Assessment of Hydration Status at a Single Time Point in Athletes

Proper hydration status is important for athletes, because of its significant influence on physical fitness and mental perception during exercise. Assessment of hydration status is conducted mainly using urinary and plasma indices, which are not always useful at a single time measurements. The study aim was to compare different markers of hydration status in athletes, measured at a single time point. Eighty-nine volleyball players (37 women, 52 men, mean age 16.9y) were recruited. Serum and urine samples were analysed for specific gravity, osmolality, urine and serum osmolality ratio and water deficit. We found a strong correlation between urine specific gravity and osmolality, and only weak correlations with serum osmolality. Urine indices identified 61.8 to 70.8% cases of dehydration, while serum osmolality only 32.6%. Water-deficit was calculated using two different equations which gave contrary results (1.38±0.74% vs -0.33±0.88%). In conclusion, the urinary and plasma indices identified a high occurrence rate of dehydration in athletes, but results derived from different parameters were contradictory. The U osm :Posm ratio seems to be a useful indicator of dehydration, but should be specified with a reference range or cut-off value.


Introduction
Maintaining the correct water balance is one of the most dynamic and important process in human body [1]. This is especially important for athletes, because dehydration can have a negative influence on physical and cognitive performance [2,3]. As such, monitoring athlete hydration status is crucial for optimising performance during training sessions and competitions. The assessment of hydration is mainly based on urine and blood indices, with no consensus regarding the "gold standard" in dehydration markers [2]. One of the most popular markers is plasma osmolality, which reflects intracellular dehydration [3]. Other common indices are urine osmolality and urine specific gravity. Shirreffs [4] concluded that urinary markers are more sensitive indicators of dehydration, but they may have a delay in the reaction. Urine measurements seems to be secondary to plasma osmolality changes [5]. Searching for better indicators of dehydration, researchers have tested composite measures calculated on the basis of blood or urine tests. For example, Cheuvront et al. [6] tested a series of water-deficit equations, and they found significant improvements after replacement in the equation of sodium to plasma osmolality.
Likewise, Armstrong et al. [7] suggested that urine:plasma osmolality ratio could be good indicator of hydration status, measured at a single time point.
Parallel to the search of reliable dehydration markers, studies have investigated the utility of identifying cut-off values. The physiological regulation of plasma osmolality makes that normal values vary about 1-2% from value of 287mOsm/kg in wellhydrated individuals [8]. Based on the small deviation, a cut-off of 290mOsm/kg would seem suitable for classifying subjects as being either euhydrated or dehydrated [9]. Manz and Wentz [7] suggested cut-off value for urinary osmolality of 830mOsm/kg. Baron et al. [8]

ARTICLE INFO ABSTRACT
Proper hydration status is important for athletes, because of its significant influence on physical fitness and mental perception during exercise. Assessment of hydration status is conducted mainly using urinary and plasma indices, which are not always useful at a single time measurements. The study aim was to compare different markers of hydration status in athletes, measured at a single time point. Eighty-nine volleyball players (37 women, 52 men, mean age 16.9y) were recruited. Serum and urine samples were analysed for specific gravity, osmolality, urine and serum osmolality ratio and water deficit. We found a strong correlation between urine specific gravity and osmolality, and only weak correlations with serum osmolality. Urine indices identified 61.8 to 70.8% cases of dehydration, while serum osmolality only 32.6%. Water-deficit was calculated using two different equations which gave contrary results (1.38±0.74% vs -0.33±0.88%). In conclusion, the urinary and plasma indices identified a high occurrence rate of dehydration in athletes, but results derived from different parameters were contradictory. The U osm :Posm ratio seems to be a useful indicator of dehydration, but should be specified with a reference range or cut-off value.
suggest that osmolality over 800mOsm/kg is a more appropriate threshold for differentiation between euhydrated and slightly dehydrated status. In the case of urine specific gravity, a number of studies indicate cut-off values of 1.020-1.025 for the upper range of euhydrated state [10][11][12]. Given the discrepancies, the aim of this study was to compare different markers of hydration status in athletes, measured at a single time point.

Materials and Methods
Eighty-nine volleyball players (37 women and 52 men, anthropometric data in Table 1) participated in this study. Testing was conducted in a laboratory at the Department of Biochemistry, Institute of Sport-National Research Institute in Warsaw, Poland.
This study was approved by the Ethical Research Committee at the Institute of Sport-NRI in Warsaw. Participants were arrived at laboratory between 7:00 and 9:00am. The players were at the beginning of the league season and had two training sessions daily.
Venous blood and urea samples were collected after arrival on an empty stomach and transferred to the laboratory within 1 hour of collection.

Statistical Analyses
All data are presented as mean±SD. The Shapiro-Wilk test was employed to determinate data normality. Because most of the data did not follow a normal distribution, Wilcoxon signed-rank tests was used for a comparison of mean differences. Relationships between variables were also determined using Pearson's correlation coefficient. The level of p≤0.05 was considered significant. Variables  Table 2 shows the measured parameters, and Table 3 shows correlation between them. Significant but weak correlations were observed between U SG and S osm or WD 2 . Interestingly, S osm strongly correlated with WD 2 but there were only weak correlation with U osm :S osm ratio. Calculated osmolality was significantly higher than measured one, and correlation between them was statistically important but rather weak. We observed statistically significant differences between water deficit equations WD 1 and WD 2 but with opposite mean values and weak correlation between them. In approximately half of the athletes (49.3%) WD 1 results were above 2%, what can be considered as important dehydration, while by WD 2 only 1.1% reached this threshold.   Note: Cut-off values adopted from: a Sawka et al [14], b Manz and Wentz [9], c Kenefick et al [8].

Discussion
Maintaining proper hydration status is important for athletes, because its impact on physical fitness and mental perception [13].
A common practice in sport is the monitoring of hydration using different biomarkers [14]. Problem of maintaining appropriate hydration status is often studied in disciplines with weight categories, e.g. combat sports [15]. In our study, more than 60% athletes had results of urine indices above cut-off values, shows dehydration status. Considering serum osmolality, only about one-third of the athletes tested were classified as dehydrated. It is noteworthy that the urine indices identified a similar percentage of euhydrated subjects. The greatest percentage of above cut-off values was observed in OSM cal . It is interesting, because only in 33.7% of cases [Na + ] concentration were above 145mmol/L, what indicates hypertonicity [10]. The results show that the problem of dehydration applies even in those sports that seem less exposed to the occurrence of this phenomenon. One of the most accepted indicators of hydration is plasma osmolality [11]. However, Hamouti et al. [12] suggested that P osm is less sensitive after an overnight recovery, and urine specific gravity and osmolality are better under these circumstances. In turn, Cheuvront and Kenefick [16] concluded that urine-based measurements are secondary to changes in P osm for detecting dehydration. Shirreffs [4] stated that urinary measures could have a time lag over the short term.
In the current study, we found significant correlations between the urinary indices and Sosm. The percentage of urine results above the adopted cut-off values shows more dehydration cases when compared with data for Sosm. This finding partly confirms the assertion that plasma osmolality could better describe acute changes of hydration status. Although, we measured serum not plasma osmolality, the results are interchangeable [10]. For an accurate assessment of hydration status in sport, it is important to determine appropriate cut-off values for different hydration biomarkers. Unfortunately, wide discrepancies exist in the proposed values for urine osmolality, ranging from 700mOsm/kg [17] to above 1000mOsm/kg [18]. We adopted a value of 800mOsm/kg as a meaningful threshold for differentiation between euhydrated and dehydrated status [7]. In reference to urine specific gravity, most authors apply 1.020 as a cut-off value [11]. Similarly, in case of plasma osmolality, a threshold value of 290mOsm/kg is widely accepted.
A lack of consensus around what constitutes the "gold standard" index for assessing hydration status has promoted studies in this area. Armstrong et al. [19] proposed the U osm :P osm ratio as a potential hydration biomarker, which can be measured at a single time point.
Results obtained in our study are similar to Armstrong's data on a low-volume drinkers group. To our knowledge, there are no specific reference range for this ratio, with values <1.0 only indicating the existence of relative water excess. Our data highlight the need to determine reference range or cut-off value for this parameter. Two water-deficit equations were replicated in this work; the most common (WD 1 ) and the best according to the authors (WD 2 ). We found significant differences and in relation to other obtained results, the WD 1 equation seems to be more relevant. The percentage of results above 2% of water deficit, that is considered as significant for performance [20], were 49.4% for WD 1 and 1.1% for WD 2 . Thus, the WD 1 results are more convergent with other measures taken in this study. In conclusion, the occurrence frequency of dehydration in the athletes confirms the need for determination of reliable markers of hydration status in different sports disciplines. Urine is a more accessible material for research, but seems to be less effective with only single measurements obtained. The U osm :P osm ratio seems to be a useful indicator of dehydration status, but should be applied with a specified reference range or cut-off value. In future study should be investigated the need of determining the specific cut-off values for different sports disciplines.