Assessment of fat mass and fat-free mass in Portuguese youth and adults: calibration of bioelectrical impedance with dual-energy x-ray absorptiometry

Abstract

Abstract Background/Objectives: Bioelectrical impedance analysis (BIA) is more commonly employed in outpatient evaluations and epidemiological studies, as its costs are lower and it is less time-consuming. So, we aimed to compare fat mass (FM) and fat-free mass (FFM) estimated by BIA and by dual-energy x-ray absorptiometry (DXA), in Portuguese youth, and to calibrate BIA estimates. Subjects/Methods: We analysed data from two population-based cohorts: EPIPorto (n = 392; <65y) and EPITeen (n = 974; 27y). Both completed single-frequency BIA (Tanita TBF-300) and DXA (QDR 4500A Hologic). Agreement between BIA and DXA (FM/FFM) was evaluated by Bland and Altman. To calibrate the BIA estimates, the coefficients were estimated by linear regression using univariate (FM or FFM, separately) and multivariate models (FM and FFM in the same model). Results The correlation between BIA and DXA was high for FFM and for FM (FFM: r = 0.954/r = 0.965; FM: r = 0.939/r = 0.940, for EPIPorto and EPITeen, respectively). However, BIA underestimated FM (14.5% EPIPorto; 28.4% EPITeen) and overestimated FFM (8.7% EPIPorto; 16.4% EPITeen). Using the multivariate constrained model to have an absolute equal intercept for FFM and FM, the following calibration equations for BIA measurements were obtained for EPITeen: FFMcalibrated = -6.621 + 0.984 x FFMBIA; FMcalibrated = + 6.620 + 0.974 x FMBIA; and for EPIPorto: FFMcalibrated = -4.089 + 0.998 x FFMBIA; FMcalibrated = + 4.089 + 0.970 x FMBIA. Conclusion FM assessed by BIA was largely underestimated and FFM overestimated, in comparison to DXA, and therefore the calibration should be applied to BIA estimates in clinical settings and research studies.