Bioelectrical impedance analysis method for body composition assessment
Pyšná Dominika, Balcarová Ivana, Hajer Miroslav, Pyšná Jana
V mnoha zemích světa se globálně rozšířila nadváha a obezita. Bioelektrická impedanční analýza /BIA/ je jednou z hojně užívaných metod pro zjišťování tělesného složení. Působí tak jako prevence obezity. BIA je dostupnou, přesnou, non-invazivní metodou. Základním principem je průchod elektrického impulzu skrz pravou polovinu těla nebo jeho části. Impedanci, tělesný odpor, určujeme přístrojem Bodystat různého typu.
In many countries overweight and obesity disorders have been expanding. Bioelectrical impedance analysis method /BIA/ is one of the methods used for assessing the body composition as a means of obesity prevention. BIA is an accessible, rapid and non-invasive method. The basic principle is passing of low-level electrical current throught the whole body or its certain parts. The impedance is measured with BIA analyzer.
Key words: bioelectrical impedance analysis- electrical current- impedance
Human adipose tissue quantity is growing as a result of the comfortable modern life style. Overweight and obesity are very widespread. There are many methods for evaluating the body composition in clinical settings. One of them is a relatively inexpensive bioelectrical impedance analysis /BIA/ method. BIA is the most commonly used body composition technique in published studies. This technique is relatively simple, save, accurate, quick and noninvasive (Amani, 2006; Heyward, Wagner, 2004). Low-level electrical current is passed throught the individual`s body and the resistance, impedance /Z/ is determined by a BIA analyzer. Each human tissue contains different water quantity, and therefore has distinct resistance. For example, adipose tissue is a poor conductor of electrical current. Persons with large fat-free mass /FFM/ and total body water /TBW/ have less resistance to electrical current, they are better conductors than those with smaller FFM (Lukaski, 1987).
Predicted accuracy of a selected method is similar to the skinfold method. BIA can be prefered, because it does not require a high level of technical skills, it is more comfortable and can be used to asses the body composition of obese individuals (Gray et al., 1989; Segal, 1996).
The traditional BIA method is very frequently used. It is dependent on the whole-body /wrist-to-ankle/ measurement of impedance at a single /50-kHz/ frequency. This article deals with the basic assumptions and principles of the traditional BIA method, as well as recommends some American reviews.
Basic Principles of BIA Method
The TBW or FFM quantity are estimated indirectly from the bioelectrical impedance measures. Electrical conductivity presumes geometric shape of the body and dependance of impedance to the lenght and degree of the conductor (Kushner, 1992).
We presume that a body is an excellent cylinder at a single frequency 50 kHz. The impedance, or opposition to the flow of current, depends on the lenght and cross-sectional area. Body tissues have a specific resistivity and are supposed to be constant. But the geometric shape of the body is not perfect (Van Loan, 1990). Also, the specific resistivity is not constant, because of the variations in human tissues` structure, momentary hydration and electrolyte concentrations (Kushner, 1992). The resistivity of trunk is greater than that of arms and legs. That is why the specific resistivity of limbs is greater in the obese population when compared to that of normal weight (Chumlea et al, 1988; Fuller, Elia, 1989).
Some biological tissues are very good conductors, some are rather insulators. The flow of current is running over the path of best conductivity. The FFM is a good conductor, it contains 75% of water quantity. Anhydrous fat mass is a poor electrical conductor. Very important is the frequency level. At low frequencies /under 1 kHz/, the current goes through the extracellular water /ECW/ only, at higher frequencies /500- 800 kHz/, it passess through the intracellular water /ICW/ and ECW too (Lukaski, 1987). The single-frequency impedance measurements are carried out at 50 kHz low-level excitation current in order to assess the whole body impedance (Schoeller, 2000).
The traditional BIA model measures the whole-body resistance. The electrodes are usually located on the right wrist and the ankle opposite to the heart. There are many different types of BIA analyzers. The variations use gadget models to measure the total body structure. Some models can actually be used at home and in individual monitoring. For example the upper- and lower-body BIA analyzers according to the measurement location (Wotton et al., 2000), single- and multiple frequency BIA according to the frequency level and other (Buchholz et al., 2004).
Segmental BIA divides the body into segments with the same impedance. For example, the forearm constitutes more than 1% of the whole body weight, but 25% of the total impedance (Organ et al., 1994). Segmental BIA involves resistance measuring of each part of the body in particular and can be used in people with abnormal constitution or altered hydration /individuals after hemodialysis/ (Organ et al., 1994; Wotton et al., 2000).
The parallel model may be used in people with undernourishment or hydration imbalance. One of the best BIA parallel models is the Cole model, which can predict changes in ECW, ICW and the total body water volume very well (Gudivaka et al., 1999).
Using the BIA Method
For the total body tetrapolar BIA measuring we use four electrodes at the frequency of 50 kHz. The client must not eat or drink within the four hours of the test, must not exercise for twelve hours, drink alcohol for fourty eight hours and consume any diuretic medication for seven days before the monitoring. Women should not undergo any testing during the menstruation, because of the aqua decrement (Cornish et al., 1999; Kyle et al., 2004). We applied electrodes to the hand, wrist, foot and ankle after ten minutes in lying position. We placed the electrodes after cleaning the skin on the right side of the body. There are many factors that may create an assessment error (Gonzalez et al., 2002). In healthy population with normal fluid balance and hydration the BIA analyzer estimates the TBW, FFM /bones, muscles, water/ first, all of the values are represented in percents or units of mass (Cornish et al., 1999).
Instances of incorrect measurement
Many reviews have predicated about the very good reliability of the BIA method in clinical as well as nonclinical settings.(Gupta et al., 2009; Kushner, 1992; Kyle et al., 2004; Van Loan, 1990). The difference in reliability is not demonstrated by BIA analyzer variations. In some cases, a theoretical error can be assumed in persons, who gave wrong weight information in the data entry. There are also small divergences in the same-day assessments /1-2%/ and in the multiple-day measures /2- 3,5%/ (Kushner, 1992; Van Loan, 1990). The accurancy depends on the client factors /dehydration, eating etc./, technical skills /electrode displacement, deviations in body position etc./ enviromental conditions /skin and room temperature etc./ (Liang et al., 2000) and the prediction equation used to predict the FFM. Factors affecting BIA prediction should be chosen with respect to the individual`s age, gender, weight and physical activity level. Someone can give distinct data, therefore we predict error in BIA monitoring (Kushner, 1992; Van Loan, 1990).
BIA and Obesity
BIA measurement can be used as
prevention of overweight and obesity. This method is an acceptable, but not
valid technique in comparison to other methods assessing the body composition of
the obese population. A reivew reported, that BIA-estimated adipose tissue was
It depends on the type of obesity. BIA is valid for the population with BMI ≤ 34 kg/m². In extremely obese persons it is not possible to use this analyzer, because of the variable, non-constant, body data (Kyle et al., 2001; Kyle et al., 2004). The disparity between mass and conductivity of the body evoke lower measurement accurancy. The trunk contains about 50% of the total body mass, but only about 10% of the whole-body impedance (Bracco et. al., 1996). We can use the segmental BIA, which uses different frequencies, according to the body weight, for different body externities (Tagliabue, 2001).
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