Heterogeneous and nonlinear associations between depressive symptoms and functional disability in older Chinese adults

BACKGROUND: Existing studies on the association between depressive symptoms and functional disability in older adults have mainly focused on average effects. However, limited evidence is available on how this association varies across different levels of disability, and the potential nonlinear relationship between the two remains unclear. METHODS: This study used data from the 2015, 2018, and 2020 waves of the China Health and Retirement Longitudinal Study (CHARLS). A balanced panel of 1,189 older adults with disability was constructed. A two-way fixed-effects model was used to estimate the association between depressive symptoms and functional disability. Quantile regression (QR)was employed to examine heterogeneity across different quantiles of disability. Quantile random forest (QRF)was further used to explore the nonlinear relationship between depressive symptoms and functional disability. RESULTS: Depressive symptoms were significantly positively associated with functional disability (β = 0.171, p < 0.001). QR analyses showed that this association differed substantially across quantiles of disability and increased progressively from lower to higher quantiles (from 0.034 to 0.267). QRF analysis further revealed the effect became markedly stronger at higher levels of depressive symptoms. SHAP analysis at the individual level further supported these findings. CONCLUSION: Depressive symptoms are an important factor affecting functional disability in older adults. Their effect shows significant distributional heterogeneity and nonlinear characteristics. Strengthening depression intervention among older adults with higher levels of disability may help delay the progression of functional decline.