Human hydration level monitoring using embedded piezoresistive microcantilever sensors

Salivary Osmolality, Function, and Hydration Habits in Community-Dwelling Older Adults

The aim of this study was to examine the relationship between hydration status as measured by salivary osmolality and personal hydration habits, selected demographic characteristics, and performance on a walking and balance test in older community-dwelling adults. This study used a descriptive observational design in a convenience sample of multiethnic, community-dwelling older adults (N = 53). We collected saliva for analysis on 3 days both in the morning and early afternoon, along with a hydration habit questionnaire, get up and go test and demographic information. An exploratory factor analysis of the hydration habit questionnaire revealed a two-factor solution including physical barriers and psychological barriers to drinking fluids. A linear mixed-model approach revealed that time of day (p < .01), race (p = .015), mobility (p < .01), and cognitive barriers (p = .023) are all significant predictors of salivary osmolality among noninstitutionalized seniors. There is also a significant interaction between psychological barriers to drinking fluids and time of day (p < .01). Average salivary osmolality was higher in this group of older adults than has been reported in younger adults. Controlling for all other variables, salivary osmolality is higher in the morning than in the afternoon, lower among Black or African American seniors than among White or Caucasian seniors, and higher among seniors with decreased mobility. An interaction between psychological barriers and salivary osmolality showed that those participants with more psychological barriers to drinking had higher salivary osmolality in the morning and an inverse relationship in the afternoon.

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.

Engineering Approaches to Assessing Hydration Status

Dehydration is a common condition characterized by a decrease in total body water. Acute dehydration can cause physical and cognitive impairment, heat stroke and exhaustion, and, if severe and uncorrected, even death. The health effects of chronic mild dehydration are less well studied with urolithiasis (kidney stones) the only condition consistently associated with it. Aside from infants and those with particular medical conditions, athletes, military personnel, manual workers, and older adults are at particular risk of dehydration due to their physical activity, environmental exposure, and/or challenges in maintaining fluid homeostasis. This review describes the different approaches that have been explored for hydration assessment in adults. These include clinical indicators perceived by the patient or detected by a practitioner and routine laboratory analyses of blood and urine. These techniques have variable accuracy and practicality outside of controlled environments, creating a need for simple, portable, and rapid hydration monitoring devices. We review the wide array of devices proposed for hydration assessment based on optical, electromagnetic, chemical, and acoustical properties of tissue and bodily fluids. However, none of these approaches has yet emerged as a reliable indicator in diverse populations across various settings, motivating efforts to develop new methods of hydration assessment.

Salivary Markers for Quantitative Dehydration Estimation During Physical Exercise

Salivary markers have been proposed as noninvasive and easy-to-collect indicators of dehydrations during physical exercise. It has been demonstrated that threshold-based classifications can distinguish dehydrated from euhydrated subjects. However, considerable challenges were reported simultaneously, for example, high intersubject variabilities in these markers. Therefore, we propose a machine-learning approach to handle the intersubject variabilities and to advance from binary classifications to quantitative estimations of total body water (TBW) loss. For this purpose, salivary samples and reference values of TBW loss were collected from ten subjects during a 2-h running workout without fluid intake. The salivary samples were analyzed for previously investigated markers (osmolality, proteins) as well as additional unexplored markers (amylase, chloride, cortisol, cortisone, and potassium). Processing all these markers with a Gaussian process approach showed that quantitative TBW loss estimations are possible within an error of 0.34 l, roughly speaking, a glass of water. Furthermore, a data analysis illustrated that the salivary markers grow nonlinearly during progressive dehydration, which is in contrast to previously reported linear observations. This insight could help to develop more accurate physiological models for salivary markers and TBW loss. Such models, in turn, could facilitate even more precise TBW loss estimations in the future.

Chemical sensor platform for non-invasive monitoring of activity and dehydration

A non-invasive solution for monitoring of the activity and dehydration of organisms is proposed in the work. For this purpose, a wireless standalone chemical sensor platform using two separate measurement techniques has been developed. The first approach for activity monitoring is based on humidity measurement. Our solution uses new humidity sensor based on a nanostructured TiO₂ surface for sweat rate monitoring. The second technique is based on monitoring of potassium concentration in urine. High level of potassium concentration denotes clear occurrence of dehydration. Furthermore, a Wireless Body Area Network (WBAN) was developed for this sensor platform to manage data transfer among devices and the internet. The WBAN coordinator controls the sensor devices and collects and stores the measured data. The collected data is particular to individuals and can be shared with physicians, emergency systems or athletes' coaches. Long-time monitoring of activity and potassium concentration in urine can help maintain the appropriate water intake of elderly people or athletes and to send warning signals in the case of near dehydration. The created sensor system was calibrated and tested in laboratory and real conditions as well. The measurement results are discussed.

Quantifying dehydration in the fire service using field methods and novel devices

OBJECTIVE

This study reports measurements of hydration status among firefighters prior to training, documents changes in hydration status after prolonged firefighting training, and reports the utility of salivary measurements to assess changes in hydration in field environments.

METHODS

Nude body mass measurements as well as urinary and salivary measurements of hydration status were taken before and after approximately three hours of firefighting training activities. Initial hydration status was assessed via urinary and salivary measures. Changes in body mass and total body water were measured following firefighting activity and correlated with changes in urinary and salivary measures of hydration.

RESULTS

The most important findings of this study were that a high percentage of firefighters arrived at training in a significantly or seriously dehydrated state; that firefighters lost a significant amount of body mass because of firefighting operations; and that portable salivary osmolality measurements showed much stronger correlation with changes in hydration status after firefighting operations than standard urinary measurements did.

CONCLUSIONS

Firefighters arriving in a dehydrated state are at risk for heat injuries and may be in a physically and/or psychologically compromised state at the outset of firefighter training. Even during cool autumn days with ample fluids available, firefighters experience dehydration during typical firefighting activities, so the ability to measure hydration status throughout such activities may be important. Our data suggest that quantification of changes in hydration status through salivary osmolality measurements may provide a viable field measurement tool for such activities.

Hydration and Health

Daily adequate water intake to maintain euhydration is arguably the most important nutrient requirement for humans. Within a margin of error, the body regulates the maintenance of body fluid balance and especially that of the plasma volume, through mechanisms that stimulate thirst and/or modify the rate of urine production. However, there are circumstances such as with excessive sweating during exercise in the heat, osmotic diarrhea, or excessive fluid consumption, or water intoxication where normal mechanisms of regulation may be inadequate to compensate for acute changes in hydration status and result in life threatening consequences. Health and onset of disease may be affected by the chronic hydration state of individual. The risks of colorectal cancer, nephrolithiasis in those with a history of kidney stones, and bladder cancer may be reduced by more frequent water consumption. Recent research suggests that appropriate timing of water intake around meal occasions may help reduce energy intake and contribute to maintenance of body weight in overweight individuals. Definitive benefits of hydration on cardiovascular and oral health and general immune system function remain to be determined. It is also unclear whether the health benefits of water and fluid ingestion are a function of the process of frequent fluid intake or the maintenance of a potentially expanded state of hydration.

Spit: Saliva in Nursing Research, Uses and Methodological Considerations in Older Adults

Over the last 10 years, interest in the analysis of saliva as a biomarker for a variety of systemic diseases or for potential disease has soared. There are numerous advantages to using saliva as a biological fluid, particularly for nurse researchers working with vulnerable populations, such as frail older adults. Most notably, it is noninvasive and easier to collect than serum or urine. The authors describe their experiences with the use of saliva in research with older adults that examined (a) osmolality as an indicator of hydration status and (b) cortisol and behavioral symptoms of dementia. In particular, the authors discuss the timing of data collection along with data analysis and interpretation. For example, it is not enough to detect levels or rely solely on summary statistics; rather it is critical to characterize any rhythmicity inherent in the parameter of interest. Not accounting for rhythmicity in the analysis and interpretation of data can limit the interpretation of associations, thus impeding advances related to the contribution that an altered rhythm may make to individual vulnerability.

Biological variation and diagnostic accuracy of dehydration assessment markers

BACKGROUND

Well-recognized markers for static (one time) or dynamic (monitoring over time) dehydration assessment have not been rigorously tested for their usefulness in clinical, military, and sports medicine communities.

OBJECTIVE

This study evaluated the components of biological variation and the accuracy of potential markers in plasma, urine, saliva, and body mass (B(m)) for static and dynamic dehydration assessment.

DESIGN

We studied 18 healthy volunteers (13 men and 5 women) while carefully controlling hydration and numerous preanalytic factors. Biological variation was determined over 3 consecutive days by using published methods. Atypical values based on statistical deviations from a homeostatic set point were examined. Measured deviations in body fluid were produced by using a separate, prospective dehydration experiment and evaluated by receiver operating characteristic (ROC) analysis to quantify diagnostic accuracy.

RESULTS

All dehydration markers displayed substantial individuality and one-half of the dehydration markers displayed marked heterogeneity of intraindividual variation. Decision levels for all dehydration markers were within one SD of the ROC criterion values, and most levels were nearly identical to the prospective group means after volunteers were dehydrated by 1.8-7.0% of B(m). However, only plasma osmolality (P(osm)) showed statistical promise for use in the static dehydration assessment. A diagnostic decision level of 301 plusmn 5 mmol/kg was proposed. Reference change values of 9 mmol/kg (P(osm)), 0.010 [urine specific gravity (U(sg))], and 2.5% change in B(m) were also statistically valid for dynamic dehydration assessment at the 95% probability level.

CONCLUSIONS

P(osm) is the only useful marker for static dehydration assessment. P(osm), U(sg), and B(m) are valid markers in the setting of dynamic dehydration assessment.