The Age-Varying Association Between BMI and Height-for-Age in Vietnamese Schoolchildren: A Sex-Stratified Quantile Regression Study

Background: BMI and height are usually related through mean-based models, which can mask how this relationship varies across the height distribution and across age. Whether BMI relates to height differently at different quantiles, and whether this differs by sex, is not well studied in Southeast Asian children. Methods: We analyzed longitudinal school health data from 32,006 Vietnamese children (16,489 boys, 15,517 girls) aged 5 to 18 with at least 3 annual visits from 2018 to 2025. Using sex-stratified linear quantile mixed models, we estimated the association between BMI-for-age Z-score (BMIZ) and height-for-age Z-score (HAZ) at 5 quantiles, adjusting for a flexible age spline and study site, with a child-level cluster bootstrap for inference, plus a pooled model with a formal sex interaction test. Results: A 1SD increase in BMIZ was linked to higher HAZ at every quantile in both sexes (boys 0.078 to 0.100, girls 0.055 to 0.089, all p < .01), with no significant sex interaction (p = .35), contrasting with a small negative association from a linear mixed model (-0.023). This association did not differ significantly across quantiles within either sex, pointing to a fairly uniform effect across the height distribution itself, but it did vary strongly by age, strongest in early and mid-childhood and reversing to negative by late adolescence in both sexes, so children with overweight or obesity were taller for age through most of childhood but shorter than leaner peers by the oldest ages studied. Within-group variability in HAZ narrowed with age in both sexes, and boys with thinness showed a consistently wider height spread than boys in the other BMI categories through most of childhood, a pattern that was less pronounced in girls. Conclusions: BMI's association with linear growth in Vietnamese children is positive through most of childhood but weakens and reverses by late adolescence, a pattern mean-based models obscure. BMI and height should be interpreted together in light of a child's age, not as one fixed relationship.