Abstract
<title>Abstract</title> <p>Introduction: Social interaction, physical frailty, and subjective cognitive difficulties are important public health concerns in aging societies because they are relevant to community screening, health promotion, and preventive planning. This study examined whether longitudinal associations among social interaction (SI), Kihon Checklist-defined physical frailty (PF), and Kihon Checklist-defined subjective cognitive decline (SCD) mainly reflected stable between-person differences or within-person temporal processes among community-dwelling older adults. Methods This 6-year prospective cohort study used data from 307 adults aged 65 years and older in a Japanese community cohort in 2014, 2017, and 2020. SI was measured using the 18-item Index of Social Interaction. PF and SCD were operationalised using official Kihon Checklist domain-based cut-offs and treated as binary screening-defined states. A Bayesian random intercept cross-lagged panel model separated trait-like differences from within-person temporal deviations, adjusting for baseline covariates. Results Model fit was acceptable (PPP = 0.605; 95% CrI for the observed–replicated chi-square difference: -55.25 to 41.20; final PSR = 1.001). Higher trait-like SI was associated with lower trait-like PF (β = -0.445, 95% CrI − 0.649 to -0.230) and lower trait-like SCD (β = -0.354, 95% CrI − 0.636 to -0.065). Within-person autoregressive paths were observed, but all cross-lagged credible intervals included zero. Conclusion SI, PF, and SCD were associated mainly at the stable between-person level. Clear within-person cross-lagged associations were not observed; however, this finding should be interpreted cautiously because of the modest sample size, binary screening-defined outcomes, and limited temporal resolution. The findings support the use of social interaction as part of community-based functional aging profiles for population monitoring, early risk identification, and preventive public health planning, rather than as evidence of short-term temporal effects.</p>