The Range and Direction of Changes in the Classification of the Body Mass Index in Children Measured at the Age of 6 and 10 (Longitudinal Studies)

Background/Objectives: Body Mass Index (BMI) is widely used to assess nutritional status in children and serves as a key indicator for identifying risks of overweight and underweight during development. Understanding how BMI classifications evolve over time is crucial for early intervention and public health planning. This study aimed to determine the scope and direction of changes in BMI classification among children between the ages of 6 and 10. Methods: This longitudinal study included 1,026 chil-dren (497 boys and 529 girls) from Gdansk, Poland. Standardized anthropometric measurements were collected at ages 6 and 10. BMI was calculated and classified using international reference systems (IOTF and OLAF). Changes in BMI classification were analyzed using rank transformations and Pearson correlation coefficients to assess linear relationships between somatic features. Results: Most children (76.51%) re-tained their BMI classification over the four-year period. However, 23.49% experi-enced changes, with boys more often moving to a higher BMI category (15.29%) and girls more frequently shifting to a lower category (14.03%). The prevalence of obesity declined between ages 6 and 10, while both overweight and underweight classifica-tions slightly increased. Strong correlations were observed between somatic features and BMI at both ages. Conclusions: The stability of BMI classification over time un-derscores the importance of early identification and sustained monitoring of nutri-tional status. Gender-specific patterns suggest the need for tailored health promotion strategies. In this context, incorporating dietary interventions—such as promoting balanced meals and reducing unhealthy food intake—could play a significant role in maintaining healthy BMI trajectories and preventing both obesity and undernutrition during childhood.