The Role of Saliva in Swallowing: A Systematic Review.

Saliva plays a critical role in swallowing by lubricating tissues and helping form and transport the food bolus. While low saliva production (hyposalivation) is linked to difficulty swallowing, emerging evidence shows that changes in saliva's physical and chemical properties may also affect swallowing. However, the mechanisms underlying these relationships remain poorly understood. The aim of this systematic review was to explore relationships between saliva and swallowing by reviewing studies that provided objective measurements of both saliva production and swallowing biomechanics. A comprehensive search of Medline (Ovid and PubMed), CINAHL, Web of Science, and Scopus was conducted, covering all records from database inception through August 22, 2025. We included studies that reported objective measures of saliva-such as flow rate, volume, thickness, pH, and biomarkers-and swallowing biomechanics. Three independent reviewers carried out two stages of screening, reviewed full texts, extracted data, and evaluated the quality of the methods using a modified Oxford Centre for Evidence-Based Medicine (OCEBM) scale. Most studies used a cross-sectional study design and were rated a "4" according to the OCEBM scale. In many studies, higher salivary flow rates were linked to increased spontaneous swallowing frequency in healthy adults. However, associations between saliva flow and specific swallowing biomechanical measures were not consistent. Lower levels of the neuropeptide substance p were associated with swallowing difficulties in several patient groups, but results varied across studies. The identified evidence suggests that salivary flow and neuropeptides may impact swallowing function, especially sensory function and spontaneous swallowing frequency. However, these relationships were inconsistent. Differences in the patient populations, saliva collection methods, and swallowing assessments made data synthesis between studies difficult. More standardized, long-term research is necessary to elucidate saliva's role in swallowing and improve clinical assessments for people with salivary dysfunction and dysphagia.