What Is the Normal Salivary Flow Rate in Healthy Adults? A Systematic Review with Meta-Analyses

Common reference values for defining healthy salivary flow rates were established decades ago. However, these parameters may no longer accurately reflect normal conditions in contemporary populations. This study aimed to establish average unstimulated (USF) and stimulated (SSF) salivary flow rates in adults as reference values for normality and to compare healthy individuals with those presenting hyposalivation-associated conditions (HACs).

Searches were conducted across 8 databases and the gray literature. Included studies were those with a group of healthy adult individuals (>18 and <60 y old) who underwent a quantitative salivary profile assessment presented in milliliters per minute (mL/min). Studies were excluded if they lacked a healthy group or included children, adolescents, or elderly, among other criteria. The methodological quality of the included studies was assessed using a JBI instrument. An effect size meta-analysis was performed to estimate the average salivary flow in healthy individuals using the restricted maximum likelihood method. Pairwise meta-analyses comparing salivary flow between individuals with HACs and healthy controls were performed using a random effects model. Subgroup analyses and meta-regression were conducted to examine the influences of latitude, study year, and quality. Prospero: CRD42024449389.

A total of 63 studies, including 3,167 healthy individuals and 2,470 individuals with HAC (ages 18 to 60 y, 60% female), were included. The average USF in healthy individuals was 0.82 mL/min (95% confidence interval [CI] = 0.63-1.01), and the average SSF was 1.48 mL/min (95% CI = 1.31-1.65). The mean difference in salivary flow between individuals with HAC and healthy controls was 0.43 mL/min for SSF (95% CI = 0.04-0.76, P = 0.03) and 0.42 mL/min for USF (95% CI = 0.14-0.71, P = 0.004). Meta-regression and subgroup analysis revealed that latitude, study year, and methodological quality did not explain the heterogeneity.

The average salivary flow in healthy individuals may be substantially higher than the current threshold values for USF but not for SSF. These values may represent the actual global salivary flow rates.Knowledge Transfer Statement:The limit of the confidence intervals could be adopted as the threshold for normality. Nearly 0.4 mL/min might be the clinically relevant salivary flow difference between healthy individuals and those with HAC.