ty
ISOTOPE DETERMINATION OF BODY COMPOSITION .
41
Rubidium™has been suggested as a substitute but dees not appear to be
satisfactory.���**? Total body potassium (K**) can also be measured in
total body counting facilities. In general, the measurement of total
exchangeable potassium with K* and total potassium with K'? are in
good agreement.'**Calibration of total body counters and geometric
factors relating to the distribution of the K’? in the body and the
position of the crystal or crystals are the principal factors determining
the precision of the measurement of total body K*°.®*
Exchangeable total potassium can be used to calculate lean body
mass (LBM) from
,
LBM = ue
where k has a value of 60-70 mEq, kg.”
The division by Moore et al."° of the tissues into K rich and K poor
groups, and also physiologically into actively metabolizing, supporting,
and transporting categories. provides an additional usefulness for total
body potassium measurements, They have been used as a reference for
other physiologic measurements, such as basal caloric expenditure and
total red cell volume.’
Exchangeable Chloride: Exchangeable chloride has been measured
with CP (t
= 3 X 10* years).*°°* These values are generally taken
as representing the volume of extracellular water, Because of the long
physical half-life of Cl°* and the short physical half-life of the other
isotopes of chlorine, radioactive bromine (Br°*) has been substituted
and appears to be equivalent.°*"4
Exchangeable Sodium: Two isotopes of sodium are available (Na®,
tx, = 2.6 years and Na**.t,, = 15 hours). Sodium is generally administered intravenously (as the chloride) and 18 to 24 hours allowed for
mixing. However. at the end of this time. mixing is not complete
within the total body sodium and the quantity measured is called
exchangeable sodium.*?"** There is a slowly exchanging component but
the short half-life of the isotope used generally precludes its measurement.
In addition. there is a nonexchangeable sodium that amounts to approximately 30 per cent of total hody sodium.” Correlations have been made
of exchangeable sodium with various abnormal physiologic states and
with other body composition parameters.°!°
Exchangeable Calcitum: Fxchangeable calcium has been measured
with Ca? tty, — 4.7 dayst but this represents only a small fraction of
the total body calcium.”* After intravenous adiministration. there are
a eis
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