ASSESSMENT OF HYDRATION

For vital functions to be sustained, it is important that intracellular and extracellular fluids in the body remain balanced. Healthy individuals regulate body fluid balance through physiological and behavioral adaptations (Cheuvront et al., 2010). This balance is maintained by electrolytes, primarily intracellular potassium and extracellular sodium, along with other minerals and proteins (Demirkan et al., 2010).

Loss of body water leads to decreases in physical and cognitive performance, thermoregulation, and cardiovascular function. A loss of 10% or more of body water can be fatal (EFSA, 2017). Hydration refers to having an adequate amount of fluid within body tissues (Riebl and Davy, 2013). Optimal hydration is achieved when an individual consumes enough fluids to compensate for normal daily and unexpected losses, thereby supporting physical and mental health as well as overall well-being (Hydration Toolkit, 2016).

Although the effects of positive fluid balance on morbidity and mortality are well established, little is known about how fluid status should be monitored (Malbrain et al., 2014).

The main principles of body fluid regulation suggest the following indicators of dehydration: plasma osmolality (Posm), urine osmolality (Uosm), urine specific gravity (USG), urine color (Ucol), and potentially saliva osmolality (Sosm) (Cheuvront et al., 2010). Additionally, body weight and thirst can also be used as assessment methods.

Body weight is often used in both laboratory and field settings to evaluate rapid changes in hydration, particularly in athletes. Acute changes in hydration are calculated as the difference between pre- and post-exercise body mass. This method assumes that 1 g of body mass loss is equivalent to 1 ml of water loss (Cheuvront and Sawka, 2005).

Osmolality (osmol/kg solvent) is often used instead of osmolarity (osmol/L solution). In dilute aqueous solutions, the two terms are generally interchangeable (Jequier and Constant, 2010). Plasma osmolality is typically maintained within a narrow range of 280–290 mOsm/kg (Riebl and Davy, 2013). An increase of just 1% is sufficient to trigger the sensation of thirst and double the plasma concentration of antidiuretic hormone (ADH). Therefore, plasma osmolality measurement is the most commonly used hematological index of hydration (Jequier and Constant, 2010).

Since changes in osmolality largely reflect changes in sodium, plasma sodium may serve as an alternative measure of hydration (Cheuvront and Sawka, 2005). It is unclear whether a single hematocrit measurement can provide a valid static indicator of hydration status (Armstrong et al., 2012).

Current evidence and consensus suggest that urine indices—particularly urine osmolality—are among the most promising markers of hydration. A Uosm greater than plasma osmolality indicates functional water deficit, whereas a Uosm lower than plasma osmolality reflects functional water excess (Manz and Wentz, 2003). A Uosm ≥ 800 mmol indicates a state of hypohydration (Kavouras et al., 2016).

USG is a fast and accurate indicator of hydration status. A urine sample is placed on the glass plate of a refractometer for measurement. Normal values range between 1.013 and 1.029; a USG ≥ 1.030 indicates dehydration, while values between 1.001–1.012 suggest hyperhydration. It has been emphasized that USG reflects recent fluid intake more clearly, and therefore it is recommended to be used together with body weight changes (Riebl and Davy, 2013). As plasma osmolality (Posm) increases, urine osmolality also rises as an expected physiological response to dehydration. In this context, plasma osmolality, urine osmolality, and urine specific gravity are considered among the most widely used markers of hydration (Marcos et al., 2014).

Lighter urine colour indicates adequate hydration, whereas darker shades suggest a need for fluid intake (Marcos et al., 2014). Although more subjective, urine colour can serve as a hydration marker when combined with more objective methods such as USG (Riebl and Davy, 2013). At the individual level, in the absence of laboratory analysis or when a quick estimation of hydration is needed, morning urine colour can be used as a reasonably accurate indicator (Marcos et al., 2014). However, it is widely accepted that urine variables tend to reflect recent fluid intake rather than long-term hydration status (Jequier and Constant, 2010).

Saliva is less frequently studied compared to other body fluids for hydration monitoring, but saliva osmolality has been shown to track hydration changes resulting from sweating (Cheuvront and Sawka, 2005).

Thirst is defined as the desire for fluids and foods due to dryness in the mouth and throat (Kara, 2013). In most healthy populations, fluid balance is acutely regulated by central and peripheral mechanisms and maintained through the feedback variable of thirst (Marcos et al., 2014). Hydration status can be approximated by measuring thirst using a simple numerical scale (Jequier and Constant, 2010). Thirst has been reported to be triggered by a 1–2% loss of body water (Riebl and Davy, 2013).

Despite growing interest in the Visual Analog Scale (VAS) as a strong psychometric tool, the preferred visual or categorical method for assessing thirst, in terms of sensitivity and validity, remains unclear—especially for specific populations such as older adults and children. The best method for assessing thirst is still uncertain and requires further research (Millard-Stafford, 2012).

Hydration is critical for maintaining healthy bodily functions. Intracellular and extracellular fluid balance is regulated by electrolytes and minerals; inadequate fluid intake, however, negatively affects physical performance, cognitive function, and the cardiovascular system. Hydration assessment can utilize plasma osmolality, urine parameters (osmolality, specific gravity, color), body weight changes, saliva osmolality, and thirst perception. However, relying on a single method does not provide a definitive evaluation, so using multiple indicators together increases reliability.

Plasma and urine parameters should be used as primary biological indicators, while practical applications can be supported with urine color and body weight changes.

In sensitive populations, such as athletes, older adults, and children, thirst alone is insufficient and should be combined with objective measurements.

Daily fluid intake should be tailored to individual needs, physical activity levels, and environmental conditions.

Further research is needed to develop simple, non-invasive, and valid methods for hydration assessment.

Health professionals and the general public should be made aware of the importance of hydration, and regular fluid intake should be encouraged.