Serial associations of depressive symptoms and functional impairment in the social isolation-cognitive function relationship: Findings from the Hubei Memory and Aging Cohort Study

BACKGROUND: Depressive symptoms and functional impairment link social isolation to poor cognitive function in older adults, though their serial indirect associations are unquantified. OBJECTIVE: To examine the association between social isolation and domain-specific cognitive function, and to hypothesize "depressive symptoms → functional impairment → cognitive domains" as a theoretically derived serial association model for statistical decomposition of correlations. METHODS: We conducted a cross-sectional analysis of 4,987 baseline participants (mean age 73.01 ± 5.55; 57.0% female) from the Hubei Memory and Aging Cohort Study. We examined the indirect association between social isolation and cognitive domains via depressive symptoms and functional impairment using structural equation modeling (R Lavaan) with 5,000 bootstrap resamples, treating this analysis as a theory-based statistical decomposition of correlations among variables rather than a causal mediation analysis. RESULTS: The prevalence of social isolation was 34.5% among community-dwelling older adults. Depressive symptoms showed indirect associations with language (β = -0.003; 3.19%) and attention (β = -0.002; 2.25%). Functional impairment was associated with all cognitive domains: memory (β = -0.012; 12.77%), language (β = -0.014; 11.02%), attention (β = -0.008; 8.99%), and executive function (β = -0.017; 10.43%). The serial indirect association showed small coefficients across domains (β = -0.002 to -0.003; 1.84%-2.36%). Subgroup analyses indicated significance among rural residents and males. Joint analysis showed a stronger association in socially isolated individuals with low socioeconomic status. CONCLUSIONS: Social isolation, depressive symptoms, and functional impairment correlate with domain-specific cognitive function, yet the serial structure implies no causality. Interventions should prioritize rural, male, and socioeconomically disadvantaged older adults.