Body Composition: Bioelectrical Impedance Analysis
(BIA)
Testing Procedure | Equations for BIA
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Electrical impedance is a newer method used
for body composition assessment. A major advantage of the bioelectrical
impedance analysis (BIA) is its simplicity. This procedure involves sending a
very small current through the body—800 mA at 50 kHz, which
is unable to be felt—and measuring its resistance. The underlying theory to
this procedure is based on the subject's height and his/her resistance to a
current. Lean tissue offers less resistance to a current as it contains more
water and electrolytes than adipose tissue. Another advantage of this procedure
is that no special training or skill is required. While the accuracy of BIA is
reported to be similar to skinfold measurements, research is continuing in
order to improve the accuracy of the equations. This procedure holds promise
for making accurate measurements both easily and quickly.
There is an excellent web site produced at
the University of Vermont on BIA which is worth viewing (http://nutrition.uvm.edu/bodycomp/bia/bia-toc.html). For a more technical explanation of how
bioelectrical impedance estimates body composition, view the RJL Systems web
site (http://rjlsystems.com/research/bia-principles.html), which is manufacturers these analyzers.
- The
subject should not have exercised or taken a sauna within 8 hours of the study.
- The
subject should refrain from alcohol intake for 12 hours prior to the
study.
- The
subject's height and weight should be accurately measured and recorded.
- The
subject should lie quietly during the entire test.
- The
subject should remove the right shoe and sock and lie supine with the arms
30 degrees from the body and legs not touching.
- The
electrode sites may need to be cleaned with alcohol, particularly if the
skin is dry or covered with lotion.
- Attach
the electrodes and patient cables as shown in the illustration.
- Turn
the analyzer on and make sure the subject refrains from moving. When the
measurements have stabilized, record the displayed Resistance (R) and
Reactance (Xc) (if needed) with the subject's name, age, gender, height
and weight.
Several equations can be used to predict
fat-free mass. The following equations have a low standard error for predicting
fat-free body mass and is appropriate for the general population (Lohman T.G. Advances
in Body Compositions. Champaign, IL: Human Kinetics, 1992.)
females: Fat-Free
Mass (kg) = 0.475 × [(ht2 × (cm2) / R (ohms)]
+ 0.295 × wt (kg) + 5.49
males: Fat-Free
Mass (kg) = 0.485 × [(ht2 × (cm2) / R (ohms)]
+ 0.338 × wt (kg) + 3.52
Note that the equation estimates fat-free
mass which can be used to estimate percent fat.