Bioelectrical impedance analysis
Encyclopedia
Bioelectrical impedance analysis (BIA) is a commonly used method for estimating body composition. Since the advent of the first commercially available devices in the mid-1980s the method has become popular owing to its ease of use, portability of the equipment and its relatively low cost compared to some of the other methods of body composition analysis. It is familiar in the consumer market as a simple instrument for estimating body fat. BIA actually determines the electrical impedance
, or opposition to the flow of an electric current through body tissues which can then be used to calculate an estimate of total body water (TBW). TBW can be used to estimate fat-free body mass and, by difference with body weight, body fat.
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. In recent years technological improvements have made BIA a more reliable and therefore more acceptable way of measuring body composition. Nevertheless it is not a "gold standard" or reference method. Like all assessment tools, the result is only as good as the test done. Although the instruments are straightforward to use, careful attention to the method of use (as described by the manufacturer) should be given.
Simple devices to estimate body fat, often using BIA, are available to consumers as body fat meters. These instruments are generally regarded as being less accurate than those used clinically or in nutritional and medical practice. They tend to under-read body fat percentage.
In recent years, more accurate methods of Bioelectrical Impedance Analysis have been developed.
The use of bioelectrical impedance analysis (BIA) is widespread both in
healthy subjects and patients, but suffers from a lack of standardized method and
quality control procedures. BIA allows the determination of the fat-free mass (FFM)
and total body water (TBW) in subjects without significant fluid and electrolyte
abnormalities, when using appropriate population, age or pathology-specific BIA
equations and established procedures. Published BIA equations validated against a
reference method in a sufficiently large number of subjects are presented and
ranked according to the standard error of the estimate.
measured total resistance, modern BIA devices have phase sensitive electronics.
The principle of measurement is based upon the fact that the condensers in the alternating
current circuit lead to a time delay Δt: the current maximum is in advance of the voltage
maximum.
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
, or opposition to the flow of an electric current through body tissues which can then be used to calculate an estimate of total body water (TBW). TBW can be used to estimate fat-free body mass and, by difference with body weight, body fat.
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. In recent years technological improvements have made BIA a more reliable and therefore more acceptable way of measuring body composition. Nevertheless it is not a "gold standard" or reference method. Like all assessment tools, the result is only as good as the test done. Although the instruments are straightforward to use, careful attention to the method of use (as described by the manufacturer) should be given.
Simple devices to estimate body fat, often using BIA, are available to consumers as body fat meters. These instruments are generally regarded as being less accurate than those used clinically or in nutritional and medical practice. They tend to under-read body fat percentage.
In recent years, more accurate methods of Bioelectrical Impedance Analysis have been developed.
The use of bioelectrical impedance analysis (BIA) is widespread both in
healthy subjects and patients, but suffers from a lack of standardized method and
quality control procedures. BIA allows the determination of the fat-free mass (FFM)
and total body water (TBW) in subjects without significant fluid and electrolyte
abnormalities, when using appropriate population, age or pathology-specific BIA
equations and established procedures. Published BIA equations validated against a
reference method in a sufficiently large number of subjects are presented and
ranked according to the standard error of the estimate.
Historical background
Electrical properties of tissues have been described since 1871. These properties were further described for a wider range of frequencies on larger range of tissues, including those that were damaged or undergoing change after death. Thomasset conducted the original studies using electrical impedance measurements as an index of total body water (TBW), using two subcutaneously inserted needles. Hoffer et al. and Nyboer first introduced the four-surface electrode BIA technique. A disadvantage of surface electrodes is that a high current (800 uA) and high voltage must be utilized to decrease the instability of injected current related to cutaneous impedance (10 000 Ω/cm2). By the 1970s the foundations of BIA were established, including those that underpinned the relationships between the impedance and the body water content of the body. A variety of single frequency BIA analyzers then became commercially available, and by the 1990s, the market included several multi-frequency analyzers. 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. More recently, segmental BIA has been developed to overcome inconsistencies between resistance (R) and body mass of the trunk.Phase angle
In order to be able to differentiate between the two components resistance and reactance from themeasured total resistance, modern BIA devices have phase sensitive electronics.
The principle of measurement is based upon the fact that the condensers in the alternating
current circuit lead to a time delay Δt: the current maximum is in advance of the voltage
maximum.