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Measuring
Body Composition
The Importance
of Body Cell Mass
In order to
accurately diagnose HIV-associated wasting, clinicians need to evaluate
more than the total body weight of the patient. Total body weight
may not change for years after a diagnosis of HIV infection. At
the same time, however, body composition changes may be occurring
even while the patient is asymptomatic. As lean body mass decreases,
it is usually replaced by water and fat. After body cell mass reaches
a value of 54% of normal, death ensues.4
It is particularly important to remember that patients may show
signs of wasting even when their body weight remains unchanged.
As such, assessment of body cell mass is key to prevention, early
intervention, and the monitoring of responses to treatment for HIV-associated
wasting.
Bioelectrical
Impedance Analysis
Bioelectrical
impedance analysis (BIA) is the preferred method for assessing body
cell mass depletion, even in the presence of normal weight.25
BIA can be easily performed with portable equipment in the office
setting, involves no radiation, is inexpensive, painless, and has
a high degree of accuracy. A low-level current of 800 microamps
is introduced into the patient, and voltage changes are recorded
via electrodes attached to their hands and feet. Lean tissue contains
most of the body's conducting electrolytes, and therefore, conducts
electric current more readily than fat tissue, which has a lower
water content. The numerical results of BIA are combined with the
patient's other statistics (height, weight, sex, and age) to calculate
body cell mass, fat-free mass, and other body composition measurements.
Body cell mass
consists of metabolically active tissue, which includes muscle tissue,
organ tissue, and intracellular water.26
Body cell mass is a component of lean body mass, which also includes
extracellular water and bone tissue. Therefore, it is important
to remember that lean body mass contains more than just muscle.
BIA is the
most accessible, reliable, efficient and accurate test for measuring
body cell mass. Not all BIA machines use the same software. Differences
among machines and methodologies can result in calculations that
can vary by as much as 10%. BIA results may also be affected by
the patient's level of hydration. As such, BIA is much less reliable
for measuring fat mass. BIA is most effective as an ongoing assessment
tool to longitudinally measure changes in body composition.27
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