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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