What is Bioelectrical Impedance
Analysis?
Bioelectrical impedance analysis (BIA) measures the impedance or opposition
to the flow of an electric current through the body fluids contained mainly in
the lean and fat tissue. Impedance is low in lean tissue, where intracellular
fluid and electrolytes are primarily contained, but high in fat tissue. Impedance
is thus proportional to body water volume (TBW). In practice, a small constant
current, typically 800 uA at a fixed frequency, usually 50 kHz, is passed
between electrodes spanning the body and the voltage drop between
electrodes provides a measure of impedance. Prediction equations, previously
generated by correlating impedance measures against an independent
estimate of TBW, may be used subsequently to convert a measured
impedance to a corresponding estimate of TBW. Lean body mass is then
calculated from this estimate using an assumed hydration fraction for lean
tissue. Fat mass is calculated as the difference between body weight and lean
body mass.
The impedance of a biological tissue comprises two components, the
resistance and the reactance. The conductive characteristics of body fluids
provide the resistive component, whereas the cell membranes, acting as
imperfect capacitors, contribute a frequency-dependent reactive component.
Impedance measurements made over a range of low to high (1 MHz)
frequencies therefore allow development of prediction equations relating
impedance measures at low frequencies to extracellular fluid volume and at
high frequencies to total body fluid volume. This is known as multi-frequency
bioelectrical impedance analysis (MFBIA).