Human Hydration Status Monitoring

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.

This study evaluated the components of biological variation and accuracy of potential markers in plasma, urine, saliva, and body mass, for static and dynamic dehydration assessment. Design: Eighteen healthy volunteers (13M, 5F) were studied while carefully controlling hydration and numerous pre-analytical factors. Biological variation was determined over three consecutive days using published methods. Atypical values based on statistical deviations from a homeostatic set-point were examined. Measured deviations in body fluid were produced using a separate, prospective dehydration experiment and evaluated by ROC analysis to quantify diagnostic accuracy.

All dehydration markers displayed substantial individuality and half displayed marked heterogeneity of intra-individual variation. Decision levels for all dehydration markers were within one standard deviation of the ROC criterion values and most were nearly identical to the prospective group means after dehydrating volunteers by 1.8 - 7.0% of body mass. However, only plasma osmolality (Posm) showed statistical promise for use in static dehydration assessment. A 301 ± 5 mmol/kg diagnostic decision level is proposed. Reference change values (RCV) of 9 mmol/kg (Posm), 0.010 (urine specific gravity, Usg), and 2.5% change in body mass (Bm) were also statistically valid for dynamic dehydration assessment at the 95% probability level.

Posm is the only useful marker for static dehydration assessment. Posm, Usg, and Bm are valid markers in the setting of dynamic dehydration assessment.

Inclusion criteria

• You are a member of the U.S. Army between 18-39 years of age
• You have passed the APFT within the previous 12 months
• You have completed and passed a recent medical physical exam
• You are willing to discuss with OMSO and the Principle Investigator (PI) all medications and supplements you are taking and you are willing to stop taking any supplements not approved by OMSO and the PI.

Exclusion criteria

• You have any physical problems that would make exercise difficult
• You have ever had a heat injury or have a history of having trouble in the heat
• You have an allergy to sulfa drugs
• You have been treated for dry eyes
• You are pregnant, planning on becoming pregnant during the study, or are presently lactating