Volume 86, Issue 1, July (2001), pp. 45-52 © The Author 2001

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

N. J.  Fuller^{1, 3}*, J. C. K.Wells¹ and M.Elia^2,3
1MRC Childhood Nutrition Research Centre, Institute of Child Health, 30 Guilford Street, London WC1N 1EH, UK
²Addenbrooke’s Hospital, Hills Road, Cambridge CB2 2QQ, UK
³MRC Dunn Clinical Nutrition Centre, Hills Road, Cambridge CB2 2DH, UK

 (Received 20 October 2000–Revised 6 February 2001–Accepted 28 February 2001)

The aim of the present study was to evaluate a model of body composition for assessing total body protein (TBP) mass using dual-energy X-ray absorptiometry (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 were 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 (²H-dilution space or predicted using an assumed hydration fraction of fat-free mass (HF_ffm)), bone mineral content (BMC; DXA) and fat-free soft tissue (FFST; DXA). TBP was calculated using the 4-CM (TBP = 3·050BWt-0·290TBW-2·734BMC-2·747BV) and the DXA model (TBP = FFST-0·2305BMC-TBW). DXA measurements were obtained using the Lunar DPX (Lunar Radiation Corporation, Madison, 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 slightly worse than the 4-CM (4·0–5·4 %), whereas that modelled with assumed HF_ffm was more precise (2·4–5·2 %) because it obviated imprecision associated with measuring TBW. Agreement between the 4-CM and DXA model with measured TBW 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 HF_ffm was assumed (3·7 % and ±21 % respectively). Most of the variability in agreement between these various models was due to interpretation of biological factors, rather than to measurement imprecision. Therefore, the DXA model, which is less complex and demanding than the 4-CM, is of value for assessing TBP in groups of healthy subjects, but is of less value for individuals in whom there may be substantial differences from reference 4-CM estimates.

Keywords:
Body composition: Bone mineral: Fat-free mass: Total body water