Assessment of adult body composition using bioelectrical impedance: comparison of researcher calculated to machine outputted values.

OBJECTIVES To explore the usefulness of Bioelectrical Impedance Analysis (BIA) for general use by identifying best-evidenced formulae to calculate lean and fat mass, comparing these to historical gold standard data and comparing these results with machine-generated output. In addition, we explored how to best to adjust lean and fat estimates for height and how these overlapped with body mass index (BMI). DESIGN Cross-sectional observational study within population representative cohort study. SETTING Urban community, North East England PARTICIPANTS Sample of 506 mothers of children aged 7-8 years, mean age 36.3 years. METHODS Participants were measured at a home visit using a portable height measure and leg-to-leg BIA machine (Tanita TBF-300MA). MEASURES Height, weight, bioelectrical impedance (BIA). OUTCOME MEASURES Lean and fat mass calculated using best-evidenced published formulae as well as machine-calculated lean and fat mass data. RESULTS Estimates of lean mass were similar to historical results using gold standard methods. When compared with the machine-generated values, there were wide limits of agreement for fat mass and a large relative bias for lean that varied with size. Lean and fat residuals adjusted for height differed little from indices of lean (or fat)/height(2). Of 112 women with BMI >30 kg/m(2), 100 (91%) also had high fat, but of the 16 with low BMI (<19 kg/m(2)) only 5 (31%) also had low fat. CONCLUSIONS Lean and fat mass calculated from BIA using published formulae produces plausible values and demonstrate good concordance between high BMI and high fat, but these differ substantially from the machine-generated values. Bioelectrical impedance can supply a robust and useful field measure of body composition, so long as the machine-generated output is not used.