resulting in fewer players losing ≥2% BM in Camp 2 (0%-8%), as compared with Camp 1 (36%-43%; p = .017). Our field data suggest that elite female field hockey players experience substantial sweat losses during competition in the heat regardless of the season. In agreement with previous findings, we observed substantial interindividual variation in sweat and hydration indices, supporting the use of individualized athlete hydration strategies.

Fluid Status Vulnerability in Older Adults.

There is a growing body of evidence about physiological changes with age that impact fluid and electrolyte balance. It is important that infusion nurses have knowledge in managing care for geriatric patients so they can identify these changes when they are exhibited. Knowing how to minimize the effect of these changes on the health of older adults is critical. The infusion nurse with knowledge of geriatric-focused care can avoid complications and critical illness in older adults. In addition, it is important to provide specific patient education that is grounded in geriatric best practices. This information will assist older adults to better protect themselves from dehydration, kidney injury, and other complications associated with fluid balance, such as delirium. This article reviews the literature on specific changes with aging that predispose older adults to adverse complications with fluid imbalance. New technology in geriatrics that can improve management of fluid status, such as dehydration and electrolyte monitors, are also discussed. This review included searches of the Medline®/PubMed® Database using MeSH terms (National Library of Medicine). Search terms included the following: aging-biological; aging kidney; water-electrolyte imbalance; dehydration; hypo-hypernatremia; hypo-hyperkalemia; delirium; wearable technology; and hydration monitors.

Effect of Personalized Sodium Replacement on Fluid and Sodium Balance and Thermophysiological Strain During and After Ultraendurance Running in the Heat.

PURPOSE: To investigate the effect of personalized sweat sodium replacement on drinking behavior, sodium and water balance, and thermophysiological responses during and after ultraendurance running in hot conditions. METHODS: Nine participants (7 male, 2 female) completed two 5-hour treadmill runs (60% maximum oxygen uptake, 30°C ambient temperature), in a double-blind randomized crossover design, consuming sodium chloride (SODIUM) capsules to replace 100% of previously assessed losses or placebo (PLACEBO). Fluid was consumed ad libitum. RESULTS: No effect of SODIUM was observed for ad libitum fluid intake or net fluid balance (P > .05). Plasma sodium concentration increased in both trials, but to a greater extent in SODIUM at 2.5 hours (mean [SD]: 4 [4] mmol·L-1 vs 1 [5] mmol·L-1; P < .05) and postexercise (4 [3] mmol·L-1 vs 1 [5] mmol·L-1; P < .05). Plasma volume change was not different between trials (P > .05) but was strongly correlated with sodium balance in SODIUM (r = .880, P < .01). No effect of sodium replacement was observed for heart rate, rectal temperature, thermal comfort, perceived exertion, or physiological strain index. During the 24 hours postexercise, ad libitum fluid intake was greater following SODIUM (2541 [711] mL vs 1998 [727] mL; P = .04), as was urinary sodium excretion (NaCl: 66 [35] mmol, Pl: 21 [12] mmol; P < .01). CONCLUSIONS: Personalized sweat sodium replacement during ultraendurance running in hot conditions, with ad libitum fluid intake, exacerbated the rise in plasma sodium concentration compared to no sodium replacement but did not substantially influence overall body-water balance or thermophysiological strain. A large sodium deficit incurred during exercise leads to substantial renal sodium conservation postexercise.

Tangram of Sodium and Fluid Balance.

Homeostasis of fluid and electrolytes is a tightly controlled physiological process. Failure of this process is a hallmark of hypertension, chronic kidney disease, heart failure, and other acute and chronic diseases. While the kidney remains the major player in the control of whole-body fluid and electrolyte homeostasis, recent discoveries point toward more peripheral mechanisms leading to sodium storage in tissues, such as skin and muscle, and a link between this sodium and a range of diseases, including the conditions above. In this review, we describe multiple facets of sodium and fluid balance from traditional concepts to novel discoveries. We examine the differences between acute disruption of sodium balance and the longer term adaptation in chronic disease, highlighting areas that cannot be explained by a kidney-centric model alone. The theoretical and methodological challenges of more recently proposed models are discussed. We acknowledge the different roles of extracellular and intracellular spaces and propose an integrated model that maintains fluid and electrolyte homeostasis and can be distilled into a few elemental players: the microvasculature, the interstitium, and tissue cells. Understanding their interplay will guide a more precise treatment of conditions characterized by sodium excess, for which primary aldosteronism is presented as a prototype.

Repeatability of Ad Libitum Water Intake during Repeated 1 h Walking/Jogging Exercise Sessions Conducted under Hot Ambient Conditions.

A drinking strategy aiming to replace a given percentage of the sweat losses incurred during exercise should result in reproducible fluid intake volume and, hence, fluid balance from one exercise session to the other performed under similar scenarios. Whether this may also be the case with ad libitum drinking during exercise is unclear. We characterized the repeatability of ad libitum water intake during repeated 1 h exercise sessions and examined its effect over time on fluid balance and selected physiological functions and perceptual sensations. Twelve (3 women) healthy individuals participated in this study. At weekly intervals, they completed four 2 × 30 min walking/jogging exercise bouts (55% V˙O2max, 40 °C, 20-30% relative humidity) interspersed by a 3 min recovery period. During exercise, participants consumed water (20 °C) ad libitum. There were no significant differences among the four exercise sessions for absolute water intake volume (~1000 mL·h-1), percent body mass loss (~0.4%), sweat rate (~1300 mL·h-1) and percent of sweat loss replaced by water intake (~80%). Heart rate, rectal temperature, and perceived thirst and heat stress did not differ significantly between the first and fourth exercise sessions. Perceived exertion was significantly lower during the fourth vs. the first exercise session, but the difference was trivial (<1 arbitrary unit). In conclusion, ad libitum water intake during four successive identical 1 h walking/jogging sessions conducted in the heat will result in similar water intake volumes and perturbations in fluid balance, heart rate, rectal temperature, and perceived thirst, heat stress and exertion.

Free amino acids in response to salinity changes in fishes: relationships to osmoregulation.

Free amino acids (FAAs) are believed to play important roles in osmoregulation and buffer capacity in some aquatic animals, such as fishes. However, the potential roles of FAAs have not been systematically summarized and characterized until now. In the present study, the meta-analysis was conducted to investigate the relationships between FAAs and environmental salinities. Twenty published documents were included, accounting for 106 study cases. The effect sizes of total free amino acids (TFAAs), total essential amino acids (TEAAs), and total non-essential amino acids (TNEAAs) to salinity increase were calculated and determined by the restricted maximum likelihood (REML) method. It clearly showed that the elevated salinities significantly induced the contents of TFAAs, TEAAs, and TNEAAs at the ratio of 36%, 27%, and 29%, respectively. Faced to the salinity changes, the contents of FAAs in fishes under freshwater and seawater varied significantly, while the individuals under brackish water displayed relatively constant contents of FAAs. When salinity elevated, the contents of 17 amino acids in muscles significantly increased, suggesting the important roles of FAA metabolism in osmoregulation in fishes. The results also indicated that the effect sizes of TFAAs were positively related to the rates of salinity increases, and exhibited a significant quadratic linear relationship with temperatures. Additionally, the contents of FAAs also showed positive correlation with osmotic pressure, concentrations of plasma Na+, Cl-, and urea, implying their potential roles of FAAs in osmoregulation in fishes. These findings suggested that elevated salinities greatly induced the contents of FAAs in fishes, making a great contribution to maintaining the homeostasis of fishes in response to environmental salinity changes.

The 2023 Sir David Cuthbertson Lecture. A fluid journey: Experiments that influenced clinical practice.

This review summarises some of my work on fluid and electrolyte balance over the past 25 years and shows how the studies have influenced clinical practice. Missing pieces in the jigsaw are filled in by summarising the work of others. The main theme is the biochemical, physiological and clinical problems caused by inappropriate use of saline solutions including the hyperchloraemic acidosis caused by 0.9% saline. The importance of accurate and near-zero fluid balance in clinical practice is also emphasised. Perioperative fluid and electrolyte therapy has important effects on clinical outcome in a U-shaped dose response fashion, in which excess or deficit progressively increases complications and worsens outcome. Salt and water overload, with weight gain in excess of 2.5 kg worsens surgical outcome, impairs gastrointestinal function and increases the risk of anastomotic dehiscence. Hyperchloraemic acidosis caused by overenthusiastic infusion of 0.9% saline leads to adverse outcomes and dysfunction of many organ systems, especially the kidney. Salt and water deficit causes similar adverse effects as fluid overload at the cellular level and also leads to worse outcomes. Serum albumin is shown to be affected mainly by dilution and inflammation and is not a good nutritional marker. These findings have been incorporated in the British consensus Guidelines on Intravenous Fluid Therapy for Adult Surgical Patients (GIFTASUP) and National Institute for Health and Care Excellence (NICE) guidelines on intravenous fluid therapy in adults in hospital and are helping change clinical practice and improve outcomes.

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.

Osmoregulatory Performance among Prickly Sculpin (Cottus asper) Living in Contrasting Osmotic Habitats.

AbstractDuring the colonization of freshwater by marine fish, adaptation to hypoosmotic conditions may impact their ability to osmoregulate in seawater. The prickly sculpin (Cottus asper) is a euryhaline fish with marine ancestors that postglacially colonized many freshwater habitats. Previous work on C. asper suggested that isolation in freshwater habitats has resulted in putative adaptations that improve ion regulation in freshwater populations compared with populations with current access to estuaries. To determine whether long-term colonization of freshwater is associated with a reduced ability to ion regulate in seawater, we acclimated C. asper populations from three habitat types that vary in the extent to which they are isolated from marine habitats and compared their seawater osmoregulation. Seawater acclimation revealed that lake populations exhibited a reduced capacity to osmoregulate in seawater compared with coastal river populations with ongoing access to estuaries. In particular, when acclimated to seawater for several weeks, lake populations had lower gill Na+/K+-ATPase activity and lower intestinal H+-ATPase activity than coastal river populations. Lake populations also had a reduced ability to maintain plasma ion concentrations, and they produced lower quantities of intestinal carbonate precipitates in seawater than coastal river populations. Furthermore, there was a positive relationship between the anterior intestinal Na+/K+-ATPase activity and the amount of precipitate produced by the intestine, which suggests that the anterior intestine plays a role in seawater osmoregulation. Our results suggest that the extent of isolation from the sea could, in part, explain the reduced osmoregulation in seawater in postglacial freshwater populations of C. asper.

Managing fuels and fluids: Network integration of osmoregulatory and metabolic hormonal circuits in the polymodal control of homeostasis in insects.

Osmoregulation in insects is an essential process whereby changes in hemolymph osmotic pressure induce the release of diuretic or antidiuretic hormones to recruit individual osmoregulatory responses in a manner that optimizes overall homeostasis. However, the mechanisms by which different osmoregulatory circuits interact with other homeostatic networks to implement the correct homeostatic program remain largely unexplored. Surprisingly, recent advances in insect genetics have revealed several important metabolic functions are regulated by classic osmoregulatory pathways, suggesting that internal cues related to osmotic and metabolic perturbations are integrated by the same hormonal networks. Here, we review our current knowledge on the network mechanisms that underpin systemic osmoregulation and discuss the remarkable parallels between the hormonal networks that regulate body fluid balance and those involved in energy homeostasis to provide a framework for understanding the polymodal optimization of homeostasis in insects.

A randomised trial to assess fluid and electrolyte balance responses following ingestion of different beverages in young and older men.

BACKGROUND: Older adults are susceptible to dehydration and fluid overload due to a reduced ability to maintain homeostatic control of fluid and electrolyte balance. PURPOSE: To assess fluid and electrolyte balance responses in young and older men following ingestion of commonly consumed beverages differing in composition. METHODS: 12 young and 11 older men were recruited. Euhydrated body mass was recorded. Participants consumed 1L (250 ml every 15 min) of water, fruit juice, a sports drink or low-fat milk in a randomized cross-over design. Urine and blood samples were obtained before and after the drinking period and every hour thereafter for 3-h. Samples were used to determine osmolality, electrolytes (Na+ and K+), water clearance, and glomerular filtration rate. RESULTS: Free water clearance was significantly higher in Young than Older at 1 and 2 h after the ingestion of W and S (p < 0.05). Net Na+ and K+ balance were not different between Young and Older (p = 0.91 and p = 0.65) adults, respectively. At 3 h Na+ balance was negative after ingesting water and fruit juice, but neutral after sport drink and milk. Net K+ balance was neutral at 3 h after ingesting milk, but negative after water, fruit juice and sport drink. CONCLUSIONS: Milk was retained longer than other beverages in Young, but not in Older, despite similar net electrolyte balance responses. Older had higher fluid retention in the first 2 h after the ingestion of all beverages, except for milk when compared to Young, indicating an age-related loss of ability to regulate fluid balance under current study conditions.

Effects of saline immersion on the physiological alterations of grass goldfish (Carassius auratus) during subsequent recovery in freshwater.

The present work aims to evaluate the tolerance, osmoregulation, metabolism, and antioxidant ability of saline water immersed grass goldfish (Carassius auratus) during the recovery in freshwater. Grass goldfish (38.15 ± 5.48g) acclimated in freshwater were immersed by salinities (0‰, 20‰ and 30‰) for different time durations (10, 20, 30 and 60 min); and the physiological responses were measured during freshwater recovery. The blood osmolalities were not significantly different at any group fish, while whereas the decline of Na+ concentration and the ratio of Na+/Cl- as well as the rise of Cl- concentration was observed in saline treated fish. Soon after freshwater recovery, the transcription of NKA-α and NKA-ß mRNA in gills of salinity 20 immersed fish elevated significantly and then decreased, whereas no obvious changes were detected in salinity 30 treated fish. Till 24h post freshwater recovery, gill Na+/K+-ATPase activities in saline treated fish were lower than control group except for the fish immersed by salinity 20 for 10-30 min. At 24h of recovery, cortisol levels in salinity 20 immersed fish were lower than salinity 30 treated fish, but remained higher than control. As for serum lactic acid, fish treated by salinity 20 for 10 or 20 min did not show any fluctuation. However, higher lactic acid contents were detected in all other five salinity treated groups during recovery. Generally, at 24 h of recovery, salinity 20 treated fish exhibited higher SOD and CAT activities than fish immersed by salinity 30. In summary, grass goldfish could survive by immersion in salinity 20 less than 60 min or salinity 30 less than 30min, even though immersion by salinity 20 could minimize the negative effects.

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.

Syndrome of Inappropriate Antidiuresis: From Pathophysiology to Management.

Hyponatremia is the most common electrolyte disorder, affecting more than 15% of patients in the hospital. Syndrome of inappropriate antidiuresis (SIAD) is the most frequent cause of hypotonic hyponatremia, mediated by nonosmotic release of arginine vasopressin (AVP, previously known as antidiuretic hormone), which acts on the renal V2 receptors to promote water retention. There are a variety of underlying causes of SIAD, including malignancy, pulmonary pathology, and central nervous system pathology. In clinical practice, the etiology of hyponatremia is frequently multifactorial and the management approach may need to evolve during treatment of a single episode. It is therefore important to regularly reassess clinical status and biochemistry, while remaining alert to potential underlying etiological factors that may become more apparent during the course of treatment. In the absence of severe symptoms requiring urgent intervention, fluid restriction (FR) is widely endorsed as the first-line treatment for SIAD in current guidelines, but there is considerable controversy regarding second-line therapy in instances where FR is unsuccessful, which occurs in around half of cases. We review the epidemiology, pathophysiology, and differential diagnosis of SIAD, and summarize recent evidence for therapeutic options beyond FR, with a focus on tolvaptan, urea, and sodium-glucose cotransporter 2 inhibitors.

Salinity-dependent changes in branchial morphometry and Na+ , K+ -ATPase responses of euryhaline Asian sea bass, Lates calcarifer.

Asian sea bass (Lates calcarifer Bloch, 1790) is a euryhaline fish widely cultured in Asia and Australia. Although it is common to culture Asian sea bass at different salinities, osmoregulatory responses of Asian sea bass during acclimation to various salinities have not been fully observed. In this study, we used scanning electron microscopy to observe the morphology of the ionocyte apical membrane of Asian sea bass acclimated to fresh water (FW), 10‰ brackish water (BW10), 20‰ brackish water (BW20), and seawater (SW; 35‰). Three types of ionocytes were identified in FW and BW fish: (I) flat type with microvilli, (II) basin type with microvilli, and (III) small- hole type. Flat type I ionocytes were also observed in the lamellae of the FW fish. In contrast, two types of ionocytes were identified in SW fish: (III) small-hole type and (IV) big-hole type. Furthermore, we observed Na+ , K+ -ATPase (NKA) immunoreactive cells in the gills, which represent the localization of ionocytes. The highest protein abundance was observed in the SW and FW groups, whereas the highest activity was observed in the SW group. In contrast, the BW10 group had the lowest protein abundance and activity. This study demonstrates the effects of osmoregulatory responses on the morphology and density of ionocytes, as well as protein abundance and activity of NKA. In this study, we found that Asian sea bass had the lowest osmoregulatory response in BW10, because the lowest amounts of ionocytes and NKA were required to maintain osmolality at this salinity.

Staying Hydrated in Seawater.

Reduction of intestinal lumen osmotic pressure by the formation of Ca(Mg)CO3, "ichthyocarbonate," is essential for osmoregulation by the only vertebrate group, ray-finned fishes, widely capable of hydrating by ingesting seawater. Ichthyocarbonate formation and excretion are under elaborate physiological control and play an important, yet still poorly defined, role in the oceanic carbon cycle.

Potential role of skeletal muscle glycerophosphocholine in response to altered fluid balance in humans: an in vivo nuclear magnetic resonance study.

Many cells adapt to hyperosmolal conditions by upregulation of organic osmolytes to maintain cell function and integrity. Glycerophosphocholine (GPC), a recognized osmolyte in renal medullary cells, is the major phosphodiester (PDE) in human skeletal muscle, wherefore we hypothesized muscular GPC to be associated with surrogate parameters of fluid status and osmolality in healthy humans. The objective of this study was to investigate the relationship of muscular GPC with surrogate parameters of body fluid status and osmolality. We analyzed data of 30 healthy volunteers who underwent noninvasive 31P-magnetic resonance spectroscopy of either calf (n = 17) or thigh (n = 13) muscle. Therefore, we conducted correlation analyses between phosphor metabolites, and blood values depicting body fluid status and osmolality. Relevant parameters were further implemented in a multivariable regression model to evaluate if GPC concentrations can depict variations in fluid and electrolyte balance. Uric acid (0.437, P = 0.018) and urea (0.387, P = 0.035) were significantly correlated with GPC, which in case of uric acid was independent of sex. Considering sex, following multivariable regression reported GPC as suitable parameter to predict uric acid (R2 = 0.462, adjusted R2 = 0.421; P < 0.001). Our data indicate a connection between muscular GPC concentrations and uric acid, which is a marker of body fluid status, in healthy human subjects, suggesting that skeletal muscle might regulate GPC content in adaptation to changes in fluid status.NEW & NOTEWORTHY Using in vivo magnetic resonance spectroscopy, our study is the first one indicating fluid balance-dependent properties of glycerophosphocholine concentrations in human skeletal muscle. In vivo examination of GPC as organic osmolyte in human skeletal muscle marks a novel approach, which might give further insight on how water and electrolyte balance affect muscle tissue. Beside this main finding, glycerophosphocholine of both calf and thigh muscle correlated remarkably with blood laboratory parameters of lipid metabolism in our study population.