Curvilinear association between loneliness and hair cortisol concentrations in community-dwelling older adults at elevated cardiovascular risk.

Loneliness, a distressing subjective experience that may become a chronic psychosocial stress exposure when persistent, has been linked to dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis, yet empirical findings linking loneliness to cortisol regulation remain inconsistent. One largely unexplored possibility is that loneliness may exert non-linear association on HPA-axis activity. This study examined the curvilinear association between loneliness and hair cortisol concentration (HCC) in community-dwelling older adults. Participants were 126 older adults aged ≥60 years at elevated cardiovascular risk, drawn from the baseline assessment of a randomized controlled trial in China. Loneliness was assessed using the 6-item De Jong Gierveld Loneliness Scale. HCC was quantified using liquid chromatography-tandem mass spectrometry. Linear regression models tested quadratic associations between loneliness and log-transformed HCC, adjusting for demographic, methodological, psychological, social, and health-related covariates. Loneliness showed a significant curvilinear association with HCC, with HCC appearing higher at low-to-moderate loneliness and lower at higher loneliness. The quadratic term remained significant after full covariate adjustment, but sensitivity analyses indicated that statistical significance was sensitive to the exclusion of high-influence cases. These preliminary findings suggest that loneliness may be nonlinearly associated with long-term cortisol output among older adults at elevated cardiovascular risk. The findings may help contextualize prior inconsistencies in the loneliness-cortisol literature and highlight the need for longitudinal studies to clarify temporal and mechanistic pathways.