Bioelectrical impedance analysis is a simple, non-invasive and portable method for estimating fluid compartments and fat-free mass. This method is based on the bioelectrical principle that lean tissues containing the majority of the body's water and electrolytes are good electrical conductors, whereas fat mass (which is almost dry) acts as an insulator and is a poor electrical conductor (Fig. 2).
The human body is considered as a conductor, which opposes an obstruction of defined impedance (Z) to an alternating current. Impedance is the vectorial sum of the resistance (R) and reactance (Xc). Resistance is defined as the pure opposition of the body to the alternating current and reactance (Xc) is the resistive effect related to capacities produced by tissue interfaces and cell membranes .
The volume (V) of a cylindrical conductor through which an electric current flows can be calculated according to Ohm's second law as a function of the length of the conductor (L), the bioelectrical resistance (R), and the specific resistivity (p, i.e. indicating the tissue intrinsic property to behave as resistors) of the lean mass:
Applying this model to the human body, a strong correlation was found between bioelectri-cal volume (height2/resistance) and the fluid compartments determined by dilution methods.
Intracellular and extracellular fluids are electrical conductors, while cell membranes act as capacitors . At low frequencies (5 kHz) the current flows almost exclusively through the extracellular compartments. At higher frequencies (> 50 kHz) it exceeds the cell membranes and flows through both the extracellular and the intra-cellular fluids. On the basis of this principle, low and high frequencies are applied respectively to estimate ECW and TBW.
Instruments for BIA generally use a single frequency (50 kHz, 800 mA) with a tetrapolar placement of electrodes on the dorsal surface of the right hand (two electrodes) and foot (two electrodes) .
The strong correlation between bioelectrical parameters and water volumes has enabled the development of reliable BIA equations for predicting TBW. Moreover, multiple regression equations for estimating fat-free mass from resistance, weight, height, gender and age have been developed by comparison with a reference method such as DEXA, hydrodensitometry, etc. Table 1 shows some common BIA formulae used for estimating TBW and FFM [25-32].
The main limits of the BIA method have to do
Table 1. Bioelectrical impedance analysis formulae used for the estimation of total body water (TBW) and fat-free mass (FFM)
Author Formulae for TBW
Kushner  M: 0.396 x (stature2/resistance) + 0.143 x weight + 8.399
F: 0.382 x (stature2/resistance) + 0.105 x weight + 8.315
Kushner  0.593 x (stature2/resistance) + 0.065 x weight + 0.04
Visser  M: 8.3 + 0.323 x (stature2/resistance) + 0.165 x weight
F: 11.9 + 0.272 x (stature2/resistance) + 0.109 x weight
Sun  M: 1.20 + 0.45 stature2/resistance + 0.18 weight
F: 3.75 + 0.45 stature2/resistance + 0.11 weight
Formulae for FFM
Segal  M: 0.0006636 x stature2 - 0.2117 x resistance + 0.62854 x weight - 0.1238 x age + 9.33285
F: 0.00064602 x stature2 - 0.01397 x resistance + 0.42087 x weight + 10.43485
Rising  13.74 + 0.34 x (stature2/resistance) + 0.33 x weight - 0.14 x age + 6.18 if M
Roubenoff  M: 9.15 + 0.43 x (stature2/resistance) + 0.2 x weight + 0.07 x reactance
F: 7.74 + 0.45 x (stature2/resistance) + 0.12 x weight + 0.05 x reactance
Kyle  -4.104 + 0.518 x (stature2/resistance) + 0.231 x weight + 0.130 x reactance + 4.229 if M
Sun  M: -10.68 + 0.65 x (stature2/resistance) + 0.26 weight + 0.02 resistance
F: -9.53 + 0.69 x (stature2/resistance) + 0.17 weight + 0.02 resistance
M, males; F, females; stature in m, weight in kg with the influence of hydration status on its reliability in estimating fat-free mass. FFM hydration is already highly variable in healthy subjects, becoming more so with age . When water imbalance occurs, BIA can underestimate and overestimate the FFM in dehydration and fluid retention states, respectively.
A recent study of ours demonstrated that BIA is reliable in evaluating body composition in underweight elderly men, but it seems to have intrinsic weaknesses in assessing underweight women .
A further application of BIA is to evaluate ECW and the distribution of intracellular and extracellular fluids, using low-frequency analysers (< 5 kHz)  or multifrequency systems .
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