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PROFESSIONAL BIO-IMPEDANCE

PRINCIPLE OF BIO-IMPEDANCE (BIA) ANALYSIS

 

Bio-impedance (BIA) has been used to determine body composition since the 1970s. It consists in passing an alternating electrical current of low intensity through the body and measuring the opposition of tissues to the passage of this current. The value of the impedances, phase angles, resistances and reactances collected by the measuring device allow, thanks to algorithms, to determine the body compartments through which the current flows and to provide a more or less precise and detailed result of the body composition, depending on the type of device.

The current used is of very low intensity. It is totally painless.

1 – The frequencies generally used


The resistance of tissues to the passage of currentvaries according to the type of tissue and the frequency used.

The best-known devices are either Mono-frequency in general at 50 kHz, or Bi-frequency in general at 20 and 100 kHz, or Multifrequency in general on 3 to 6 frequencies in a range from 5 to 500 kHz.

- The frequency at 50 kHz is one of the most important frequencies, because it is at 50 kHz that the Phase Angle value is obtained. On the one hand, it is at this frequency that it is most accurate, and, on the other hand, all published studies used as a reference have been carried out using devices with a frequency at 50 kHz.

- Frequencies between 50 and 500 kHz can measure total water. Above 200 kHz some accuracy in repeatability is lost, which is why, in general, multi-frequency devices use frequencies between 5 and 250 kHz.

- The frequencies between 1 and 5 kHz make it possible to measure extra cellular water because at these frequencies the current does not pass the barrier of the cellular membrane. It is also important to know that the current passes the skin barrier with difficulty below 5 kHz, which is why most multifrequency devices have a frequency range between 5 and 250 kHz.

International Journal of Clinical Nutrition, 2014, Vol. 2, No. 1, 1-10 Available online at http://pubs.sciepub.com/ijcn/2/1/1 © Science and Education Publishing DOI:10.12691/ijcn-2-1-1 : Poor reproducibility has been observed at frequencies below 5 kHz and above 200 kHz, especially for reactance at low frequencies ( Hannan WJ, Cowen SJ, Fearson KCH, Plester CE, Falconer JS, Richardson RA. Evaluation of multi-frequency bio-impedance analysis for the assessment of extracellular and total body water in surgical patients. Clin Sci 1994; 86: 479-85. )

- The frequencies at 20 and 100 kHz are intermediate frequencies that are of less interest, because the frequency at 20 kHz is too high to claim to measure extra cellular water and the frequency at 100 kHz too high to obtain a reference phase angle. On the other hand, they are useful in multi-frequency devices to perform a Cole-Cole curve* and analyze, more precisely, the content of the tissue traversed, provided they are associated with a low frequency (5 kHz and a high frequency (200 kHz).


2 – Body compartments in Bio-impedance


A - Results that can be obtained from a Mono frequency 50 kHz / Bi-Frequency 20 and 100 kHz / Multifrequency type device



Find out more

Model with 2 compartments:


Fat free mass (FFM) corresponds to all water and all non-greasy tissues.
Fat mass (FM) is dry fat.
This model is used to calculate the non-fat mass and the fat mass.

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Model with 3 compartments:

 

Dry non-fat tissues (Proteins and minerals (DDFM)).
Total water (TBW)
Dry fat mass (FM)
This model calculates the fat free mass, fat mass, total water, non-greasy tissues, hydration rate of FFM and non-greasy tissues.

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 B - Result obtainable from a multifrequency device comprising at least: a frequency at 5 kHz, one at 50 kHz, one at 200 kHz and a phase angle at 50 kHz  

Model with 4 compartments:


Dry non-fat tissues (Proteins and minerals (DDFM)).
Extracellular water (ECW)
Intracellular water (ICW)
Fat mass (FM)


This model is used to calculate fat free mass, fat mass, total water, non-greasy tissues, hydration rate of FFM and non-greasy tissues, intra and extra cellular water, water balance.

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A model with 5 compartments:

 

Metabolic proteins
Intracellular water (ICW)
Extracellular Water (ECW)
Extracellular solids (Bone mineral content + Extracellular proteins)
Fat mass (FM)
This model is used to calculate the fat free mass, fat mass, total water, bone mineral content, non osseus minerals, extra-cellular solids, body cell mass (BCM), hydration rate of FFM and nonfatty tissues, intra- and extracellular water, water balance, metabolic proteins...

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3 – Reference values

 They are essential for comparing the subject's results with "normality" values. It is the difference between measured values and reference values that allows the physical state of the subject to be assessed.
They depend on sex and age. 

4 – Phase angle

 It intervenes in algorithms, but it is also a fundamental raw data to judge a person's state of form and health.
See Phase Angle section. 

5 – Phase Angle/Body Cell Mass/Metabolic Proteins/Bone Mineral Content/Intra or Extracellular Water

 These 7 data are necessary to judge the state of health, form, malnutrition...

The phase angle is used, among other things, to determine the vitality level of the cells.

The active cellular mass can be assessed quantitatively (quantity of cells), but also qualitatively (proportion of water and metabolic proteins in the cell)

The water balance is also an essential data to judge the general state of the person.

In the context of sports or pathology monitoring, we will always be interested in these 6 fundamental data, which makes the 7-compartment model the most used model with a multi-frequency measurement device.