Bidirectional transitions of frailty states among middle-aged and older adults: a longitudinal cohort analysis using a multi-state Markov model based on the China Health and Retirement Longitudinal Study.

Background and Objectives: Frailty is a dynamic syndrome increasing older adults' vulnerability to adverse outcomes. Longitudinal data on frailty transitions and their influencing factors remain limited. We aimed to examine bidirectional frailty transitions among middle-aged and older adults using a multi-state Markov (MSM) model. Research Design and Methods: Data were obtained from four waves (2011-2018) of the China Health and Retirement Longitudinal Study (CHARLS), including 15 763 participants aged ≥45 years. Frailty was assessed using a 32-item frailty index. MSM models estimated transition probabilities, mean sojourn times, and covariate effects. Additional analyses examined age- and gender-specific patterns and included an age × gender interaction term. Results: Baseline prevalence of robust, pre-frail, and frail states was 44.3%, 39.4%, and 16.3%, respectively. Within one year, pre-frail participants had probabilities of 18.0% reverting to robust and 19.7% progressing to frail states. At five years, these probabilities were 23.4% and 33.4%, respectively, with mortality increasing to 19.7%. Older age increased frailty progression and mortality risks but reduced recovery likelihood. Notably, significant age × gender interactions were observed for transitions from pre-frail to robust and from frail to death. Men showed higher recovery rates but greater frailty-related mortality than women. Urban residency, higher education, and marriage were protective, while smoking and alcohol increased frailty risk. Discussion and Implications: Frailty among middle-aged and older Chinese adults demonstrates substantial bidirectional transitions, indicating notable opportunities for intervention and prevention. Age, gender, socioeconomic status, and lifestyle behaviors are key modifiable determinants influencing frailty progression and recovery. Public health strategies prioritizing targeted screening and preventive interventions-particularly among vulnerable groups-could effectively mitigate frailty progression, promote recovery, and improve overall population health outcomes.