Bioimpedance Measurement as a Research Tool

Introduction

Research, whether academic or private, has the primary objective of answering scientific questions using standardized protocols that involve a number of measurement tools. Their role is to generate objective data that are then used to address the scientific hypotheses posed upstream of the protocol. Therefore, it is necessary to know 1) the parameters measured by these tools, 2) with what precision, and 3) if these measurements are repeatable over time. Moreover, it is also possible for research work to demonstrate and develop alternative uses of these tools for measuring parameters other than those initially planned.

In this context, the use of bioimpedance analysis (BIA) in research has gradually increased in recent years, and in this dossier, we will present its current uses as well as future usage perspectives.

Bioimpedance Analysis in Body Evaluation

Research is not a closed-off field and is therefore part of a disciplinary field, where it is in constant relation with everyday field practice, both to retrieve information and to confirm scientific responses obtained. This premise also holds true for BIA, which is mainly used for body composition assessment in a clinical¹ context or in a sports² context for:

• Determining the body composition of a particular population such as athletes according to their sport³ or patients with a chronic disease^{4, 5}.
• Determining the effects of an intervention on body composition such as a surgical operation^6-8, a physical activity program^9-12 or those induced by a sports competition¹³.
• Determining the influence of changes in body composition on the physiopathology of certain chronic diseases^{14, 15}.

BIA has a variety of potential uses in a research context, which explains its increasing employment compared to other techniques for evaluating body composition. Indeed, there are numerous techniques, differing in various parameters including the number of body compartments evaluated, the duration of assessment, and subject convenience. For instance, total water estimation through deuterium dilution is highly precise, however, it requires a measurement time of several hours, whereas the same assessment through BIA will be much quicker but with lower precision. Therefore, researchers aiming for the most precise measurement, conducted in a laboratory under controlled conditions with a small number of individuals, will choose deuterium dilution. Whereas for a field study involving rapid measurements and/or a large number of individuals, where having a very precise measurement is less crucial, BIA will be used.

In 2009, in this chapter¹⁶, Deurenberg and Roubenoff summarized the principle, advantages, and disadvantages of each of these techniques, which they compiled in the table below:

If we look at the data in this table, we can observe that BIA has lower precision than reference techniques like DXA or MRI, but it is one of the least expensive, fastest, and most comfortable techniques for the subject, making it relevant for field studies. Thus, it allows for measurements under special conditions or in large populations, as demonstrated by these two studies conducted with the Biody Xpert^ZMII.

The first is a study conducted by Wekre and colleagues¹⁷ which focused on the hydration of divers during a saturation intervention, where divers are placed in a closed hyperbaric environment, i.e., with a pressure 2 or 3 times higher than atmospheric pressure, leading them to significant dehydration. Therefore, it is necessary to monitor their hydration to maintain their physical and psychological capacity, and the goal of this study was to verify if the strategies implemented during the intervention maintained good hydration in these divers. Two evaluation methods were used to answer this question: BIA and urinary density, and both confirmed that divers maintained their hydration level throughout the intervention. Additionally, the results show that both techniques have similar performances in determining this level.

The second study was conducted by Martinez-Rodriguez and colleagues¹⁸ and focused on the effects of a nutritional intervention program on 196 first-year students at the University of Alicante. This study was based on the observation that the start of a university course was responsible for many changes in the eating habits of the students, whose causes are a lack of information on nutrition, a limited budget, and/or a lack of time and skills for cooking. These changes can cause a significant weight gain and also persist over time, leading to

the possible appearance of lifestyle-related pathologies, such as obesity, diabetes, or cardiovascular diseases. Moreover, the students included in this study were enrolled in a nutrition course and it was shown that they were at higher risk of developing eating disorders. Indeed, they receive a significant amount of information about nutrition, which can create pressure on their own eating behavior. Given this different information, it is necessary that training on these issues be offered through tailored programs, and the goal of this study was to provide a nutritional education program to these students and measure its effects on body composition.

Students included in the intervention group attended several workshops on diets, lifestyle, and the risk of developing eating disorders, during which a “nutrition myth” was confronted with scientific elements. To control the effects of the intervention, students in the control group attended complementary classes to those they were already taking but whose subject was not specific. The results show that students who participated in the intervention improved their body composition, particularly by reducing their fat mass and increasing their muscle mass, while students in the control group did not show changes in these parameters at the end of the protocol. Therefore, this nutritional education program would have beneficial effects on the students’ eating habits, which translates into an improvement in their body composition.

Use of Bioimpedance: Future Perspectives

As previously explained, BIA is a technique that allows the assessment of body composition using predictive equations that include biophysical data measured by devices using this technique. However, recent research has shown that these biophysical data are also related to other biological parameters, thus BIA can assess events other than body composition.

Currently, the most described parameter in the literature is the phase angle, which corresponds to an angular transformation of resistance and reactance, and whose value at 50 kHz reflects the health status of individuals¹⁹, making it particularly interesting. Indeed, several studies have shown that it is associated with nutritional status²⁰, the risks of developing cardiovascular diseases²¹, and even mortality^22, thus it is a relevant tool for the prevention of the development of chronic pathologies. However, additional research is necessary to determine precise thresholds which, when exceeded, indicate a risk of developing a pathology. Moreover, in many diseases, it has also been shown that the phase angle is associated with the clinical state of the patients^23-25 and that its value can be used as a prognostic factor in breast cancer²⁶, upon entry into intensive care²⁷, and in many other pathologies¹⁹. Similarly, further research is needed to determine threshold values that can be used to achieve these objectives.

Two other parameters also seem interesting for studying the health state of individuals: the impedance ratio (IR), which is the ratio of impedance measured at 200 kHz to that measured at 5 kHz, and capacitance. IR appears to be a marker of inflammation since it is correlated with CRP, a blood marker of systemic inflammation²⁸, and the phase angle, making it relevant for monitoring nutritional status²⁹. Capacitance, on the other hand, is a raw value obtained only by BIA devices in spectroscopy and thus is a parameter little studied in the literature. However, a recent publication has explored the connections between capacitance and various biological events, showing that capacitance may be representative of the cell membrane exchange capacity^30. Interestingly, it also seems that the value of capacitance could detect insulin resistance. Indeed, individuals with this dysfunction of insulin metabolism have capacitance values higher than 2.67 nF for men and 1.65 nF for women, suggesting that these values could be used as a detection threshold³¹. However, these results remain to be confirmed by future studies on different populations and on a larger scale.

Finally, an alternative use of BIA that presents interest is the study of skeletal muscle using BIA, also known as impedance myography, based on the sensitivity of BIA to the state of the cell membrane and hydration. Indeed, the physiological events suffered by the skeletal muscle target its membrane, such as damage, and/or the amount of water it contains. From the BIA’s perspective, the measured resistance is directly related to tissue hydration, while reactance depends on the state of the cell membrane³², thus it is virtually possible to characterize skeletal muscle using these parameters. In 2015, Bartels et al.³³ showed, in three case studies, that raw BIA data were dependent on the state of skeletal muscle contraction and the presence of an injury. Before this date, Nescolarde et al.^34,35 had shown that resistance and reactance measured by BIA were locally altered by a muscle tear, diagnosed by MRI. Specifically, the resistance and reactance of the damaged muscle decreased compared to values measured in its non-injured equivalent, which would be explained by the local increase in water volume and the degradation of the membrane due to the tear, respectively. Although these results show the relationship between BIA values and muscle damage, it is currently not possible to quantify muscle damage solely with BIA, limiting its field use. Finally, in 2021, Cebrian-Ponce et al.³⁶ conducted a systematic review of the literature on this issue to highlight the possible uses of impedance myography in healthy subjects. Besides damage, the selected publications show:

• That BIA values, particularly the phase angle of skeletal muscles, decrease by 0.3 to 0.6° per year with aging.
• That resistance and reactance increase during muscle contraction, regardless of the level of force produced.
• That resistance and reactance decrease with the onset of muscle fatigue.

These results suggest that impedance myography could allow local assessment of skeletal muscle at rest, which would be particularly interesting for detecting sarcopenia but also for monitoring neuromuscular pathologies³⁷ where currently used measurement tools remain invasive. Additionally, they also suggest that BIA could be used to assess muscle changes during exercise, however, further research is needed to verify if it could replace existing techniques, such as surface electromyography.

Conclusion

In a research context, the main use of BIA is the study of body composition with the goal of characterizing it in specific populations, studying the effects of an intervention on it, and/or characterizing the mechanisms by which an imbalance of one or more body compartments can participate in the pathophysiology of some chronic diseases. However, other potential uses of BIA are emerging, which involve using the raw data obtained by BIA, including the phase angle, IR, or capacitance, which can be used to characterize the health status of individuals, or impedance myography for the localized study of skeletal muscle.

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