Secular Trends in Height, Body Mass, and BMI Among Polish Boys in Eastern Regions from 1986 to 2021: Cross-Decade Analysis of Nutritional Status

<b>Background/Objectives:</b> Secular trends in children's physical development are important indicators of population health, nutritional status, and socioeconomic conditions. This study aimed to assess long-term changes in the height, weight, Body Mass Index (BMI), and nutritional status of boys from Eastern Poland between 1986 and 2021. <b>Methods:</b> Anthropometric data were collected from 13,172 boys aged 8, 13, and 17 years at five time points (1986, 1996, 2006, 2016, and 2021). Standardized measurement protocols were used throughout the study. The BMI was calculated and categorized using international cut-off points for age and gender. Secular changes in the height, weight, and Body Mass Index (BMI) were analyzed using an analysis of variance (ANOVA) with post hoc tests, and differences in dietary categories were assessed using chi-square tests (<i>p</i> ≤ 0.05). <b>Results:</b> The height, weight, and BMI increased significantly across all ages. The largest height gain was seen in 13-year-olds, while the greatest BMI increase occurred between 2016 and 2021. The overweight and obesity prevalence rose sharply by an average of 21.70% across age groups, with the normal BMI prevalence decreasing by 18.41%. The underweight prevalence declined, especially among adolescents; however, this likely reflects a general upward shift in the BMI rather than a true nutritional improvement. <b>Conclusions:</b> Strong secular trends are evident, influenced by global and local socioeconomic factors, including Poland's EU accession and the COVID-19 pandemic. While an increased height suggests better living standards, the rising overweight and obesity rates indicate emerging health risks. Due to the lack of direct lifestyle and socioeconomic data, further research incorporating these factors and the pubertal BMI variability is needed to clarify underlying causes. Targeted regional strategies promoting healthy diets, physical activity, and lifestyles are urgently required.