Relationship between body mass index and fat mass percentage among adolescents from Constantine (Algeria)

Several references based on body mass index (BMI) have been used in Algeria to define adolescent obesity. No studies have been published to assess the diagnostic accuracy of these references. This study aimed to evaluate the sensitivity and specificity of the International Obesity Task Force (IOTF) and Algerian national data based on BMI to detect excess fat in adolescents. A cross sectional study of 257 adolescents aged 16-19 was conducted in Constantine, Algeria. The BMI was calculated from weight and height measurements. The body fat percentage (BF%) was estimated by impedancemetry (BIA). The IOTF and Algerian national data were evaluated for their sensitivity and specificity to detect excess body fat. The BF% thresholds used to define true positives were 25% for boys and 30% for girls. Body mass index was significantly associated with BF% in the whole population (r=0.759). Obesity is more common in girls (p<0.01), regardless of the reference used. The percentage of obesity among adolescents according to the IOTF, Algerian data and BF% was 5.84%, 5.45% and 20.62%, respectively. The obesity rate estimated by BF% was significantly higher (p<0.00001). Sensitivity of the evaluated criteria, IOTF and Algerian data, was high for males (83.3% and 100%, respectively) and low for females (19.2% for both IOTF and Algerian data). Specificity of the two criteria was high for both sexes (100%). Specificity was higher than sensitivity; therefore, the ability of BMI to detect non-obese adolescents was higher than its ability to identify obese adolescents. As BMI is an index of body weight rather than body fat, it can be a source of error. Local data and international references gave the same percentages of obesity. Body mass index is a very specific but less sensitive method to assess increased body adiposity in adolescents. It fails to identify excessive adiposity in a significant percentage of female teenagers. It seems necessary to measure body fat in addition to BMI in adolescent girls. Therefore, it is important to consider body fat mass when determining obesity in order to predict the consequences of obesity, which are mainly due to an increase in fat mass in the body. Key words: Adolescent, Obesity, BMI, Fat mass rate, IOTF, Impedancemetry, Algeria