The predominant constituent of saliva is water (97-99.5 %), which enters saliva from plasma across acinar cells (Young & van Lennep, 1979). Dehydration, induced by a 24 h period without food and water, has recently been shown to decrease parotid saliva flow rate in adults (Ship & Fischer, 1999). To identify if saliva flow rate, total protein concentration and osmolality are sensitive markers of whole body hydration status, we compared changes in these parameters with changes in body mass during progressive acute dehydration and rehydration in humans.

With local ethics committee approval, twelve healthy males (age 21 ± 1 years, body mass 76.2 ± 2.3 kg,V_O2,max 57.6 ± 2.2 ml kg^-1 min^-1, mean ± S.E.M.) volunteered to participate in the study. To ensure subjects arrived at the laboratory in a euhydrated state they were instructed to drink 30 ml (kg body mass)^-1 of water the day before the trial. Subjects reported to the laboratory following an overnight fast and cycled on a stationary ergometer at 70 % of age-predicted maximal heart rate in an environmental chamber (30 °C and relative humidity 70 %) until progressive body mass loss (BML) of 1.0 ± 0.0, 2.0 ± 0.0 and 2.9 ± 0.2 %. After exercise, subjects were given a volume of carbohydrate (CHO)-electrolyte solution (6 % CHO and 25 mmol l^-1 Na⁺) equivalent to 150 % BML to consume within 1 h. Unstimulated whole saliva samples were collected over a 2 min period into pre-weighed tubes at pre-exercise, 1.0, 2.0 and 2.9% BML and then at 1 h 15 min, 2 h 15 min and 3 h 15 min post-exercise. Saliva samples were stored at -40 °C prior to analysis. Saliva volume was estimated by weighing to the nearest mg and saliva density was assumed to be 1 g ml^-1. Saliva flow rate was determined by dividing the volume of saliva by the collection time. Saliva total protein concentration was measured using a kit (No. 610, Sigma, Poole, UK). Saliva osmolality was measured using a freezing point depression osmometer (Advanced Instruments, MA, USA). Correlations between % BML and each saliva parameter were calculated by pooling individual Pearson’s correlation coefficients and applying Fisher’s Z_r transformation. Adjustments were made to the significance of correlation coefficients according to sample size and number of correlations performed (Shavelson, 1988). Differences between correlation coefficients were determined using an adjusted z score equation (Meng et al. 1992). Results were also analysed using repeated measures ANOVA with post-hoc Tukey’s test where appropriate. Statistical significance was accepted at P < 0.05.

Saliva total protein concentration and osmolality increased (main effect of time: P < 0.01), and flow rate decreased (main effect of time: P < 0.01), during dehydration (Table 1). After consumption of the rehydration solution saliva parameters were not significantly different from pre-exercise. Saliva total protein concentration and osmolality correlated strongly with % BML during dehydration (r = 0.97 and 0.94, respectively: P < 0.01). Correlations for saliva total protein concentration and osmolality with % BML were greater (P < 0.01) than the correlation for flow rate with % BML (-0.70: P < 0.05) during dehydration (Table 1). These data show that changes in saliva total protein concentration and saliva osmolality are strongly associated with changes in body mass during progressive acute dehydration in humans.