ISOTOPE DETERMINATION OF BODY COMPOSITION
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43
ane
the ratio of whole body content to plasma content is determined when
the isotope content of the extravascular fluid is at a maximum:*" (4)
resolution of the plasma radioactivity curves into a series of exponential
curves. that are matched to mathematical models. .from which the
ratio of intravascular to extravascular protein can be determined."**""*°
In principle, the last of these methods is the most reliable but it also
is the most difficult, because it requires extensive computation and
experimental data that is sufficiently reliable to permit application of a
complex model of protein metabolism.
The total hemoglobin can be measured by the dilution of carbon
monoxide. STFA. Tao This includes both the hemoglobin in red cells and
hemoglobin in marrow. The data available indicate that this measurement
does not appear to include much myoglobin which can bind carbon
monoxide but. under the experimental conditions used. does not appear
to do so."*
Uric 4cid: Total exchangeable uric acid has been measured with both
N' and C'! labelled uric acid. This measurement is probably satisfactory
in the normal subject.
The labelled uric acid does not mix uniformly
with solid deposits in gouty subjects."
Urea and Creatinine: Urea and creatinine are uniformly distributed
within the total body water and therefore the total body urea and
creatinine can be measured with labelled molecules. Labelled urea and
creatinine have been used to measure total body water.7""S
Glucose: The glucose pool has been determined in normal and diabetic
subjects with C'‘-labelled glucose. The volumeof distribution™*? appears
to be in the approximate range reported for extracellular fluid. The total
quantity is dependent principally upon the concentration, since in the
diabetic the volume of distribution is only moderately increased.
Calculation of Body Fat: While a number of the body constituents
can be measured, the problem of determining the total amount of chemieal fat present still remains. This has been approached in two ways. One
is by direct measurement of the uptake of a gas teyclopropane or
krypton within total body lipids.“"’” The second is by calculation from
other body components.
The measurement of total body fat by the gas absorption technique
has been carried out in a limited number of individuals using cither
cyclopropane or krypton.’> The subjects were exposed to a measured
quantily of gas in a closed system and the rate of change of concentration
Is.fi.na
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