The mediating role of cognitive function in the association between physical activity and frailty risk

Frailty is a common geriatric syndrome that compromises quality of life and worsens health outcomes. The aim was to examine the effect of physical activity on frailty and to explore the potential mediating role of cognitive function. We used data from 11,751 adults aged ≥ 45 years from the 2018 China Health and Retirement Longitudinal Study (CHARLS). Frailty was measured using a Frailty Index, cognitive function with the MMSE, and physical activity categorized by total MET scores. Logistic regression assessed independent and joint associations. Restricted cubic spline analyses based on MMSE scores were conducted to explore nonlinear relationships and identify a cognition threshold (13 points). Mediation analysis with bootstrapping evaluated the mediating role of cognition, with subgroup analyses by sex, education, and residence. A total of 11,751 participants (mean age 60.46 ± 9.02 years) were included in the analysis, of whom 8.1% were classified as frail. Participants with frailty were older, had lower cognitive scores, shorter sleep duration, lower educational attainment, and were less physically active. Higher levels of physical activity were associated with a lower risk of frailty (OR = 0.30, 95% CI: 0.26-0.36), and each one-point increase in cognitive score was also linked to reduced frailty risk (OR = 0.89, 95% CI: 0.87-0.90). A nonlinear association was identified between cognitive function and frailty, with an inflection point at approximately 13 points on the MMSE-derived cognitive score, below which the risk of frailty increased more sharply. Mediation analysis showed that cognitive function partially mediated the association between physical activity and frailty, with a total effect of - 0.14 (95% CI: -0.16 to - 0.12), a direct effect of - 0.13 (95% CI: -0.15 to - 0.11), and an indirect effect of - 0.01 (95% CI: -0.02 to - 0.01), accounting for 8.37% of the overall association. Cognitive function partially explains how physical activity reduces frailty risk. This mediating pathway differs across population groups. Interventions promoting both physical and cognitive health may help delay frailty and support healthy aging.