Initially 5,614 students were eligible for the study (2,616 females and 2,998 males) but 2,289 students refused to participate (34.5% females and 47.4% males (p <0.001)). A total of 3,325 children and adolescents who presented a signed informed consent were included. No significant differences in age and years of maternal education were found between the study subjects and those who refused to participate.