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CHAPTER 2__
Isotopic Determination of Body Composition
in Man
By NATHANIEL I. BERLIN. M.D. axp WILLIAM E. SIRI
HE USE OF RADIOACTIVE ISOTOPES in physiologic and clinical studies
has made possible a number of measurements of various aspects of
hody composition, Studies of body composition have had four principal
orientations. The first has been the study of a population to determine
the changes in body composition during the life span and to assess
nutritional status. The second goal has been to relate body composition
to other physiologic measurements, particularly metabolic rate.‘ The
third goal has been to study patients with various diseases to determine |
the effect of disease upon body composition.”° A fourth goal has been
to determine the relationship between concentration of a particular
substance in the blood and the total quantity in the body. This has been
particularly investigated for electrolytes’* and for red cells and
plasma.**"" It has been amply demonstrated that for total red cell and
plasma volume, and particularly for serum electrolytes, the measurement
of the concentration in the peripheral blood need not be directly correlated to the total body content and often is misleading. This failure
to have adequate information can have important clinical consequences.
In terms of gross body composition. the body can be divided into
four chemical compartments: fat.* water. minerals. and proteins. The
total quantity of macromolecular carbohydrates and nucleic acids is
small and in this review will not be considered. Other systems of classification are discussed below. Historically, measurement of the fat content
has been of greatest interest probably because. in the normal. it is the
greatest variable and because of the pioneering work of Behnke and
his collaborators in the measurement of body density and in formulating
the concept of a lean body mass and a fat compartment.'?7°?°° This
*This should be di-ltingui-hed from adipose tissue, and in this review will always .be
taken as fat in the chemical sence.
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