• Ingrid Schifferli, Juan José Orellana-Cáceres, Gladys Morales, Jorge Inostroza, and Fernando Carrasco.
• Departamento de Salud Pública, Facultad de Medicina, Universidad de La Frontera, Temuco, Chile.
• Rev Med Chil. 2020 Oct 1; 148 (10): 1435-1443.

BackgroundEquations for the evaluation of fat-free mass (FFM) and fat mass (FM) with Bioelectrical impedance analysis (BIA) were formulated in Caucasian populations. International recommendations suggest that population-specific equations should be formulated.AimTo validate an equation previously formulated in Chileans adults and compare it to a new equation generated on an independent sample.Material And MethodsIn 108 adult volunteers aged 38.1±14.1 years (44% males), with a body mass index (BMI) of 25.1± 4.1 kg/m2, body composition was measured by BIA (Bodystat) and dual X-ray absorptiometry (DXA: Lunar Prodigy). Body composition estimated using Schifferli equation and BIA were compared with DEXA, by the Bland-Altman method and simple linear regression.ResultsFFM and FM measured by DXA were 45.2 ± 9.8 kg and 29.6 ± 11.7 % respectively. Resistance was 467.7 ± 76.3 ohm. Schifferli equation and BIA significantly overestimated FFM by 7.3 and 7.4 kg, respectively. The error was higher for high levels of FFM (slope β < 1, p < 0.01). Both equations underestimated FM measured by DXA (averages of 7.5 and 7.8%, respectively, p < 0.01), without a differential bias for Schifferli equation, but with a bias in low levels of FM measured with BIA (slope β < 1, p < 0.01). Estimation biases could be eliminated using the regression coefficients.ConclusionsBoth equations behave similarly and have biases, although less with Schifferli. Statistically correcting for biases, the new adjusted equations provide clinically valid estimates of FFM and FM. Equations should not only be population-specific, but also device-specific.

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