Is a higher body mass index associated with longer duration of survival with disability in frail than in non-frail older adults?

Background/objectives: This study investigated the hitherto unclear association of body mass index (BMI) with age at overall, disability, and disability-free survival in older adults with and without frailty.

Methods: This prospective cohort study enroled 10232 Japanese adults aged ≥65 years, who underwent follow-up for adverse events, from the Kyoto-Kameoka Study conducted from 2011-2016. BMI, calculated based on self-reported height and body weight, was classified into five categories: <18.5, 18.5-21.4, 21.5-24.9, 25.0-27.4, and ≥27.5 kg/m². Frailty was assessed using the validated Kihon Checklist. The relationships between BMI and disability and mortality were analysed using multivariate Cox proportional hazards models and Laplace regression.

Results: During the 5.3-year median follow-up period (45472 person-years), 2348 (22.9%) incidences of disabilities occurred. After adjusting for confounders, including medical history and lifestyle, individuals in the lowest and highest BMI categories had a higher hazard ratio (HR) of disability [<18.5 kg/m²: HR: 1.31, confidence interval (CI): 1.16-1.49; ≥27.5 kg/m²: HR: 1.27, 95% CI: 1.08-1.49, p for non-linearity <0.001] compared with that of those with BMI = 21.5-24.9 kg/m². In the 50th percentile differences in age at overall and disability-free survival, participants with BMI < 18.5 kg/m² were more likely to die before disability incidence [survival with disability (overall survival - disability-free survival): -10.2 months]; those with BMI ≥ 27.5 kg/m² had longer survival with disability (12.5 months). These relationships were more marked in the frailty-stratified model, where in the BMI ≥ 27.5 kg/m² group, individuals with frailty survived longer with disability (27.2 months) than did individuals without frailty (6.2 months).

Conclusion: Higher BMI is associated with a longer duration of survival with disability among older adults, especially in those with frailty. Therefore, reversing frailty should be prioritised because individuals with frailty have a shorter probability of disability-free survival than do individuals without frailty, regardless of BMI.