Trajectory of Body Mass Index and Frailty Among Older People in Southern Brazil: A Longitudinal Study

<b>Background/Objectives</b>: Frailty is a common geriatric syndrome associated with adverse outcomes such as disability, hospitalization, and mortality. This study aimed to assess the association between body mass index (BMI) trajectories over ten years and frailty among community-dwelling older adults in Brazil. <b>Methods</b>: This population-based longitudinal study used data from the COMO VAI? cohort, conducted with individuals aged ≥60 years in Pelotas, southern Brazil. Frailty was defined in 2024 using Fried's phenotype, which evaluates weight loss, exhaustion, low physical activity, slowness, and weakness. BMI categories were defined as underweight (BMI < 22.0 kg/m²), eutrophy (22.0-27.0 kg/m²) and overweight (>27.0 kg/m²). BMI trajectories were identified using group-based trajectory modeling for 789 participants with data from at least two of three assessments (2014, 2019, 2024). Only trajectory groups comprising at least 5% of the sample were retained. Associations of baseline BMI and BMI trajectories with frailty were analyzed using Poisson regression with robust variance, adjusted for confounders and calf circumference. <b>Results</b>: Baseline underweight and overweight prevalence were 9.2% and 56.2%, respectively. Trajectory modeling identified three BMI groups: eutrophic (31.6%), overweight (56.4%), and obesity (12.0%). Obesity emerged as a distinct longitudinal trajectory rather than a baseline BMI category. Underweight did not emerge as a distinct BMI trajectory due to its low prevalence over time. Frailty prevalence in 2024 was 36.5%. Overweight trajectory participants had lower frailty prevalence after ten years (PR = 0.73; 95% CI: 0.54-0.99), while baseline underweight was associated with higher frailty ten years later (PR = 1.73; 95% CI: 1.05-2.84), consistent with the known risk of underweight and the potential protective effect observed in overweight older adults. <b>Conclusions</b>: Baseline underweight increased frailty risk, whereas an overweight trajectory showed a potential protective effect, consistent with the "obesity paradox" in older populations.