Objective: To examine the cross-sectional and longitudinal associations of physical activity, screen time and dietary habits on the body mass index (BMI) of children.
Methods: A cohort of 122 girls and 146 boys (age at entry 10 years) from three rural states in the western USA was studied over an 18-month period. Subjects were measured for height and weight. Habitual physical activity, screen time (television viewing, video games and computer use) and dietary variables were assessed by a questionnaire. Cross-sectional and longitudinal analyses were conducted to examine the associations between physical activity, screen time and diet with BMI at baseline and follow-up and change in BMI.
Results: At baseline, approximately 10% of boys and girls were obese and 17.8% of boys and 14.8% of girls were overweight. BMI showed a high degree of stability for boys and girls (r = 0.90), whereas physical activity, screen time and dietary habits showed moderate stability (r = 0.31-0.50) across the 18-month period. Cross-sectional and longitudinal correlations between physical activity, screen time, diet and BMI were low and non-significant (r < 0.15). The regression models explained between 8% and 22% of the variance in the change in BMI; however, none of the predictor variables were statistically significant.
Conclusion: Physical activity, screen time and dietary habits were not significantly related to the BMI in cross-sectional or longitudinal analyses. Further research is warranted to better understand the complex, multifactorial phenotype of the BMI in growing and maturing children.