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. 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 2233 adults aged 60 and above from the China Health and Retirement Longitudinal Study (2011-2015). Cross-lagged panel models (CLPMs) and random-intercept cross-lagged panel models (RI-CLPMs) were used to examine bidirectional associations between IC and functional ability. Intrinsic capacity (0-100) was assessed across 5 domains: locomotion, cognition, psychological well-being, vitality, and sensory function. Functional ability was measured using scores for basic (basic activities of daily living [BADL]: 0-18) and instrumental (IADL: 0-15) activities of daily living at 3 timepoints. 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. Intrinsic capacity decline significantly predicted BADL deterioration (β = 0.089-0.109, p < .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 < .001) being stronger than the reverse (β = 0.093-0.100, p < .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.