Estimated and measured BMI and self-perceived body image of adolescents in Germany: part 1 - general implications for correcting prevalence estimations of overweight and obesity.

Objective: This study examines the degree of divergence between BMI calculated from subjective assessments and BMI calculated from measured height and weight as a function of gender and body image. Methods: In the German National Health Interview and Examination Survey for Children and Adolescents (KiGGS) the height and weight of 17,641 children and adolescents aged 0–18 years were measured. Participants were also questioned about their subjective body image (whether they considered themselves much too thin, a bit too thin, exactly the right weight, too fat or much too fat). A representative subsample of adolescents between 11 and 17 years old (N = 3,436: 1,663 boys and 1,773 girls) was asked additionally to self-report their body weight and height before being measured. Results: The bias in the self-reported BMI yielded an underestimation of overweight and obesity prevalence. Girls who considered themselves much too fat or too fat and boys who considered themselves as much too fat underestimated their BMI. This was taken into account using a correction procedure for prevalence estimates of overweight and obesity based on the concept of conditional probabilities. Conclusion: The proposed correction formula using data from the KiGGS study can be applied to other German studies of adolescents in which weight, height and body image are only determined by self-report. Furthermore the correction procedure in principle can be transferred to other studies in other countries as long as a parallel validation study has been conducted to assess both subjective and objective BMI and body image