Estimated Cardiorespiratory Fitness and Risk of Incident Frailty in Middle-Aged and Older Adults: A Cross-National Longitudinal Cohort Study

(1) Background: Frailty is a major geriatric syndrome associated with adverse health outcomes, while direct assessment of cardiorespiratory fitness (CRF) is often impractical in routine clinical settings. This study investigated the association between estimated cardiorespiratory fitness (eCRF) and incident frailty in middle-aged and older adults from three nationally representative aging cohorts. (2) Methods: We analyzed longitudinal data from the Health and Retirement Study (HRS; 2006–2020) in the United States, the English Longitudinal Study of Ageing (ELSA; 2004–2018) in England, and the China Health and Retirement Longitudinal Study (CHARLS; 2011–2018) in China. Participants aged 50 years or older were included. eCRF was calculated using validated sex-specific non-exercise algorithms. Frailty was assessed using a 30-item Frailty Index (FI), and incident frailty was defined as FI ≥ 0.25. Cox proportional hazards models were used to evaluate the association between baseline eCRF and incident frailty. (3) Results: A total of 8152 participants (3982 women and 4170 men) were included in the longitudinal analysis. Each 1-SD increase in eCRF was associated with a lower risk of incident frailty in HRS (HR = 0.60, 95% CI: 0.54–0.68), ELSA (HR = 0.54, 95% CI: 0.46–0.64), and CHARLS (HR = 0.74, 95% CI: 0.63–0.87). Compared with the low-eCRF group, the moderate- and high-eCRF groups had progressively lower risks of incident frailty across all three cohorts, indicating a graded inverse dose–response relationship. Findings were generally consistent across subgroup and sensitivity analyses. (4) Conclusions: Higher eCRF was associated with a lower risk of incident frailty among middle-aged and older adults across three nationally representative cohorts. As an accessible, non-invasive metric, eCRF may be useful for identifying individuals at elevated risk of incident frailty.