Limitations of salivary osmolality as a marker of hydration status

Salivary osmolality (Sosm) is a potentially useful hydration marker but may be confounded by oral artifacts.

This study aimed to determine the efficacy of Sosm for detecting hypohydration and evaluate the effect of a simple mouth rinse.

Eight healthy volunteers (six males and two females; age = 22 ± 7 yr, body mass = 83.7 ± 14.9 kg, height = 176.9 ± 9.2 cm) were measured for nude body mass (BM), plasma osmolality (Posm), and Sosm when euhydrated (EUH) and again when hypohydrated (HYP) by exercise-heat exposure with fluid restriction. After the initial saliva sample during HYP, a 10-s mouth rinse with 50 mL of water was provided, and saliva samples were obtained 1 min (RIN01), 15 min (RIN15), and 30 min (RIN30) after rinse. The ability of Sosm to detect HYP was compared with Posm.

Volunteers were hypohydrated by -4.0% ± 1.2% of BM (range = -2.2% to -5.3%). Sosm was elevated above EUH after hypohydration (EUH 58 ± 8 mmol · kg vs HYP 96 ± 28 mmol · kg, P < 0.05). Sosm baseline and change values displayed more variability than Posm based on ANOVA and regression analyses. After the oral rinse, saliva decreased in concentration (RIN01 = 61 ± 17 mmol · kg, P < 0.05) but returned to prerinse values within 15 min (RIN15 = 101 ± 25 mmol · kg) and remained similar 30 min after (RIN30 = 103 ± 33 mmol · kg).

Sosm was remarkably altered 1 min after a brief water mouth rinse. Fifteen minutes proved an adequate recovery time, indicating that the timing of oral artifacts and saliva sample collection is critical when considering Sosm for hydration assessment. Given the inherent variability and profound effect of oral intake, use of Sosm as a marker of hydration status is dubious.