Body Composition: Bioelectrical Impedance Analysis (BIA)
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.
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.