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Abstract
Acknowledging that total body water (TBW) turnover is complex, and that no measurement is valid for all situations, this review evaluates 13 hydration assessment techniques. Although validated laboratory methods exist for TBW and extracellular volume, no evidence incontrovertibly demonstrates that any concentration measurement, including plasma osmolality (Posm), accurately represents TBW gain and loss during daily activities. Further, one blood or urine sample cannot validly represent fluctuating TBW and fluid compartments. Future research should (a) evaluate novel techniques that assess hydration in real time and are precise, accurate, reliable, non-invasive, portable, inexpensive, safe, and simple; and (b) clarify the relationship between Posm and TBW oscillations in various scenarios.
Key teaching points:
• All hydration assessment techniques provide singular measures of a complex and dynamic fluid matrix, containing interconnected compartments.
• A single gold standard, including plasma osmolality, is not possible for all hydration assessment requirements.
• In the laboratory, measurement resolution and accuracy are essential.
• Field assessment of hydration requires techniques that are easy-to-use, safe, portable, and inexpensive.
• Total body water approximates “euhydration” when morning body weight is near the normal baseline, fluid intake is adequate, urine color is pale yellow, and urine volume is normal.
• Body weight change provides the simplest and most accurate index of hydration status in real time, when serial measurements are made in close proximity.
Key teaching points:
• All hydration assessment techniques provide singular measures of a complex and dynamic fluid matrix, containing interconnected compartments.
• A single gold standard, including plasma osmolality, is not possible for all hydration assessment requirements.
• In the laboratory, measurement resolution and accuracy are essential.
• Field assessment of hydration requires techniques that are easy-to-use, safe, portable, and inexpensive.
• Total body water approximates “euhydration” when morning body weight is near the normal baseline, fluid intake is adequate, urine color is pale yellow, and urine volume is normal.
• Body weight change provides the simplest and most accurate index of hydration status in real time, when serial measurements are made in close proximity.
Notes
Conflict of Interest Disclosure: The author declares that no present corporate interest presents a conflict of interest with this review paper. The author previously has received research funding from ILSI North America and The Coca Cola Company.
Presented at the ILSI North America 2006 Conference on Hydration and Health Promotion, November 29–30, 2006 in Washington, DC.