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Bioimpedance analysis – it’s all about the algorithms!

Bioimpedance analysis (BIA) is a method to determine a person’s body composition, for which the fields of medicine, fitness and nutrition have utilized more progressively lately. As it is an indirect method, it fundamentally depends on the applied mathematical equations used to calculate the body composition, taken from measurement parameters. The development and validation of these equations is a complex and time-consuming process, but it ultimately determines the accuracy of a BIA device.

The basis for the bioimpedance analysis (also referred to simply as body analysis) is the different electrical conductivity of tissues. While the electrolyte-containing body water conducts electricity very readily, adipose tissue behaves like an insulator. These properties allow tissue to be differentiated by BIA and a person’s body composition to be determined.

For this purpose, a weak alternating current electric field is generated via hand and foot electrodes, and the resulting drop in voltage is then measured. The bioelectrical charge and the strength of the current can thus be used to calculate the impedance, which describes the resistance level of the body’s tissue against the current. The impedance is composed of two partial resistances - the ohmic resistance (resistance R) of the electrolyte-containing intra- and extracellular fluid, and the capacitive resistance (reactance Xc) produced by the cell membranes.

The physical basis of body composition analysis

Cell membranes can store charges and in a simplified way, they act as capacitors. Their capacitor properties cause a phase shift between current and voltage. This leads to a time difference between the current’s and the voltage’s maximum strength - the current rushes ahead of the voltage. This shift is measured in degrees and referred to as the phase angle (What is a phase angle?). Modern phase-sensitive BIA devices are able to measure the phase angle and use it to distinguish the partial resistances.

The more intact cells the body has, the larger the phase angle becomes.

The resistance is inversely proportional to the proportion of total body water (TBW) and can thus be used for its assessment. Reactance, on the other hand, is a measurement of body cell mass (BCM).

Using appropriate algorithms, the exact proportion of fat and muscle tissue and total body water can be calculated from the impedance, its partial resistances and the phase angle.

In addition, the phase angle provides further important information about the state of health of the organism and depends on the number, size and functional state of the body cells. A large phase angle is associated with many intact body cells in a good nutritional state, whereas a small angle indicates malfunction and cellular damage. The decrease in the phase angle is due to a breakdown of the membrane’s integrity and the resulting fluid shifts from intracellular to extracellular.

The phase angle describes the relationship between water resistance and cell resistance.

With the development of the multifrequency BIA, this method is able to determine not only the percentage of total body water, but also to distinguish between intracellular and extracellular water.

Depending on the frequency of the alternating current, the resistance of the biological conductors also changes. Low frequencies in the range of 1 to 5 kHz expatiate, especially in the extracellular space, because they can hardly overcome the cell membranes and are therefore particularly suitable for the determination of extracellular fluid (ECW). Using higher frequencies, the total body water can be determined from the measured resistance, and the difference between total body water and intracellular water can be calculated by the previously measured percentage of extracellular fluid.

With different AC frequencies, a distinction can be made between intracellular and extracellular water.

One of the key benefits of BIA is that the entire measurement process takes only a few seconds, is non-invasive and very well tolerated. Therefore, this particular method is characterized by a high degree of patient and user friendliness.

At present, a large number of BIA devices are used for fitness, but the knowledge of exact body composition is also of great benefit in the medical field. For the medical application, a high accuracy of the data is crucial, which depends directly on the mathematical equations.

The heart of a BIA device: the mathematical equations

Based on comparative measurements with scientific reference methods, the necessary algorithms are generated, ensuring a high quality of measurement results. Depending on each compartment, certain algorithms and methods may prove more effective than others. The most accurate method is often referred to as the "gold standard".

Magnetic resonance tomography is the most accurate method for the quantification of skeletal muscle mass. It also makes it possible to further differentiate fatty tissue and also to determine the proportion of visceral adipose tissue. Here, a few hundred MRI images per person must be evaluated by hand or with the help of software in order to calculate the muscle mass or the visceral fat mass.

To determine the fat mass, the four-compartment model (4C model) is the gold standard. This method divides the body into the four compartments: fat, water, protein and minerals. The analysis is based on the combination of different methods and allows the best possible differentiation of fat mass, mainly due to the inclusion of the individual state of a body’s hydration.

By means of air displacement plethysmography (ADP), the volume of the body is measured in a closed chamber. The density of the body can be calculated from a person’s body weight with the percentage of adipose tissue deduced from the results. By taking further measurements (total body water, bone mineral content and protein content), this value can be more accurately differentiated.

Total body water is determined by using deuterium dilution. In this case, deuterium oxide (D2O) is administered orally. After a set time, the concentration in the body is measured, usually via blood sample. Assuming that the D2O distributes evenly in the body water over time, the body water content can be accurately calculated from the measured concentration and administered dose - with a minimal deviation of one to two percent.

To further differentiate the fat-free mass, bone mineral content (BMC) is measured by dual-energy X-ray absorptiometry (DXA). Total body water and bone mineral content then serve as the basis for calculating the protein content.

Yet the use of these different methods makes the four-compartment model time-consuming and expensive, which is why it is mainly used for scientific purposes.

Conclusions about the amount of extracellular water allow the use of sodium bromide, similar to deuterium dilution. 

The calculation formula for the fatty or fat-free mass according to Fuller et. al (1992). Since the individual state of hydration is included in the calculation, the method (4C model) is considered to be the most common and is often referred to as the "gold standard".

BIA devices are also used in standard care and allow a sound assessment of the nutritional and general state, as well as the hydration state. Due to the continuous development and increased efficiency of the BIA equations, the method now also meets the high-level requirements of medical research and can be used to investigate a variety of different issues.

seca mBCA - validated for medical use

Specifically developed for medical use, the seca mBCA has been intensively validated using the 4C model and whole-body MRI. With great effort and in the scope of a multicentre study, the body composition of several hundreds of subjects was determined with the use of the 4C model, the whole-body MRI and NaBr dilution. From the data obtained, precise BIA equations were developed. The results were published in the Journal of Clinical Nutrition in 2013 and 2017, and proved that the measuring of body composition in healthy adults via seca mBCA is comparable in accuracy to the respective gold standard.

Compared to the 4-compartment model, the fat mass results of the seca mBCA had an accuracy of 98%. When determining skeletal muscle mass, the results matched with a whole-body MRI by 97%. The measurements of the body water compartments also resulted in similar findings. The total body water was consistent with results from the method of using deuterium dilution by 97%, and the measurement of extracellular water using sodium bromide dilution by 95%.

The seca mBCA is the only BIA device that has been validated against several gold standards (whole-body MRI, 4C model, NaBr dilution) and whose results have also been reviewed and published by medical journals (Journal of Clinical Nutrition 2013 and 2017).

In further studies, each with 1,000 healthy volunteers, standard values ​​for different ethnic groups were also determined. They provide a secure basis for the interpretation of the seca mBCA in a daily routine.

Printout of the seca mBCA with a series of measurements. Using normal ranges (green), the health status can be estimated immediately.

Precise and reliable results make the seca mBCA an innovative instrument that also meets the high demands of medical research. In contrast to many common reference methods, it is characterized by a simple application, as well as being highly economic and time efficient. Its range of applications is not limited to inpatient and outpatient standard care, but also extends to a variety of other applications such as weight management or rehabilitation, making it a versatile and universally applicable device.

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