Models in Body Composition

The use of models in the assessment of body composition allows for the indirect assessment of compartments in the body. Typically, a compartment is homogenous in composition (e.g., fat), however, the simpler the model the greater the assumptions made and the greater the likelihood of error. The sum of components in each model is equivalent to body weight (Figure 1). These models make assessments at the whole-body level and do not provide for regional or specific organ/tissue assessments.

The basic two-compartment (2C) model (Table 1) is derived from measuring the density of fat-free mass (FFM) by hydrodensitometry and subtracting FFM from total body weight thereby deriving fat mass (body weight — FFM = fat mass). FFM is a heterogeneous compartment consisting of numerous tissues and organs. A 2C approach becomes inadequate when the tissue of interest in included within the FFM compartment. Nevertheless, the 2C model is routinely and regularly used to calculate fat mass from hydrodensi-tometry, total body water, and total body potassium.

A three-compartment (3C) model consists of fat, fat-free solids, and water. The water content of FFM is assumed to be between 70% and 76% for most species and results from cross-sectional studies in adult humans show no evidence of differences in the hydration of FFM with age. The fat-free solids component of FFM refers to minerals (including bone) and proteins. The 3C approach involves the measurement of body density (usually by hydrodensitometry) and total body water by an isotope dilution technique. Assumptions

Table 1 Multicompartment body composition models

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