Bioelectrical impedance analysis is based on the conductive and non-conductive properties of various biological tissues. Most of the body's fat-free mass is composed of conductive tissues such as muscle, while fat is part of the non-conductive tissue mass. The volume of these tissues can be estimated from the measurement of the resistance to an applied electric current flowing through the body.

Some basic theory:
The body can be considered as a series of cylinders.

	For a cylinder, resistance (R) is proportional to length (L).
	Resistance in inversely proportional to cross-sectional area (A).
	Volume is equal to length times area (L x A).

Therefore, knowing resistance and length, one can calculate volume.

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These relationships are much more complex in biological systems, and various correction factors are included in the final calculations. Changes in volume, length and area (geometry), electrolytes, and temperature all have an effect on bioelectric resistance.

Some conductors can briefly store some of the flowing electrical charge (a property called capacitance, the reciprocal of reactance), causing a phase shift between voltage and current. This is quantified by the measurement of a phase angle (ø). Resistance and phase angle are measured by BIA analyzers. Reactance can then be determined. Resistance and reactance combine to give impedance.

Resistance and reactance are frequency dependent. Varying frequency allows one to determine extra- and intra-cellular water. Low frequencies do not penetrate cells and measured impedance is of the extra-cellular compartment. Impedance at high frequencies represents both compartments.

Water is a constant fraction of fat-free mass, usually about 73 percent. The water measurement can therefore be used to estimate levels of fat-free mass. Water ratios, however, can change with the onset of certain illnesses (mostly in the extra-cellular compartment). Accurate body water measurements can be important in studying disease.

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