Uniformity and stability of saliva composition based on glucose concentration analysis

Saliva holds promise as a biomarker for glucose monitoring due to its non-invasive and self-collection characteristics. However, its viscosity may lead to uneven glucose distribution, reflecting the uniformity of saliva, and individual glycemic control factors such as diet may affect salivary glucose concentration, reflecting the stability of saliva. To address these concerns, this study aims to evaluate the uniformity and stability of salivary glucose concentrations. Macro-rheological properties (shear viscosity, elastic modulus, and viscous modulus) and micro-morphological characteristics of saliva were analyzed using a rheometer and Scanning Electron Microscopy and Energy-Dispersive X-ray Spectroscopy (SEM-EDS) techniques. Salivary glucose uniformity and stability were assessed under room-temperature static conditions, fasting saliva over a one-month period, and in postprandial states. Results showed that saliva exhibits shear-thinning behavior, with shear viscosity decreasing from 0.1 Pa∙s to 0.001 Pa∙s as shear rate increases from 0.1 to 150 s^-1. This shear-thinning property, linked to the porous glycoprotein network observed through micro-morphological analysis, may influence analyte distribution. Despite the inherent viscosity, salivary glucose concentrations demonstrated high uniformity: 89 % of fasting samples and 92 % of postprandial samples showed less than 10 % variation when divided into three portions. Regarding stability, salivary glucose levels in healthy individuals remained consistently more stable than those in diabetic patients across various conditions, including static room-temperature storage of fasting saliva, fasting saliva over one month, and postprandial saliva. For example, fasting glucose levels over one month in diabetic patients (7.38 ± 6.3 mg/dL) were higher and less stable than those observed in healthy individuals (1.674 ± 0.72 mg/dL). This study confirms that fasting salivary glucose is both homogeneous and sufficiently stable, supporting its potential as a viable biomarker for blood glucose monitoring.