Body mass index versus bioelectric impedance analysis for the assessment of obesity and excess body fat among adolescents in Abia State, Nigeria

Abstract

Background: The body mass index (BMI) Z- score is widely used in children and adolescents to assess overweight/obesity, but does not determine the body fat percentage (BF%). The bioelectric impedance analysis (BIA) can be used to determine body fat, because it provides information on body mass composition.

Objective: To evaluate the ability of the BMI Z-scores to predict BF% in adolescents.

Methods: This cross sectional survey included 277 randomly selected adolescent (10-19 years) boys and girls in Umuahia Metropolis, Abia State, Nigeria. Height and weight were measured and BMI-for-age Z-scores calculated using WHO standards, while BF% was estimated using BIA. Sensitivity and specificity of BMI Z-score ≥+1.00 were calculated and Receiver Operating Characteristic (ROC) analysis and Area Under the Curve (AUC) were used to determine the diagnostic ability of BMI Z-score to predict BF%. Inferential statistics was performed with the Student's t - test, Pearson's correlation and Chi Square test.

Results: Overall, 4.7% of participants have excess BF% and 9% were obese using BIA and BMI Z-score, respectively. Mean BMI-Z score and BF% were -0.36±1.08 and 16.08±6.83%, respectively. Percent BF was significantly correlated with BMI Z-score (r= 0.626). The AUC was > 0.89 and >0.75 in girls and boys, respectively. Sensitivity was high (80% and 84%) and specificity was moderate (65% and 60%) in boys and girls, respectively.

Conclusion: The high sensitivity but moderate specificity in detecting excess BF% underscores the need for more direct measurement of body fat, to improve the diagnostic accuracy of overweight and obesity.