Assessment of hydration biomarkers including salivary osmolality during passive and active dehydration

BACKGROUND/OBJECTIVES: Hydration state can be assessed via body mass change (BM Delta), serum and urine osmolality (S-osm, U-osm), urine-specific gravity (U-sg) and urine volume (U-vol). As no hydration index has been shown to be valid in all circumstances, value exists in exploring novel biomarkers such as salivary osmolality (V-osm). Utilizing acute BM Delta as the reference standard, this research examined the efficacy of S-osm, V-osm, U-osm, U-vol and U-sg during passive (PAS) and active (ACT) heat exposure. SUBJECTS/METHODS: Twenty-three healthy men (age, 22 +/- 3 years; mass, 77.3 +/- 12.8 kg; height, 179.9 +/- 8.8cm; body fat, 10.6 +/- 4.5%) completed two randomized 5-h dehydration trials (36 +/- 1 degrees C). During PAS, subjects sat quietly, and during ACT, participants cycled at 68 +/- 6% maximal heart rate. Investigators measured all biomarkers at each 1% BM Delta. RESULTS: Average mass loss during PAS was 1.4 +/- 0.3%, and 4.1 +/- 0.7% during ACT. Significant between-treatment differences at - 1% BM Delta were observed for S-osm (PAS, 296 +/- 4; ACT, 301 +/- 4 mOsm/kg) and U-osm (PAS, 895 +/- 207; ACT, 661 +/- 192 mOsm/kg). During PAS, only U-osm, U-vol and U-sg increased significantly ( - 1 and - 2% BM Delta versus baseline). During ACT, V-osm most effectively diagnosed dehydration >= 2% (sensitivity = 86%; specificity = 91%), followed by S-osm (sensitivity = 83%; specificity= 83%). Reference change values were validated for S-osm, U-sg, and BM Delta. CONCLUSIONS: The efficacy of indices to detect dehydration >= 2% differed across treatments. At rest (PAS), only urinary indices increased in concert with body water loss. During exercise (ACT), S-osm, and V-osm, exhibited the highest sensitivity and specificity. S-osm,S- U-sg and BM Delta exhibited validity in serial measurements. These findings indicate hydration biomarkers should be selected by considering daily activities.