BEHIND THE SCENESWhat is Bioelectrical Impedance Analysis (BIA)?

InBody devices use a method called Bioelectrical Impedance Analysis (BIA) to measure body composition, which divides your weight into different components, such as lean body mass and fat mass, to assess your health and help guide interventions.

The Human Body and Impedance

Bioelectrical Impedance Analysis (BIA) measures impedance by applying alternating low-level electrical currents through the water in the body.

The Concept of Reactance

To better illustrate how this works, imagine the flow of cars in traffic. Your car is the voltage source, or current, and the highway you are on represents the water inside your body. If there were no other cars, you could zoom along the highway; similarly, if the human body were full of water and nothing else, there would be little to no resistance.

But water is not the only element in the human body, just as you are not the only car on the freeway. As more cars move onto the freeway, traffic builds up, lanes get closed, etc. the longer it takes for you to move towards your destination, creating resistance. Similarly, the components of your body other than water, such as fat, muscle, and bone, create resistance to the electrical current that is going through your body.

In BIA, the more water in your body, the less resistance. Unlike fat, the muscles in your body contain a high percentage of water. So the more muscle you have, the more body water. And the more body water you have, the less resistance to the electrical current there is.

The Concept of Reactance

Reactance, also known as capacitive resistance, is the opposition to the instantaneous flow of electric current, caused by the cell membranes in the human body. Because of the interaction between the electrical currents and cell membranes, reactance represents the cell’s ability to store energy and is considered to directly reflect cellular strength and integrity.

Putting It All Together

Impedance is the vector sum of resistance and reactance and is the measurement BIA devices use to determine your body composition. This measurement is taken in ohms (Ω). BIA applies a cylinder model for the relationship between impedance and body water.

Body water is determined using two mathematical concepts:

  • Volume of a cylinder (Volume = Length x Area)
  • Characteristic of impedance: Impedance is inversely proportional to cross-sectional area and directly proportional to the length of the cylinder (Impedance = Length ➗ Area) Using the impedance and length of the cylinder, we can determine the volume of total body water.

In the human body, the same formula applies, where the length would be the height of the person. This is why it is imperative to have an accurate height measurement when using BIA.

The History of BIA Technology

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Source: Hoffer, E. C., Meador, C. K., & Simpson, D. C. (1969). Correlation of whole-body impedance with total body water volume. Journal of Applied Physiology, 27(4), 531-534.

1969 - Hoffer et al. and the Impedance Index

In 1969, Hoffer et al. carried out a series of experiments to prove that total body water and bioelectrical impedance were highly correlated, suggesting that impedance measurements could be used for determining total body water.

Impedance of the right half of the body was measured, including the right arm, torso, and right leg. The squared value of this length measure divided by impedance had high correlations with total body water (r = 0.92). This correlation was more in agreement with the gold standard technique when compared to other indices, including body weight. The equation Hoffer et al. showed to be correlated to body water is the impedance index (Height2➗ Impedance) used in BIA today.

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Image Credit: RJL Systems

1979 - RJL Systems and the First Impedance Meter

In 1979, RJL Systems commercialized the impedance meter for the first time and the BIA method began to gain popularity. The device measured impedance by attaching electrodes to the back of the right hand and on top of the right foot. These electrodes then controlled the flow of a 50kHz current through the right half of the subject’s body.

Prior to this, body composition could only be measured by caliper or underwater weighing. Such methods needed to be carried out by skilled technicians, were uncomfortable, required complicated installation or use of equations, and could not accommodate a wide variety of populations. Alternatively, BIA was easy, fast, less expensive, and non-invasive. Therefore, many body composition researchers, nutritionists, and medical experts began to use BIA.

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Sample empirical estimation equation Source: Schoeller, D. A., & Kushner, R. F. (1989). Determination of body fluids by the impedance technique. IEEE Engineering in Medicine and Biology Magazine, 8(1), 19-21..

1980's - Discovering limitations to BIA with Empirical Data

In the 1980’s, research accelerated the evolution of BIA. Studies proved BIA measures had high correlations with gold standard methods, such as underwater weighing and DEXA. However, technical limitations of BIA began to surface in the late 1980s.

Two primary limitations of BIA were its assumption of the human body as a single cylinder and its use of a single frequency (50 kHz). This technique may have worked for users with standard body types, but it was not as accurate for other populations that might not fit a conventional mold, such as fit elderly adults and most medical patients. To increase the accuracy of results, researchers derived various population-specific equations for determining body composition. These equations were based on what is known as empirical data.

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Sample empirical estimation equation Source: Lukaski, H. & Bolonchuk, W.W. (1989). Estimation of body fluid volumes using tetrapolar bioelectrical measurements. Aviation Space and Environmental Medicine, 59(12), 1163-1169.

1980's - Development of Empirical Equations

Empirical data is knowledge acquired by means of observation or experimentation. By collecting data from a sample population deemed to represent the expected characteristics of the entire population, researchers can derive equations that may be used to predict outcomes. In body composition, researchers have identified trends in muscle and fat mass and have used this data to predict body composition based on specific variables.

In 1986, research was published in which the impedance index was combined with factors such as body weight and gender into empirical equations. Over time, numerous other equations were developed based on additional factors such as age, ethnicity, and body type.

Although empirical estimations have the potential to provide an accurate estimate of a healthy individual’s body composition, significant problems arise when they are used for medical purposes in which accurate and precise assessments are a requirement.

For instance, age is a common factor in empirical equations used for body composition. In general, most individuals tend to lose lean body mass with age due to a sedentary lifestyle. Based on this trend, empirical equations often skew lean body mass up for younger individuals and down for older individuals. However, such data manipulation can cause inaccuracies and significant misassessments regarding health risks in population outliers such as obese youth or fit older adults.

Suppose a device that relies on empirical equations to estimate body composition is used on two people who have the same amount of lean body mass, but one person is 30 years old and the other is 40 years old. Because most individuals tend to lose lean body mass with age, even though the two individuals have the same amount of lean body mass, the empirical equations will skew the 40 year old’s lean body mass down, resulting in a higher percent body fat.

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1980's - Home-use BIA devices

In 1969, Hoffer et al. carried out a series of experiments to prove that total body water and bioelectrical impedance were highly correlated, suggesting that impedance measurements could be used for determining total body water.

Impedance of the right half of the body was measured, including the right arm, torso, and right leg. The squared value of this length measure divided by impedance had high correlations with total body water (r = 0.92). This correlation was more in agreement with the gold standard technique when compared to other indices, including body weight. The equation Hoffer et al. showed to be correlated to body water is the impedance index (Height2➗ Impedance) used in BIA today.

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1992 - Kushner and the Proposal of Multi-frequencies with Segmental Analysis

In 1992, Dr. Robert Kushner proposed that the technical limitations of BIA could be improved by measuring the human body as five separate cylinders (right arm, left arm, torso, right leg, left leg) instead of one. Each of these cylinders have different lengths and cross sectional areas, resulting in varying impedance values.

When considering the single cylinder model, the thinness and smaller cross-sectional area of the limbs reduce their impact on whole body impedance. However, since the torso makes up 50% of lean body mass, measuring the impedance of the torso separately is crucial to accurately determine body composition.

According to Kushner, measuring segmental impedance alone would not be sufficient; instead, all five body cylinders would also need to be measured at different frequencies to distinguish intracellular, extracellular and total body water. This distinction would allow for a better understanding of fluid distribution, providing an accurate measure of the hydrated state of lean mass.

In other words, the technical limitations of BIA could be overcome by measuring the different body segments at different frequencies.

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1996 - Dr. Cha creates the InBody Body Composition Analyzer

In 1996, Dr. Kichul Cha developed InBody, the world’s first 8-point tactile electrode system with direct segmental analysis to measure impedance in the five different body cylinders using multiple frequencies.

By doing so, the impedance in the limbs and torso were measured separately, yielding highly accurate results without using empirical data based on factors like age, gender, ethnicity, athleticism, and body shape. Thus, the InBody DSM-MFBIA body composition analyzer is a precision medical device.

Many BIA products today provide segmental measures of muscle and fat mass, but most of these products are still unable to take segmental impedance measurements, particularly in the torso.

The InBody measure each segment separately and shows the impedance values of all five cylinders of the body at each frequency in the Impedance Section of the InBody Result Sheet.

Revolutionizing BIA Technology with InBody

nBody’s medical-grade body composition analyzers rely on four pillars of technology to provide clients with accurate and precise direct segmental measurement multi-frequency bioelectrical impedance analysis (DSM-MFBIA) results that have been extensively validated to gold-standard methods.

InBody uses multiple currents at varying frequencies to provide precise body water analysis.

Multiple Frequencies

When measuring impedance with electrodes, contact resistance occurs. InBody accounts for contact resistance with strategically placed electrodes to ensure that measurements are accurate and reproducible.

8-Point Tactile Electrode System

InBody provides independent measurements for each of the body’s 5 cylinders (left arm, right arm, torso, left leg, and right leg) to provide you with accurate and detailed results.

Direct Segmental Measurements

InBody measures your impedance independently, so your results are not affected by your age, gender, ethnicity, athleticism, or body shape.

No Empirical Estimations

BIA TECH PROBLEM

The ability to distinguish between extracellular and total body water is important to identify fluid imbalances related to acute inflammation or edema. Many BIA devices use only one frequency at 50 kHz to measure impedance. However, since frequencies at 50 kHz do not fully pass through the body’s cells, accurate measurement of total body water is not possible. As a result, patients with increased extracellular water may be misidentified as being healthy.

INBODY SOLUTION

InBody uses a combination of low and high frequencies to determine extracellular, intracellular, and total body water. The use of multiple frequencies allows InBody devices to achieve a high level of precision. Medical practitioners can use InBody for measurements of body composition and fluid status.

Multiple Frequencies

Total body water (TBW) is stored throughout the body and can be separated into 2 compartments:

  • Intracellular water (ICW) – aka cytosol; water located inside cells of muscles, bones, organs, etc., comprising the majority of TBW.
  • Extracellular water (ECW) – water in the blood and interstitial fluids

Early BIA devices used a single 50 kHz frequency to calculate TBW. However, the 50 Khz frequency is not strong enough to fully pass through the body’s cells and therefore cannot give an accurate measurement of ICW. Therefore, ICW was estimated proportionally based on the ECW. This estimation was used to determine TBW, lean mass, and fat mass.

The estimation of intracellular water was based on the assumption that the ratio of ICW to ECW in healthy adults is about 3:2. However, individuals with body compositions that differ from conventionally healthy adults, such as elderly, obese or chronic disease patients, often have a higher ratio of ECW. Thus, in these patient populations, relying on the 3:2 ICW:ECW ratio could result in significant error.

InBody uses multiple frequencies ranging from 1 kHz to 1 MHz to provide precision body water analysis. Electrical currents interact differently with the cells at different frequencies, which allows the InBody to quantify the different fluid compartments. Low frequencies are better suited for measuring ECW, while high frequencies can pass through cell membranes to measure ICW and therefore TBW.

An accurate measure of TBW and the ability to analyze ICW versus ECW allows for a deeper analysis of individual body composition. Compartmental water measures can be used to properly quantify and identify changes in fluid balance to reflect nutritional status and fitness progress.

BIA TECH PROBLEM

If the starting measurement position changes, the length of the measured cylinder also changes. This directly impacts impedance and introduces error.

INBODY SOLUTION

8-Point Tactile Electrode System with Thumb Electrodes

8-Point Tactile Electrode System with Thumb Electrodes

When the human body comes in contact with an electrode, resistance occurs. To accurately measure the resistance in the human body, it is important to control the measurement location. Competitors’ products usually lack the thumb electrode or have the hand electrodes close together. These designs can cause measurements to start in the palm, which has a high impedance and can cause inaccuracies, or lead to inconsistent measurement starting points, reducing the reliability of results.

The anatomical design of the hand electrode creates a simple holding position that is easy to reproduce. Utilizing the anatomical characteristics of the human body, when an InBody user grasps the hand grip, current flows from the palm electrode and the electrical energy, or voltage, is initiated at the thumb electrode..

When current and voltage overlap, impedance can be measured. By separating current and voltage into the hand and foot electrodes, the point of overlap can be controlled to isolate the five cylinders of the body (limbs and torso) and consistently start at the same location on the wrists and ankles for reproducible results. With this design, the point of measure stays the same even when the user changes the holding position of the hand electrode or the contact points on the hands and feet.

BIA TECH PROBLEM

Traditional BIA views the human body as one cylinder. However, the torso of the body needs to be measured separately because its short length and large cross-sectional area mean that even a small measurement mistake can lead to substantial error.

INBODY SOLUTION

Direct segmental measurement bioelectrical impedance analysis regards the human body as five cylinders: left arm, right arm, torso, left leg, and right leg. InBody independently measures each cylinder to provide accurate measurements for the entire body.

Direct Segmental Multi-frequency Bioelectrical Impedance Analysis (DSM-BIA)

Traditional BIA systems viewed the human body as a single cylinder, using whole-body impedance to determine total body water. However, this method had a number of flaws: It assumed the distribution of lean body mass and body fat across all segments of the body is consistent.

The shape and length of the arms, legs, and torso differ; as such, the body should not be considered one cylinder, but rather as five separate parts. Since impedance is based on length and cross-sectional area, the calculation of TBW is inaccurate because each segment of the body has different length and cross-sectional area. One of the biggest problems with the single cylinder method is the lack of a separate torso measurement.

The torso has the shortest length and highest cross-sectional area, which results in a very low impedance (typically 10-40 ohms). However, the trunk comprises about 50% of an individual’s lean body mass (LBM). Therefore, small errors in torso impedance have significant impact on body composition results.

With whole-body impedance measurement, the torso impedance is not observed separately and thus, changes in torso impedance cannot be quantified. Because of the large amount of lean mass in the torso, small variability in impedance measures can have a drastic effect on how the results are interpreted.

For example, when isolating the torso, a change of just 3 ohms can lead to a difference of several pounds of lean mass; however, using a single cylinder model, this significant distinction would be observed as less than 1% difference in the whole-body impedance measurement.*

*Based on hypothetical example. Differences and percentages may vary based on the individual. Some BIA devices avoid the torso measurement entirely. For example, with many BIA scales, only the impedance of your legs and a small part of your torso are measured. Similarly, with handheld BIA devices, only the impedance of your arms and a small portion of your torso are measured.

With this design, the rest of the body must be estimated. Such devices that exclude the torso and estimate the majority of the body’s impedance may result in great errors in total body water and, in turn, lean body mass and fat mass. With InBody’s direct segmental technology and 8-point tactile electrodes, these potential errors are removed, providing more accurate body composition results.

BIA TECH PROBLEM

In many bioimpedance technologies today, empirical equations are incorporated to compensate for technological flaws, including the lack of torso impedance (due to whole-body impedance measurement), single frequency measurements (which are unable to differentiate between water compartments), and lack of reproducibility (from electrode placement or positioning). These empirical equations utilize data, such as age, gender, and ethnicity, to calculate body composition based on common trends rather than relying solely on the individual’s actual body composition.

INBODY SOLUTION

InBody measures body composition without relying on empirical assumptions based on age, gender, ethnicity, or body shape, producing accurate and precise results that are validated to gold standard methods. Put simply, InBody provides individualized feedback for better tracking of progress to help you achieve your goals.

No Estimations or Empirical Equations

Traditional BIA systems viewed the human body as a single cylinder, using whole-body impedance to determine total body water. However, this method had a number of flaws: It assumed the distribution of lean body mass and body fat across all segments of the body is consistent.

The shape and length of the arms, legs, and torso differ; as such, the body should not be considered one cylinder, but rather as five separate parts. Since impedance is based on length and cross-sectional area, the calculation of TBW is inaccurate because each segment of the body has different length and cross-sectional area. One of the biggest problems with the single cylinder method is the lack of a separate torso measurement.

The torso has the shortest length and highest cross-sectional area, which results in a very low impedance (typically 10-40 ohms). However, the trunk comprises about 50% of an individual’s lean body mass (LBM). Therefore, small errors in torso impedance have significant impact on body composition results.

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High Correlation to Gold Standard Methods

Because of its technology, InBody has been found to be one of the most accurate BIA devices on the market. In fact, it has been found to have a high correlation of 0.99 to DEXA for lean body mass in a population of normal weight adults.

Article: Accuracy of direct segmental multi-frequency bioimpedance analysis in the assessment of total body and segmental body composition in middle-aged adult population

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