Association Between the Serum Creatinine to Cystatin C Ratio, Physical Activity, and Frailty in Middle-Aged and Older Adults in China: A Nationwide Cohort Study

SI is independently associated with a lower risk of incident frailty, particularly among less physically active individuals. These findings support the potential use of SI as a feasible biomarker for early frailty risk stratification and highlight the importance of integrating biomarker-based screening with lifestyle interventions to prevent frailty.