Metabolic indicators of hydration status in the prediction of parotid salivary-gland function

Salivary glands require body fluids for normal function. It has been suggested that decreased levels of hydration (dehydration) may cause diminished salivary output, but it is not known whether salivary flow rates are related to biological measures of hydration status. The purpose of this study was to determine whether reduced parotid salivary flow rates, as a result of dehydration, can be assessed through physiological markers of hydration status. Twenty-four healthy and unmedicated adults abstained from eating and drinking for 24 h and then underwent intravenous rehydration to replace all lost weight. Unstimulated (UPFR) and 2% citric acid-stimulated (SPFR) parotid flow rates, body weight, and markers of hydration status (haematocrit, haemoglobin, serum sodium, plasma protein, creatinine, serum and urine osmolality) were assessed at baseline, at the completion of the 24-h dehydration period, and 1 h after rehydration had been completed. Correlation coefficients (r) and coefficients of determination (r2) were obtained between: (a) UPFR and each metabolic variable at each time point, (b) the change in UPFR and the corresponding change in each metabolic variable due to dehydration and rehydration, and (c) the percentage change in UPFR and the corresponding percentage change of each metabolic variable due to dehydration and rehydration. These tests were then repeated for SPFR. There were several isolated significant correlations but no consistent trends between salivary flow rates and metabolic markers of hydration status. These findings suggest that metabolic indicators of hydration status are not accurate predictors of parotid salivary flow rates.