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ISOTOPE DETERMINATION OF BODY COMPOSITION
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A reasonable goal of studies of body composition would be to sort
out these various systems of compartmentalization of body components
and evolve an acceptable scheme. Perhaps the best would be either a
separation into fat, water. mineral. and protein as chemical entities, or in
a physiologic sense. a system consisting of a body cell mass. supporting
structures, and fat. From the quantitative standpoint, there does not
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appear to be much advantage to the use of a reference man to which
is appended “excess” adipose tissue or fat. For anthropometric or nutritional studies where it is desired to have some measure of the adequacy
of caloric intake. a measurement of the excess adipose tissue could be
useful.
,
Changes in Gross Body Composition: Changes in gross body composition in a given individual can be estimated in two ways, either from
metabolic balance data‘**'"* or changes in body density,'’*’ body water. or total body potassium. The net change in body nitrogen. derived from
metabolic balance data, can be used to calculate the protein gained or
lost. From this and an assumed hydration of the lean tissue, the total
lean tissue mass gained or lost can be estimated:”
A fat = AM + kAN
where k = 6.25 Gm protein, gm N
M = bodyweight
N <= nitrogen {Gm}
If the change in total body water is also measured. a moresatisfactory
measurement of change in gross body composition can be obtained
from
;
“A fat = AM + 6.234N + ATBW
where TBW = total body water (Gm)
In a study of changes in body composition with weight reduction.
it was estimated that changes of the order of approximately 500 Gm of
bodyfat can be determined with body density-body water determination."
The Future
It should be recognized that only a few laboratories have attempted
to measure body composition in terms of more than one or two components. Studies of body density, body water. and blood volume have
been reported ‘Huff and Feller.’’ Siri®*). Moore and his colleagues”
measure total red ceil volume, plasma volume. total body water, extracellular water. total exchangeable sodium, and potassium. This requires