te.
ISOTOPE
DETERMINATION OF BODY COMPOSITION
.
37
Generally, after mixing is completed. the system may be sampled repeatedly and the specific activity extrapolated to the time of administration. If the isotope is excreted by a single route. or by known routes.
and can be collected quantitatively, then the amount excreted can be
subtracted from that administered and the volume of dilution calculated
from:
Q-q
Q = amount of isotope administered (in dpm}
Y= —-—__q <= amount of isotope excretedto timet.
dpm ‘cc
when dpm. cc of compartment under study
is Measured.
As knowledge regarding the distribution and transport of specific
chemical compounds has increased. it has become necessary to develop
more complex metabolic models. This subject has been reviewed in The
New York Academyof Sciences. Volume 108 {May 10. 19631. These
models generally treat the compound being studied as being transported
within a series of compartments, These models have had extensive apphication in iron and albumin metabolism. 11.44.52-68-51 Models of calcium
metaholism have also been developed.” The application of these models
has led to more complex methods for determination of rate of transfer
between various anatomical and biochemical compartments and have
also yielded information regarding the size of the compartments under
study.
Of the major body constituents. only total body water can be measured
directly with a suitable isotopic diluent either in the form of deuterated
or tritiated water. In terms of the mineral components of the body the
following can be measured: exchangeable sodium. exchangeable potassium. exchangeable chloride « bromide), iron. and exchangeable calcium.
Tn terms of the fluid compartments. the total body water can be measured.
Methods of limited value are available for measurement of extracellular
water and by difference between total body water and extracellular
water. the intracellular water. The plasma volume and the total red cell
volume can be measured. A number of the plasma proteins have been
labelled and the total circulating quantities measured. Fxchangeable
uric acid and total urea have been measured. The vast hulk of the
principle mineral constituent. calcium. cannot be measured directly. A
dilution method utilizing cyclopropane has been developed for measurement of total hody fat but has not been widely applied.”