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ISOTOPE DETERMINATION OF BODY COMPOSITION
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butions due to water and again assumes a two-compartment. system: one
consisting of the dry, lean tissue constituents: and the second fat. again
with very different densities.” In the normal. the best estimation of total
body fat is derived from a combined measurement of body density and
body water rather than either alone.“ The body density method alone
or the body water method alone both assume constancy for the hydration
of the lean tissue or a constant composition of the lean tissue. and thus
are not applicable in disease states affecting hydration.
The constants of the equations are listed in Table 1. Often in the
determination of these constants other empirical constants or experimental methods are used that are not entirely free of significant criticism.
Thus, the numerical values in some instances can be considered only
approximations.
There are a number of assumptions underlying these equations that
affect in varying degrees their reliability. They are as follows:
Equation 1—assumes that the lean tissue (Jean body mass) has a
constant composition, Since water is the principal component, this is
tantamount to assuming a constant hydration of the lean tissue, In
general. variations in this mineral-protein ratio are probably less significant in their effect on the value of the constants used.
Equation 2—requires the same assumptions as equation 1.
Equation 3-—assumes a mineral-protein ratio of 0.35. Parenthetically.
it should be noted that if a standard deviation of this ratio of 0.10 is
applied, the calculated fat content is not appreciably affected.*' Allen
et al.” have modified this equation by subtracting the volume and mass
associated with bone mineral. Good agreement in the calculated fat
content was obtained when the constants proposed by Allen et al. were
compared to those previously published. This was, in a sense. to be anticipated because of the small effect a large variation in mineral-protein
ratio has on the calculated fat content.
Equation 4—the principal assumption is normal hydration of cells.
Equation 3—1this method has been evaluated™* and does not appear
to be of value. In fact. in the normal the factors used cancel out one
measurement or the other.
Equation 6—assumes a constant potassium content of all lean tissues.
Equation 7—implies a constant potassium content of selected tissues
ithe body cell mass of Moure et aly" and requires a measurement of the
mass of the other tissues for which an independent direct method of