The Longitudinal Association Between Intrinsic Capacity and Functional Ability in Older Adults: Comparing Findings from the Cross-Lagged Panel Model and the Random-Intercept Cross-Lagged Panel Model

Intrinsic Capacity (IC) is a multidimensional construct defined by the World Health Organization (WHO). Although IC has been established as a significant predictor of functional decline in older adults, emerging evidence suggests that functional impairment may, in turn, accelerate the degradation of IC, indicating a potential bidirectional relationship. Therefore, this study aims to systematically examine the potential bidirectional relationship between IC and functional ability. This study analyzed longitudinal data from 2,233 adults aged 60 and above from the China Health and Retirement Longitudinal Study (CHARLS, 2011-2015). Cross-lagged panel models (CLPM) and random-intercept cross-lagged panel models (RI-CLPM) were used to examine bidirectional associations between IC and functional ability. IC (0-100) was assessed across five domains: locomotion, cognition, psychological well-being, vitality, and sensory function. Functional ability was measured using scores for basic (BADL: 0-18) and instrumental (IADL: 0-15) activities of daily living at three time points. Models were adjusted for sociodemographic, health, and behavioral factors. Compared to CLPM, RI-CLPM better captured the dynamic trajectory of functional status in older adults. IC decline significantly predicted BADL deterioration (β = 0.089-0.109, p < 0.05), but BADL changes did not significantly affect IC. A bidirectional relationship existed between IC and IADL, with IC's effect on IADL (β = 0.206-0.236, p < 0.001) being stronger than the reverse (β = 0.093-0.100, p < 0.001). This study employed CLPM and RI-CLPM to investigate the dynamic relationship between IC and functional ability. These results offer new directions for developing interventions to promote healthy aging.