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.

Background: 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.

Methods: 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.

Results: 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).

Conclusions: 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.