Salivary Versus Circulating Insulin in Humans: Quantitative Evidence and Validation Requirements

Salivary insulin offers a non-invasive approach to repeated metabolic assessment, but its quantitative relationship with circulating insulin remains incompletely defined. Paired human evidence was critically examined across the domains of cross-matrix association, temporal correspondence, scale, quantitative mapping, agreement, validation, and transferability. Salivary insulin tracked serum or plasma insulin under defined fasting and dynamic conditions, although association strength, temporal correspondence, and saliva-to-blood scale varied across populations, metabolic states, protocols, and salivary measurands. The available paired human evidence includes one explicit saliva-to-plasma mapping equation. Calibration, direct individual-level agreement, and independent external validation of a fixed mapping were not demonstrated, and individual prediction error was not characterized. Current evidence therefore supports salivary insulin as a context-dependent indicator of circulating insulin dynamics rather than a generally transferable quantitative substitute for serum or plasma insulin. Progression toward quantitative use requires explicit definition of the target measurand and temporal relationship, adequate analytical validation, calibration, characterization of individual prediction error, agreement assessment when quantitative substitution is intended, and independent external validation.