ISOTOPE DETERMINATION OF BODY COMPOSITION
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larger due to exchange of the hydrogen isotope with other hydrogen
containing compounds." While a large number of values of total body
water have been reported”’ as a function of age and sex. the wide range
of total body water in the normal generally precludes an unequivocal
interpretation of any single measurement. When expressed as percentage
of body weight. total body water diminishes with increasing fat content.
Consequently, in an individual with normally hydrated Jean tissue. the
body water can range from 35-65 per cent. The studies of Pace and
Rathbun’’ indicated that in the guinéa pig the fat-free carcass (lean
tissues) had a mean water content of 72.4 = 2.1 per cent. This was
extended to man and a number of workers utilized this factor to obtain
the total lean tissue mass from a measured total body water. However.
in man the range of hydration of the lean tissue is greater and the error
in calenlating the lean tissue mass is thus Jarger than in the guinea
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pig. 57°
The methods of determining extracellular water fall into two
classes." "'""- The first group consists of bromide (Br*).” chloride
iCh™1. and thiocyanate.”* These substances probably overestimate extracellular water. In the second group are inulin. mannitol. sulfate (S°°) 27S
and sucrose." These materials do not equilibrate with all the extracellular
water. The methods of determining extracellular water volume can also
be divided into ‘kinetic’ and ‘equilibration’ methods." In the kinetic
methods (sulfate. mannitol. Cl, Bri a single dose is administered. time
allowed for mixing and the concentration, usually in blood. is extrapolated to the time of administration. In the equilibration methods. the
material (inulin, sucrose) is infused continuously to produce a constant
level in the extracellular fluid from which the volume of dilution can
he calculated.
None of the methods of estimating extracellular water is free from
significant criticism.” The extracellular Auid volume varies from
approximately 13 per cent body weight to approximately 30 per cent
body weight depending upon the method used. Until agreement can be
obtained on a satisfactory method for measuring extracellular water.
the estimation of intracellular water as the difference hetween total body
water and extracellujar water remains uncertain.”!
Blood Volume: The htood volume is the sum of the velume of total
circulating cells and the plasma. The total circulating red cell volume
may satisfactorily be measured with red cells labelled with either radio-