Longitudinal BMI-for-Age Trajectories Among Mexican School-Aged Children, 2015–2019

Background/Objectives: Childhood body mass index (BMI) is dynamic, and cross-sectional estimates do not capture within-child changes over time. This study aimed to characterize longitudinal BMI-for-age trajectories among Mexican school-aged children and examine differences according to individual and school-context characteristics. Methods: We analyzed 463,244 children aged 5–12 years with valid anthropometric measurements over four consecutive school years (2015–2016 to 2018–2019). BMI-for-age z-scores (BAZ) were calculated using the WHO 2007 growth reference. Longitudinal patterns were identified using latent class mixed models. Multinomial logistic regression estimated the proportions of children belonging to each trajectory group, and population-averaged generalized estimating equations (GEE) examined overweight/obesity across school grades and trajectory groups. Results: Four trajectories were identified, broadly corresponding to low-BAZ, mid-range BAZ, elevated BAZ, and very-high BAZ trajectories, comprising 13.08%, 43.68%, 32.01%, and 11.23% of children, respectively. Trajectory-group membership probabilities differed by sex and school-context characteristics. Girls had a higher proportion in the mid-range BAZ trajectory than boys (45.7 vs. 41.7%) but a lower one in the very-high BAZ trajectory (8.4 vs. 14.0%). Elevated BAZ and very-high BAZ trajectory proportions increased as municipal marginalization decreased. In the GEE model, overweight/obesity increased across school grades, particularly in the elevated BAZ trajectory (grade-by-trajectory interaction (OR = 1.33; p < 0.001); girls had lower (OR = 0.94; p < 0.001) and children attending urban schools higher (OR = 1.1; p < 0.001) overweight/obesity. Conclusions: Mexican school-aged children exhibited heterogeneous longitudinal BMI-for-age patterns. Individually linked repeated anthropometric measurements provide information on within-child BMI patterns that complement cross-sectional prevalence estimates and may improve the characterization of childhood nutritional status across school and geographic contexts.