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

BACKGROUND/OBJECTIVES: Hydration state can be assessed via body mass change (BMΔ), 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 BMΔ 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 °C). During PAS, subjects sat quietly, and during ACT, participants cycled at 68±6% maximal heart rate. Investigators measured all biomarkers at each 1% BMΔ. RESULTS: Average mass loss during PAS was 1.4±0.3%, and 4.1±0.7% during ACT. Significant between-treatment differences at −1% BMΔ 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% BMΔ 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 BMΔ. 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 , U sg and BMΔ exhibited validity in serial measurements. These findings indicate hydration biomarkers should be selected by considering daily activities.