Sunday, January 5, 2014

How BIA works: Bioelectrical Impedance Analysis

Bioelectrical impedance analysis (BIA) is a commonly used method for estimating body composition, and in particular body fat. 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[1]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 oftotal 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.[2]
Dehydration is a recognized factor affecting BIA measurements as it causes an increase in the body's electrical resistance, so has been measured to cause a 5 kg underestimation of fat-free mass i.e. an overestimation of body fat.[3]
Body fat measurements are lower when measurements are taken shortly after consumption of a meal, causing a variation between highest and lowest readings of body fat percentage taken throughout the day of up to 9.9%.[4]
Moderate exercise before BIA measurements lead to an overestimation of fat-free mass and an underestimation of body fat percentage due to reduced impedance.[5]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.[6] Therefore it's recommended not to perform BIA for several hours after moderate or high intensity exercise.[7]
BIA is considered reasonably accurate for measuring groups, or for tracking body composition in an individual over a period of time, but is not considered sufficiently accurate for recording of single measurements of individuals.[8]
The accuracy of consumer grade devices for measuring BIA has not been found to be sufficiently accurate for single measurement use and are better suited for use to measure changes in body composition over time for individuals.[9] Two-electrode foot-to-foot measurement is less accurate than 4-electrode (feet, hands) and eight-electrode measurement. Results for some four- and eight-electrode 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-bodymagnetic resonance imaging (MRI) and dual-energy X-ray absorptiometry (DXA).[10]

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 μA) 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.

Measurement configuration

The impedance of cellular tissue can be modeled as a resistor (representing the extracellular path) in parallel with a resistor and capacitor in series (representing the intracellular path). This results in a change in impedance versus the frequency used in the measurement. The impedance measurement is generally measured from the wrist to the contralateral ankle and uses either two or four electrodes. A small current on the order of 1-10 uA is passed between two electrodes, and the voltage is measured between the same (for a two electrode configuration) or between the other two electrodes.[11]

Principles of Bioelectrical Impedance Analysis


1. Bioelectrical Impedance Analysis (BIA)

Bioelectrical Impedance analysis is used to estimate body composition using the difference of conductivity based on the biological characteristic of tissue. Conductivity is proportional to water and electrolyte and it is decreased when cell shape is closer to a round form. Adipose tissue is composed of round shape cell and contains relatively less water than other tissues like muscle, so conductivity is decreased according to the increase of body fat.
Papers“Suitable Method to Body Fat Assessment and Follow-up Examination”,Ji-hyeon Gang
2005. The 10th Workshop of KOSSO in 2005, 261~269

When subtle alternating current signal flows in human body, electricity is flowing through water which has high conductivity. Impedance of body-composing constituents like water, fat, muscle and so on appears different from
one another and the impedance has steady relationship to body composition, therefore body composition can be evaluated using impedance. This method uses the two factors that human body is composed of highly conductive tissue (Conductor: Lean body mass) and less conductive tissue (Insulator: Body fat) and
measured impedance reflects the ratio between conductive tissue and non-conductive tissue,
so it is called bioelectrical impedance analysis.
Bioelectrical Impedance Analysis

The difference in Impedance between fat cell and muscle

Impedance of a cylindrical homogenous conductor

2. The assumed premises of BIA

BIA has assumed premises as follows ;
a. Human body is a cylinder shape or a combination of 5 cylinders of which size is determined by height and weight.
b. Body-composing elements are homogenous and evenly distributed.
c. Do not consider individual differences and variation of body composition.
d. Do not consider the changes according to environment (temperature) & body heat and stress.

The assumed premises of BIA
In reality, human body is not a homogenous tube shape conductor and the degree to reduce the flow of electricity at an appointed temperature (ρ) is not steady. Moreover, human body is composed of complex shapes with uneven body composition, has different density through age and different body composition according to gender, and is always changing in accordance with external environment.

Therefore, In order to overcome the difference between real body and assumed body of BIA, revised estimation model should be developed and applied which considers other predictable factors like weight, gender, age and so on except impedance and height.
In addition, separate clinical experiments for gender and age are needed to develop the formula calculating mass of body fat considering gender and age.

Using only one formula which calculates mass of body fat without considering gender and age is easy to use and able to reduce development costs. However, making up for the weak point in the assumed premises of BIA is necessary to develop a body composition analyzer with much higher accuracy.

Papers“Estimation of Body Fluid Volumes Using Tetra polar Bioelectrical Impedance
Measurements.”, Henry C Lukaski and William W. Bolonchuk
Aviation, Space, and Environmental Medicine. December, 1988
“Bioelectrical Impedance Analysis in Body Composition Measurement .”, NIH, USA
National Institute of Health Technology Assessment Conference Statement
December 12-14, 1994. 15.
“Comparison of total body potassium with other techniques for measuring lean body
mass in men and women with AIDS wasting.”, Colleen Cocoran, Ellen J Anderson,
Belton Burrows, Takara Stanley, Mark Walsh, Allion M Poulos and Steven Grinspoon
Am J Clin Nutr vol 72, No.4, 1053-1058. Oct. 2000
“Estimation of skeletal muscle mass by bioelectrical impedance analysis.“,
Ian Janssen, Steven B. Heymsfield, Richard N. Baumgartner, and Robert Ross
J Appl Physiol 89:465-471. 2000
“Bioelectrical Impedance Analysis(BIA) May Predict AIDS Survival”, John S. James
AIDS treatment News. 06/16/95
“Suitable Method to Body Fat Assessment and Follow-up Examination”,Ji-hyeon Gang
2005. The 10th Workshop of KOSSO in 2005, 261~269

Bioelectrical Impedance Analysis

Bioelectrical Impedance Analysis or BIA is considered one of the most reliable and accessible methods of screening body fat. In conventional BIA, a person is weighed, then height, age, gender and weight or other physical characteristics such as body type, physical activity level, ethnicity, etc. are entered in a computer. While the person is lying down, electrodes are attached to various parts of the body and a small electric signal is circulated. Simply explained, BIA measures the impedance or resistance to the signal as it travels through the water that is found in muscle and fat. The more muscle a person has, the more water their body can hold. The greater the amount of water in a person's body, the easier it is for the current to pass through it. The more fat, the more resistance to the current. BIA is safe and it does not hurt. In fact, the signal used in body fat monitors can not be felt at all either by an adult or child.

Tanita's Patented BIA Method

Tanita has patented a revolutionary new way of measuring BIA that is faster, easier, less intrusive and includes a precision scale making this a simple one-step process. In fact, Tanita was the first company to introduce the world to the body fat monitor/scale. Tanita's monitor looks just like a bathroom scale. A person inputs age, gender and height, then steps onto the platform. Electrodes in the foot sensor pads send a low, safe signal through the body. Weight is calculated automatically along with body fat content in less than a minute. All Body Fat Monitor/Scales and UltimateScales feature Tanita's patented BIA method.

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