All hydration assessment techniques evaluate a complex fluid matrix and interconnected fluid compartments. Singular measurements are inadequate because fluid gain and loss alters TBW as a sinusoidal wave that oscillates around an average. The measurement resolution and accuracy of most hydration assessment techniques (Tables 1 and 4) is not supported by a large, consistent data base. Also, no previous publication provides incontrovertible evidence that measure- ments of concentration (including Posm) validly represent body water loss or gain in all situations. Therefore, dynamic human water turnover is inadequately represented by (a) a single measurement in time, especially when fluid balance is perturbed, and (b) techniques that have poor measurement resolution and accuracy.
In the laboratory, certain hydration assessment tech- niques are effective. Under controlled conditions (i.e., when experimental, postural, activity, dietary, and environmental factors are controlled), the TBW, volume of body fluid compartments, and extracellular fluid concentration stabilize. When body fluids are equilibrated, TBW and Posm provide objective measurements of volume and concentra- tion at a single point in time.
During daily activities or exercise, when fluid compart- ments are constantly fluctuating (i.e., volume and concentra- tion), a direct evaluation of a single body fluid (Table 1) will not provide valid information about TBW and the concentra- tion of body fluids. For example, several studies are presented above in which Posm does not track the gain or loss of TBW. Body weight change provides the simplest and most accurate index of hydration status (Table 1) in real time, when serial measurements are collected in close proximity. Thus, in the field, when an estimate of hydration status is needed or when a large body water loss is anticipated (i.e., exercise), one should compare information from two or more hydration assessment
techniques, and evaluate body hydration status more than once each day.
Future research and development efforts should focus on novel hydration assessment techniques [4,6,7,38] that (a) mea- sure fluid volume and concentration in real time; (b) have excellent precision, accuracy and reliability; (c) are non-inva- sive; (d) are interpreted in concert with other hydration indices; and (e) are portable, inexpensive, safe, and simple to use. Specifically regarding Posm, future investigations should (f) evaluate the validity of the relationship between Posm and body water gain/loss in a variety of settings, and (g) compare the ability of Posm (and other hematologic indices) to track body water change versus other (i.e., urinary) hydration assessment techniques (Fig. 2).
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