The Phase Angle: What Is It and Why Is It Becoming Necessary?

Definition of Phase Angle

In bioelectrical impedance analysis (BIA), the phase angle is a raw bioelectrical parameter obtained in two ways:

• By measuring the angular difference between the voltage U and current I curves.
• From the formula: \arctan\left(\frac{X_C}{R}\right) \times \left(\frac{180}{\Pi}\right)

The calculation of this parameter suggests that the phase angle would not be associated with biological functions or structures; however, the interest in it in the scientific literature and its clinical use has strongly progressed in recent years. From a biophysical point of view, the measured electrical parameters (phase angle, resistance, etc.) are notably dependent on the integrity of the cell membrane and its ability to store electrical charges, or capacitance. Considering that membrane integrity decreases in a pro-inflammatory and pro-oxidant environment, the phase angle has been shown to be related to an increase in inflammation¹ and thus to the health status of individuals².

Consequently, a high phase angle value is associated with good health status, whereas a decrease in this parameter between two measurements generally indicates a degradation. However, the phase angle is also dependent on age and body mass index (BMI), which can make its interpretation difficult during clinical management. This has led to the establishment of reference values based on these two variables³. Nevertheless, it is interesting to note that beyond a certain BMI, the phase angle no longer increases and tends to stabilize as body mass rises. This phenomenon is mainly explained by the fact that the increase in BMI is often associated with an excess of fat mass responsible for the onset of inflammation and clinical complications⁴. Furthermore, the phase angle is higher in men than in women, the cause of which is a higher average body mass in men rather than a better overall health status.

More specifically, it has been shown in several chronic diseases that the phase angle is related to the onset and evolution of clinical complications associated with these diseases. Indeed, it is correlated with overall mortality², cardiovascular mortality⁵, in cancer⁶, and in intensive care⁷, as well as with nutritional status⁸. Moreover, in the elderly, it is also related to muscle strength⁹ and muscle mass¹⁰, making it particularly relevant in monitoring sarcopenia during aging, but also in cancer. These relationships with physiological parameters are not limited to clinical populations but also exist in healthy populations where the phase angle has been shown to be related to muscle strength and power¹¹, as well as to speed and the ability to repeat¹². Therefore, it can also be used to evaluate the fitness level of a recreational or professional athlete.

Use of Phase Angle in Clinical Practice

Due to its definition and its relationships with several physiological and physiopathological phenomena, the phase angle can be used to indirectly assess the health status during routine clinical management. Indeed, during a single measurement or a first consultation, its value can provide insight into an individual’s health status and guide their management or the prescription of complementary exploratory examinations if the value is abnormally low. Furthermore, in apparently healthy individuals, the phase angle can also help detect an intermediate health status, i.e., a state in which an individual presents one or more non-pathological physiological dysfunctions, silent or symptomatic (e.g., immune fragility, chronic fatigue), which may become pathological in the medium/long term. Thus, hygiene and dietary measures (e.g., physical activity and/or nutritional modifications) can be implemented early to prevent these dysfunctions from becoming pathological.

The phase angle can also be useful during the management of chronic diseases to evaluate the clinical status and guide management, complementing the examinations commonly performed. Indeed, it can quickly and non-invasively assess the patients’ health status and detect a deterioration during follow-up, thereby prompting consideration of complementary examinations and/or an adaptation of the management. However, as the phase angle remains relatively non-specific, a decrease should be used as a warning sign to investigate the causes of these modifications by questioning the patient and/or performing specific examinations. Its variations and value can also be used to adjust treatment according to the health status, particularly if the treatment presents significant side effects, such as chemotherapy, for example.

Finally, as explained in the previous section, the phase angle can also be used to evaluate the fitness level of recreational or professional athletes and therefore also assess the internal load during a season or preparation for a sporting event. Indeed, exercise-related adaptations are the consequence of repeated physiological stress leading to fatigue, which must be limited in the long term for health and performance. Therefore, physical preparation is the result of a balance between this physiological stress induced by exercise and recovery to avoid an overload if the stress is too high, or a lack of adaptations if it is insufficient. Training overload, or even overtraining, is notably characterized by the appearance of chronic inflammation¹³, the origin of which is the accumulation of membrane damage induced by chronic exercise. As the phase angle is modified by these phenomena, it will tend to decrease when the training load is too significant and an overload is present, necessitating a rest period or a reduction in the training load. Conversely, an improvement in fitness and performance will be reflected by an increase in the phase angle during preparation.

Conclusion

In conclusion, the phase angle is a parameter obtained quickly and non-invasively through bioelectrical impedance analysis, allowing for the assessment of individuals’ health status due to its relationship with cell membrane integrity and the presence of systemic inflammation. It is therefore particularly relevant for monitoring individuals during nutritional follow-up, the management of a chronic pathology, or during sports preparation, complementing body composition analysis.

Bibliography

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