Saliva indices track hypohydration during 48h of fluid restriction or combined fluid and energy restriction

Objective: To investigate whether unstimulated whole saliva flow rate (UFR) and osmolality (Sosm) track changes in hydration status during 48 h of restricted fluid intake (RF) or combined fluid and energy restriction (RF + RE). Following the 48 h periods, UFR and Sosm were assessed after acute exercise dehydration and rehydration.

Design: Thirteen healthy males completed three trials in a randomised order: control (CON) where participants received their estimated energy (12,154 ± 230 kJ/d: mean ± S.E.M) and fluid (3912 ± 140 ml/d) requirements, RF trial where participants received their energy requirements and 193 ± 19 ml/d water to drink (total fluid 960 ± 15 ml/d) and RF + RE where participants received 1214 ± 25 kJ/d and 962 ± 16 ml/d. After 48 h, participants completed 30 min of maximal exercise followed by rehydration (0–2 h) and refeeding (2–6 h).

Results: At 48 h body mass loss exceeded 3% on RF and RF + RE. UFR decreased during 48 h on RF (510 ± 122 to 169 ± 37 μl/min) and RF + RE (452 ± 92 to 265 ± 53 μl/min) and was lower than CON at 48 h (441 ± 90 μl/min: P < 0.05). Sosm increased during 48 h on RF (54 ± 3 to 73 ± 5 mOsmol/kg) and RF + RE (52 ± 3 to 68 ± 5 mOsmol/kg) and was greater than CON at 48 h (52 ± 2 mOsmol/kg: P < 0.05). Unlike UFR, Sosm identified the additional hypohydration associated with exercise (P < 0.05) and returned to within 0 h values with rehydration.

Conclusions: Sosm, and to a lesser extent UFR, track hydration status during a 48 h period of RF or RF + RE and after subsequent exercise and rehydration.