RESUMO
BACKGROUND: Body mass index measures excess weight for size, and does not differentiate between fat mass (FM) and fat-free mass (FFM). Bioelectrical impedance analysis (BIA) is most commonly used to assess FM and FFM as it is simple and inexpensive. Variables from BIA measurements are used in predictive equations to estimate FM and FFM. To date, these equations have not been validated for use in adolescents with severe obesity. OBJECTIVES: In a cohort of adolescents with severe obesity (SO), a BMI ≥ 120% of the 95th percentile, this study aimed to 1) derive a BIA predictive equation data from air displacement plethysmography (ADP) measurements; 2) reassess the equation in a second validation cohort; and 3) compare the accuracy of existing body composition equations. METHODS: Adolescents with SO were assessed using ADP and BIA. FM values derived from ADP measurements from the first cohort (n = 27) were used to develop a BIA predictive equation (i.e., Hamilton). A second cohort (n = 65) was used to cross-validate the new and 9 existing BIA predictive equations. RESULTS: Ninety-two adolescents (15.8 ± 1.9 y; BMI: 46.1 ± 9.9 kg/m2) participated. Compared with measured FFM using ADP: 1) the Lazzer, Hamilton, Gray, and Kyle equations were without significant bias; 2) the Hamilton and Gray equations had the smallest absolute and relative differences; 3) the Kyle and Gray equations showed the strongest correlation; 4) the Hamilton equation most accurately predicted FFM within ± 5% of measured FFM; and 5) 8 out of 9 equations had similar root mean squared prediction error values (6.03-6.64 kg). CONCLUSION: The Hamilton BIA equation developed in this study best predicted body composition values for groups of adolescents with severe obesity in a validation cohort.
