## 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

Water

Protein

Glycogen

Minerals

Skeletal muscle

Organs & remaining soft tissues

Minerals

Figure 1 Three different models for characterizing body composition compartments. Components are as labeled: FFM, fat-free body mass.

Model Equations for % fat

Reference

Table 1 Multicompartment body composition models

Model Equations for % fat

Reference

 2C 100 x (4.971/Db - 4.519) a 3C 100 x (2.118/D - 0.78 x (TBW/W) b -1.354) 4C 100 x (2.747/Db - 0.727 x (TBW/W) c +1.146 x (BMC/W) - 2.0503) 6C 100 x (2.513/D - 0.739 x (TBW/W) d + 0.947 x (TBBM/W) - 1.79)

aBehnke AR Jr, Feen BG, and Welham WC (1942) The specific gravity of healthy men. Journal of the American Medical Association 118: 495-498.

bSiri WE (1961) Body composition from fluid spaces and density: analysis of methods. In: Brozek J and Hensch el A (eds.) Techniques for Measuring Body Composition, pp. 223-224. Washington, DC: National Academy of Science. cBoileau RA, Lohman TG, and Slaughter MH (1985) Exercise and body composition of children and youth. Scandinavian Journal of Sports Sciences 7: 17-27.

dHeymsfield SB, Wang ZM, and Withers RT (1996) Multicomponent molecular level models of body composition analysis. In: Roche AF, Heymsfield SB, and Lohman TG (eds.) Human Body Composition, pp. 129-147. Champaign: Human Kinetics. Db, body density; TBW, total body water; W, body weight; BMC, bone mineral content; TBBM, total body bone mineral.

are made that both the hydration of FFM and the solids portion of FFM are constant. Since bone mineral content is known to decrease with age, the 3C approach is limited in its accuracy in persons or populations where these assumptions are incorrect.

A four-compartment (4C) model involves the measurement of body density (for fat), total body water, bone mineral content by dual-energy X-ray absorptiometry (DXA), and residual (residual = body weight — (fat + water + bone)). This model allows for the assessment of several assumptions that are central to the 2C model. The 4C approach is frequently used as the criterion method against which new body composition methods are compared in both children and adults.

The more complex 4C model involves neutron activation methods for the measurement of total body nitrogen and total body calcium, where total body fat = body weight — total body protein (from total body nitrogen) + total body water (dilution volume) + total body ash (from total body calcium). A six-compartment model is calculated as follows: fat mass (measured from total body carbon) = body weight — (total body protein + total body water + bone mineral + soft tissue mineral (from a combination of total body potassium, total body nitrogen, total body chloride, total body calcium) + glycogen (total body nitrogen) + unmeasured residuals). However, the availability of neutron activation facilities is limited and therefore the latter models are not readily obtainable by most researchers.

At the organizational level, a five-level model was developed where the body can be characterized at five levels. The following are the levels and their constituents: atomic = oxygen, carbon, hydrogen, and other (level 1); molecular = water, lipid, protein, and other (level 2); cellular = cell mass, extracellular fluid, and extracellular solids (level 3); tissue-system level = skeletal muscle, adipose tissue, bone, blood, and other (level 4); and whole body (level 5).

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