Total body protein: a new cellular level mass and distribution prediction model

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Abstract
Background: Protein is an important body component, and the presently accepted criterion method for estimating total body protein (TBPro) mass—in vivo neutron activation (IVNA) analysis—is unavailable to most investigators and is associated with moderate radiation exposure. Objective: The objective was to derive a theoretical cellular level TBPro mass and distribution model formulated on measured total body potassium, total body water, and bone mineral and to evaluate the new model with the IVNA method as the criterion. Design: The new model was developed on the basis of a combination of theoretical equations and empirically derived coefficients. TBPro mass estimates with the new model were evaluated in healthy women (n = 183) and men (n = 24) and in men with AIDS (n = 84). Total body nitrogen was measured by IVNA, total body potassium by whole-body 40K counting, total body water by tritium dilution, and bone mineral by dual-energy X-ray absorptiometry. Results: The group mean (± SD) TBPro mass estimates in healthy women and men and men with AIDS (8.2 ± 0.9, 11.0 ± 1.8, and 10.5 ± 1.1 kg, respectively) with the new model were similar to IVNA criterion estimates (8.9 ± 0.9, 11.1 ± 1.6, and 10.9 ± 1.2 kg, respectively). TBPro mass estimates with the new model correlated highly with the IVNA estimates in all subjects combined (r = 0.92, P < 0.001). The new model suggests that the composite TBPro mass within each group consists mainly of cellular protein (75-79%) and, to a lesser extent, protein in extracellular solids (19-23%) and extracellular fluid (≈2%). Conclusion: The new model provides a non-IVNA approach for estimating protein mass and distribution in vivo.