Abstract
BACKGROUND: Static body mass index (BMI) measurements overlook longitudinal weight dynamics. We examined the association between time in target range for BMI (TTR-BMI), which integrates stability and target attainment, and incident multimorbidity. METHODS: We performed a landmark analysis using the English Longitudinal Study of Ageing and China Health and Retirement Longitudinal Study. We included participants aged ≥45 years free of multimorbidity. We calculated TTR-BMI via linear interpolation based on the trapezoidal rule to quantify continuous exposure to a target weight range. We used stratified Cox proportional hazards models to estimate hazard ratios (HRs) for incident multimorbidity. To address proportional hazards violations, we employed time-dependent coefficient analysis to examine temporal heterogeneity (≤3 years vs. >3 years). Lastly, we used negative binomial regression to assess secondary outcomes (disease accumulation). RESULTS: Among 6935 participants, there were 2483 incident cases. Higher TTR-BMI was inversely associated with multimorbidity risk in a dose-response manner. In fully adjusted models, each one standard deviation increase in TTR-BMI reduced risk by 6% (HR = 0.94; 95% confidence interval (CI) = 0.90-0.99). Time-dependent analysis revealed this protection was specific to the late (>3 years) follow-up phase (HR = 0.91; P = 0.003), with no significant effect in the early phase. Furthermore, higher TTR-BMI was associated with a lower rate of disease accumulation (incidence rate ratio = 0.97; 95% CI = 0.95-1.00, P = 0.046). CONCLUSIONS: We found that TTR-BMI is an independent predictor of incident multimorbidity. Maintaining a higher TTR-BMI serves as a protective factor against disease onset. Maximising time spent in a healthy weight range offers a precise, longitudinal target for primary prevention in older adults.