Objective: To determine the validity of estimation of body fatness by dual-energy x-ray absorptiometry (DXA) and foot-foot bio-electrical impedance (BIA).
Methods: In 176, 11-12-year-olds (84 boys; 92 girls) body fatness was measured using total body water (TBW), derived from deuterium oxide dilution space. Body fatness was also estimated from DXA and BIA. Methods were compared by regression and by Bland-Altman analysis using TBW measures as the reference.
Results: In boys, mean fat mass from TBW was 9.8 kg (standard deviation, SD=6.1); bias by DXA estimated fat mass was +0.9 kg (limits of agreement -2.2 to +4.1) and bias for BIA was -5.2 kg (limits of agreement +0.5 to -10.8). In boys, regression analysis indicated significant differences in slope (p<0.001) for DXA, and both slope (p < 0.001) and intercept (p < 0.001) for BIA. In girls, mean fat mass from TBW was 12.1 kg (SD 7.7); bias for DXA was +1.2 kg (limits of agreement -1.9 to +5.1) and bias for BIA was -0.2 kg (limits of agreement -5.4 to +5.1). In girls, regression analysis indicated significant differences for slope and intercept (p<0.001 in all cases) for both DXA and BIA.
Conclusions: Errors in estimation of fat mass using BIA and DXA can be very large, and the direction of error can differ between the sexes.