Evaluation of a model for total body protein mass based on dual-energy X-ray absorptiometry: comparison with a reference four-component model

The aim of the present study was to evaluate a model of body composition fo r assessing total body protein (TBP) mass using dual-energy X-ray absorptio metry (DXA), with either measured or assumed total body water (TBW); it was intended to provide a less complex or demanding alternative technique to, for example, the four-component model (4-CM). The following measurements we re obtained in healthy adults (n 46) aged 18-62 years, and children (n 30) aged 8-12 years: body weight (BWt), body volume (BV; under-water weighing), TBW (H-2-dilution space or predicted using an assumed hydration fraction o f fat-free mass (HFffm)), bone mineral content (BMC; DXA) and fat-free soft tissue (FFST; DXA). TBP was calculated using the 4-CM (TBP=3.050BWt-0.290T BW-2.734BMC-2.747BV) and the DXA model (TBP=FFST-0.2305BMC-TBW). DXA measur ements were obtained using the Lunar DPX (Lunar Radiation Corporation, Madi son, WI, USA) or Hologic QDR 1000/W (Hologic, Waltham, MA, USA). Precision of the DXA model for TBP with measured TBW (4.6-6.8 % mean TBP) was slightl y worse than the 4-CM (4.0-5.4 %), whereas that modelled with assumed HFffm was more precise (2.4-5.2 %) because it obviated imprecision associated wi th measuring TBW. Agreement between the 4-CM and DXA model with measured TB W was also worse (e.g. bias, 15 % of the mean; 95 % limits of agreement up to +/-39 % for adults measured on the Lunar DPX) than when a constant for H Fffm was assumed (3.7 % and +/-21 % respectively). Most of the variability in agreement between these various models was due to interpretation of biol ogical factors, rather than to measurement imprecision. Therefore, the DXA model, which is less complex and demanding than the 4-CM, is of value for a ssessing TBP in groups of healthy subjects, but is of less value for indivi duals in whom there may be substantial differences from reference 4-CM esti mates.