Bioelectrical
impedance analysis method for body composition assessment
Pyšná Dominika, Balcarová Ivana, Hajer
Miroslav, Pyšná Jana
Anotace
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.
Summary
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
Preface
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).
BIA
Models
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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