Reference curves of the body fat index in adolescents and their association with anthropometric variables

Objectives: To develop reference curves for the body fat index (BFI) in the pediatric population, in adolescents from the city of Sao Paulo, Brazil, and verify their association with body mass index and body fat percentage. Methods: The study is part of the research project ‘‘Nutritional Profile of Adolescents from Public and Private Schools of Sao Paulo’’ that was performed in 2004-2005. A total of 4,686 adolescents (2,130 boys and 2,556 girls) aged 10-15 years were divided into two groups: 1012 and 13-15 years of age. Body mass, height, body mass index, hip circumference, body fat percentage, body fat index, and sexual maturation performed by the self-assessment method (prepubertal, pubertal, and postpubertal) were analyzed. ANOVA was performed, as well as percentile distribution, Pearson’s correlation, and Bland-Altman plot. Results: In boys, there was an increase in body mass, height, body mass index, and hip circumference with advancing age and Tanner stage. In girls, there was an increase in body fat index and body fat percentage with advancing age and stage of sexual maturation. An association was found between body fat index and body mass index (r = 0.67 in boys and 0.80 in girls, p < 0.001) and body fat percentage (r = 0.71 in boys and 0.68 in girls, p < 0.001). Conclusion: The body fat index seems to reflect well the phenomena of sexual dimorphism in adolescence, is easy to perform, and represents a method that should be used in population samples.