Bioelectrical Impedance Analysis

You have likely been to some health club or gym that offers body fat testing, and while sometimes it is done via skin fold testing, it is also often done via bioelectrical impedance analysis (BIA). Most likely, you would be given a hand held device to hold onto firmly after typing in specific information about yourself and then it spits out a number that is your body fat percentage. However, what do you really know about this system? Should you feel encouraged or disheartened based on the results received? In this article we will examine how bioelectrical impedance analysis works, look at a variety of versions, understand the limitations, and fully appreciate their accuracy – or lack thereof.

What is Bioelectrical Impedance Analysis?

Bioelectrical impedance analysis (BIA) is a popular method of assessing body composition (body fat, lean tissue such as muscle, bone, etc.) by measuring total body water [1].


How does Bioelectrical Impedance Analysis work?

Bioelectrical impedance analysis (BIA) works by sending an electrical current from one point to another point on the body and measuring the resistance to said electrical current as it passes through tissue [1]. Resistance is created by the amount of tissue and the type of tissue; this is measured by the amount of water is held intra and extra cellularly [1][2]. So, how does knowing how much water the body holds help in finding body composition?

Well, water conducts electricity well, so the more water held in the tissues, the faster the electrical signal will travel through said tissue. Now, we can distinguish between two types of tissue by understanding the amount of water held in each type of tissue and since we are trying to find body composition, we distinguish these tissues as fat mass and fat free mass. However, to be clear, fat free mass (also known as “lean mass”) is made up of not only muscle, but tendons, bones, and anything non-fat. Now, because fat mass has a much lower concentration of water in its tissue (about 10% water), it is a poor conductor and slows the signal from point A on the body to point B on the body. On the flip side, fat free mass tends to have moderate to high concentrations of water (20-75% water, muscle containing 75%) so the electrical signal travels quickly from point A to point B [3][4]. This means, theoretically at least, that if the signal moves quickly from point A to point B, then we have low body fat and/or higher fat free mass; and inversely, if the signal moves slowly, we have high body fat and/or low fat free mass. In this way, water concentrations help us distinguish body composition.


Variations of Bioelectrical Impedance Analysis​

There are, of course, variations of the same system. The system being an electrical signal sent from one point to another point and the resistance measured by the time it takes for the signal to reach point B from point A. Now, we will break these variations down one by one.

Hand to Hand

This is, by far, the most commercial type of bioelectrical impedance analysis and is likely the one we would encounter in a gym or health club. These tend to be cheaper, but can still be expensive running from about $20 - $5000, although your general, gym edition tends to be in the lower range in terms of cost [5]. They were originally designed for patients who were unable to stand due to malnutrition and were bed ridden so all they could do was hold their arms straight out in front of them [6].

Foot to Foot

Again, a popular variation that is typically found in homes as it is easily built into a weight scale. This version, with both feet uncovered, allows the signal to move from one foot to the other to get a reading [6]. This one can be relatively cheap running about the same as the lower end Hand to Hand models.

Foot to Hand

This final version is the most popular one used in clinical and research settings and is the least seen in gym and health club settings. This one is, likely, the most accurate of the three mentioned thus far due to it incorporating the trunk of the body unlike the other two measures – this will be discussed in more detail further along in this article.

Frequency
Considerations of Bioelectrical Impedance Analysis Accuracy​

Now, there are many considerations to take into account when addressing the topic of BIA as electrical signaling and its accuracy are dependent on several factors.

The frequency of the BIA unit is important as it dictates how well the body is penetrated by the electrical signal. For an accurate BIA measurement, the electrical signal must go through as much tissue as possible, so the frequency must be sufficient to not only map out extracellular space, but also intracellular space; certain frequencies are not sufficient to do the latter. Any frequency 0-50 kHz tends to be sufficient for extracellular electrical movement, but is insufficient to penetrate the cell membrane to take intracellular fluid into consideration [7]. So, there are shortcomings to single frequency units, especially lower frequency units (50 kHz is popular). However, there are units that are multi-frequency and attenuate this issue by having variable frequencies ranging from 0 to 500+ kHz [7].

Full & Segmented Body

The modality used, although the theory is the same, can have a significant impact on accuracy. Full body units, like the ones discussed above (Hand to Hand, Foot to Foot, and Foot to Hand) vary in accuracy, but stay within 10% of body weight [1]. This modality assumes that the whole body has similar conduction criteria and is not variable based on where the electrical current is located; however, this is a false assumption as the location of the electrical current does have an impact since certain parts of the body have proportionally more bone mass than muscle mass and vice versa, for example [6].

So, to fix this issue, the segmented modality breaks the body up into “pieces” and sets electrodes on ends of each section to get an individual reading of each section and then combines them to give a more accurate picture of total body water [6]. This analysis modality is has a slightly lower error margin.

Physiological Differences

This is a rather detailed and long winded subject when it concerns BIA, but it is important to address some of the other confounding variables related to BIA measurements. While it would be easy to say that body composition can be measured by simply measuring the speed of total conduction by assuming greater conductive speed with more water dense tissue, things are not that simple when it comes down to the physiology. Even small changes in our physiological state can change BIA results.

BIA is rather sensitive to changes in extracellular and intracellular fluid and as these both change with a series of different circumstances, it makes it difficult to standardize results, decreasing reliability of BIA measures. Positions like eating, drinking fluids, exercising, stress, among other factors all impact total body water, and therefor do have a significant impact on BIA results [1]. Not only that, certain BIA equations do not account for major differences between sexes or assume full body conductivity is the same, which is not the case as the arm only accounts, for example, for 4% of the body, but accounts for 45% of resistance while the trunk of the body accounts for 45% of the body and yet only contributes 10% resistance to the signal [1]. Even something as simple as lying down can decrease bioimpedance (resistance) [1].

The bottom line is, there is no standard by which BIA is measured and with a variety of modalities, it is difficult to establish reliability, even if accuracy could be pinpointed.


Safety

In most tests (50 kHz), the electrical signal is much less than any neurophysiological electrical signal and will not interfere with normal heart electrical conduction or neuromuscular conduction, but it has not been evaluated with abnormal situations like a pacemaker [1]. However, keep in mind that thousands of people have used BIAs with no ill effects [1].

Which Bioelectrical Impedance Analysis to use?

Obviously there are many considerations to be made so there is likely no one BIA method that is best in all situations be that due to price, effectiveness, reliability, or accuracy. However, a 50 kHz, segmented system under “normal” (as normal as you can get) conditions (pre-exercise, without food or drink, pre-bathroom break) will work fine as intracellular and extra cellular fluid levels will not change drastically and measuring intracellular levels will be less important if the measurement is standardized as best as possible; this method is a valid method [7]. On the other hand, if a person finds themselves in abnormal conditions where intracellular and extracellular fluid may be disproportionate to day to day (possibly from taking a diuretic, being dehydrated, exercising, etc.), then a Cole-Cole (0/∞ kHz) BIA unit should be used as it will attenuate the disproportional fluid between intra and extra as it is powerful enough to penetrate the cell membrane and the calculations attributed to it take into consideration other physiological factors related to conductivity [7].

SUMMARY

Bioelectrical impedance analysis (BIA) is not the most standardized system and certainly has a series of potential issues getting in the way of accuracy and certainly reliability, but a 50 kHz, segmented unit could offer decently accurate information (+/- 6% accuracy from X-Ray) if used pre-exercise, pre-nutritional intake, pre-fluid intake, pre-bowel movement. Also, while not all safety standards have been tested, there is no indication, especially with a typical 50 kHz unit, that there is enough electrical conduction for any harm for healthy populations.

Writer: Nicolas Verhoeven
This is educational material only and not meant to be prescripton, consult your physician before making any changes.

                                                                                                          Citations

[1] Bioelectrical impedance analysis in body composition measurement. (1996). Nutrition, 12(11-12), 749-759. Retrieved from http://ajcn.nutrition.org/content/64/3/524S.full.pdf+html

 

[2] Dehghan, M., & Merchant, A. T. (2008). Is bioelectrical impedance accurate for use in large epidemiological studies? Nutrition Journal, 7(1), 26. Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2543039/
 

[3] Body Fluids : Anatomy & Physiology. (n.d.). Retrieved from http://anatomyandphysiologyi.com/body-fluids/
 

[4] Niedziocha, L. (n.d.). Does Muscle Tissue Have Less Water Than Fat Tissue? | LIVESTRONG.COM. Retrieved from http://www.livestrong.com/article/550931-does-muscle-tissue-have-less-water-than-fat-tissue/
 

[5] Peterson, J. T., Repovich, W. E., Eash, M., Notrica, D., & Hill, C. R. (2007). Accuracy of Consumer Grade Bioelectrical Impedance Analysis Devices Compared to Air Displacement Plethysmography. Medicine & Science in Sports & Exercise,39(Supplement), S373. Retrieved from http://digitalcommons.wku.edu/cgi/viewcontent.cgi?article=1303&context=ijes
 

[6] Khalil, S., Mohktar, M., & Ibrahim, F. (2014). The Theory and Fundamentals of Bioimpedance Analysis in Clinical Status Monitoring and Diagnosis of Diseases. Sensors, 14(6), 10895-10928. Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4118362/
 

[7] Gudivaka, R. (1999). Single- and multifrequency models for bioelectrical impedance analysis of body water compartments. Journal of Applied Physiology, 87(3), 1087-1096. Retrieved from http://jap.physiology.org/content/87/3/1087.long#F3

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