Objective: To examine the emotional, behavioural, and social correlates of missing values for Body Mass Index (BMI) in a nationally representative sample of Portuguese youth.
Design and Methods: Sample included 6131 6th, 8th, and 10th grade public school Portuguese students, age 11-16 years, who participated in the 2002 HBSC / WHO survey of adolescent health. Those who did not report their weight and/or height, were compared with their peers. Bivariate analyses of psychosocial and behavioural variables were conducted, comparing the two groups. Multivariate logistic regression was used to determine if the variables that were significantly associated with missing BMI values at a bivariate level would predict missing BMI, when controlling for all the others in the model.
Results: From the 6131 adolescents who answered to the questionnaire, 661 (10.8%) did not report either their weight or height or both. Gender was not associated with missing BMI. Missing values were significantly predicted by a younger age (aOR=2.56, 95% CI 1.99-3.29, P< 0.001), sedentary lifestyle (aOR=1.53, 95% CI 1.16-2.01, P< 0.01), poor body satisfaction (aOR=1.34, 95% CI 1.19-1.51, P<0.001), father absence (aOR=1.62, 95% CI 1.14-2.30, P<0.01), lack of friends of the opposite sex (aOR=1.65, 95% CI 1.03-2.66, P< 0.05), and by a poor perception of academic achievement (aOR=1.23, 95% CI 1.06-1.42, P< 0.01).
Discussion: Our findings suggest that those with missing values for BMI tend to have poorer body image, health behaviours, and social networks. These results have implications for potential bias in results in studies that do not account for missing BMI.