Many of the early research studies showed that BIA was quite variable, and it was not regarded by many as providing an accurate measure of body composition.
[citation needed] Nevertheless, it is the 4-compartment model (4C) (DXA and MRI are acceptable alternatives) – and not BIA – that is regarded as the reference method in body composition analysis.
[5] Moderate exercise before BIA measurements lead to an overestimation of fat-free mass and an underestimation of body fat percentage due to reduced impedance.
[6] For example, moderate intensity exercise for 90–120 minutes before BIA measurements causes nearly a 12 kg overestimation of fat-free mass, i.e. body fat is significantly underestimated.
Multiple electrodes, typically eight, may be used located on the hands and feet, allowing measurement of the impedance of the individual body segments – arms, legs and torso.
Results for some impedance instruments tested found poor limits of agreement and in some cases systematic bias in estimation of visceral fat percentage, but good accuracy in the prediction of resting energy expenditure (REE) when compared with more accurate whole-body magnetic resonance imaging (MRI) and dual-energy X-ray absorptiometry (DXA).
In bioimpedance spectroscopy devices (BIS) resistance at zero and high frequency can be estimated and, at least theoretically, should provide the optimal predictors of ECW and TBW and hence body fat-free mass respectively.
The use of multiple frequencies or BIS in specific BIA devices has been shown to have high correlation with DXA when measuring body fat percentage.
In 1962, Thomasset conducted the original studies using electrical impedance measurements as an index of total body water (TBW), using two subcutaneously inserted needles.
The use of BIA as a bedside method has increased because the equipment is portable and safe, the procedure is simple and noninvasive, and the results are reproducible and rapidly obtained.
In 1996, an eight-polar stand-on BIA device, InBody, that did not utilize empirical equations was created and was found to "offer accurate estimates of TBW and ECW in women without the need of population-specific formulas.
The tetrapolar arrangement is preferred, since measurement is not confounded by the impedance of the skin–electrode interface[23] In bioelectrical impedance analysis in humans, an estimate of the phase angle can be obtained and is based on changes in resistance and reactance as alternating current passes through tissues, which causes a phase shift.