BIOELECTRICAL IMPEDANCE ANALYSIS (BIA)
Introduction
Bioelectrical Impedance Analysis (BIA) measures body composition by running a series of alternating electrical currents (AC Circuit) ranging from 5 kHz to 1 mHz, through the body.
These signals then interact differently with body cells and fluids, transmitting the potential difference (voltage) back to the BIA device. The resulting data is given as impedance (Ohms) which is a combination of resistance and capacitance data. Data obtained can be used either in its raw form or via a series of body composition regression equations. The raw data may be used in a number of ways, by studying capacitance, monitoring the impedance ratio (or Prediction Marker) or utilising Phase Angle. This brochure examines Phase Angle, how it is derived and the medical applications.
Phase Angle in brief:
Impedance is a ratio of the magnitude of potential difference to the magnitude of current. The phase shift is given by the time difference between voltage and current signal waves. From this we obtain the phase angle. phase angle is the time relationship between the electrical current passing through the body and the potential difference invoked by this current across body tissue. Bodystat is unique in that it MEASURES phase shift (as opposed to calculating), which is important in clinical practice.
How does this apply in clinical practice? The phase angle reflects the relative contribution of body fluid (resistance) and cellular membrane integrity (capacitance). Malnutrition reduces cellular membrane mass and integrity and promotes shifts in fluid balance. As a consequence of these changes the phase angle decreases. Conversely, a higher phase angle implies larger body cell mass and preserved membrane integrity.
WHAT IS PHASE ANGLE?Laws of physics dictate the calculation of phase angle. In a capacitor AC (impedance) circuit, reactance of the current through cells causes the current to lag behind the voltage; consequently, current (blue) and voltage (red) do not meet at the same time. The time difference in the period between this voltage peak and current peak is called the phase shift.
Current flows by the movement of ions; movement impeded by the viscosity of the medium is known as resistance and measured in Ohms. It should be noted that the human body does not adhere to Ohm’s Law (Current = Voltage / Resistance) due to the presence of dielectrics (or insulators) in the body. Groups of cells perform specialised functions and form part of a complex communicative network, sending signals throughout the body via an ion concentration or gradient. These electromagnetic gradients or dielectrics absorb some of the current causing the cells to become electronically charged, an essential function for cellular survival. It is these cells that store electricity that act as capacitors (measured in Farad F).
The impedance obtained by the BIA device is a combination of capacitive and resistive type elements, the calculation of which is shown opposite. Consequently, BIA measures resistance (R) and capacitance in combination. Reactance (Xc) is subsequently calculated from capacitance and representative of the opposition to the electrical current or voltage. Furthermore, the phase shift is always relative
to the resistance line as the resistance line is always in phase with the voltage. When the device receives this signal it is ‘decoded’ using a series of electronics called synchronous demodulators. This is why the quality of the electronics within the device is fundamental in obtaining accurate and consistent measurements.
Once the phase shift data is received, a series of complex number equations are then applied to derive the resistance and the reactance and thus the Impedance measurements (refer to figure). From this the phase angle (φ) is calculated.
φ = Arctangent x (Reactance/ Resistance )
Bioelectrical Impedance Analysis calculates body composition by measuring resistance (R) and reactance (Xc) (through capacitance). By recording the voltage drop between the applied current and the two output sites, the phase shift is measured and the phase angle calculated.
In electromagnetic terms, the current at 5 kHz has not been strong enough to penetrate the cellular membrane. This resistance is associated with a measure of extracellular water. When the current enters the cell at a high frequency, the reactance (Remember: reactance calculated from capacitance) to the current is associated with a measure of intracellular water.
The greater the cell’s capacitance, the greater the difference in phase shift between voltage and current. Consequently, the higher the phase angle. Equally, when the cell has less ability to store electrical current, the capacitance of the cell membrane is lower and the phase shift lower. The degradation of the cellular membrane is due to shifts in the gradient, that is, cellular losses of essential nutrients. This protein leakage is commonly associated with illness and muscle wasting. Conversely, the integrity of the cell membrane can be greatly improved with nutritional intervention and resistance-type training programmes.
TO SUMMARISE: Phase angle is positively associated with capacitance and negatively associated with resistance. That is, phase angle reflects the relative contribution of fluid (resistance) and cellular membranes (capacitance) of the human body.
A PROGNOSTIC INDICATOR
Phase Angle is commonly accepted as a prognostic indicator of morbidity and mortality. As health status changes, so does the relationship between Intra/extracellular water. With bioelectrical impedance analysis measurements, volume of TBW can be ascertained from the resistance, in turn reactance reflects the ability of the cell membrane to act as an imperfect capacitor. Therefore the phase angle is an indicator of the intra and extra cellular area. This relationship is represented by changes in the phase angle. A low phase angle is associated with a reduced survival time, conversely a higher phase angle is associated with improved cellular health.
This has been demonstrated in various population groups including oncology, HIV, liver cirrhosis, COPD, heart failure, haemodialysis and sepsis.
Additionally, muscle strength and phase angle correlate which is suggestive of a lower phase angle being associated with decreasing functioning status.
POINTSTO CONSIDER
+ Phase Angle provides information regarding an individual’s cellular health. The greater the data points collected, the better informed the physician in their clinical decisions
+ Phase Angle is independant of age, weight or body fat, given the characteristics of cellular capacitance (therefore phase angle)
+ It is also widely accepted that male phase angle tends to be significantly higher than females, due to differing physiology (i.e. fat to lean ratios).
NUTRITIONALSTATS
Many research papers have examined the relationship between phase angle and malnutrition and have found a correlation between low phase angle and higher nutritional risk. Population groups used in both research and clinical practice include nephrology, HIV, oncology and surgical patients.
Bioelectrical Impedance Analysis is becoming a preferred method to establish and monitor malnutrition. Alternative methods, such as blood tests, arm circumferences and skin-fold tests are time consuming, require training and maybe affected by other nutritional changes. Traditional methods may also miss subtle changes in body cell mass (intracellular water and metabolic tissue).
Malnutrition is characterised by changes in the integrity of the cellular membrane, marked by fluid shifts. Study of phase angle, as a reflection of water distribution between ICW/ECW water, is an easy, quick, non-invasive way to ascertain nutritional status.
"Phase Angle, BIVA and the Prediction MarkerTM are obtained directly from resistance, reactance or impedence, and evidence in the literature indicated that they could be ued as prognostic or nutritional markers."
ESPEN, "Blue" Book, Basics in Clinical Nutrition Fourth Edition Page 20 (2011)
AN ALTERNATIVE METHOD THE PREDICTIONMARKER
The Prediction Marker or impedance ratio, is simply the ratio of raw impedance values. We know that at a low frequency this is not strong enough to penetrate the cellular membrane and therefore gives the extracellular water. Higher frequencies allow penetration of the cellular membrane and therefore give total body water. The Prediction Marker is simply the ratio between the high and low frequencies. This is considered a more ’pure’ form of data, and as a direct measurement, i.e. devoid of complex numbers and equations. Many research papers have recognised this as a substitute for phase angle and it is thought to be more accurate in oedematous patients as phase angle only uses 50 kHz.
Recent research papers on the Prediction Marker suggest that not only is the correlation between the Prediction Marker and phase angle ‘very tight’ but that it is also easier to use in a clinical situations as well as easier to understand and explain the origins of the data. As with phase angle, the Prediction Marker is a non-invasive, objective, direct, quick method to determine nutritional status and morbidity risks in patients. It can also be used on any population, age or gender and is independent of weight or height.
GLOSSARY
Pulsating
Direct
Alternating
Variable
An electric circuit, in this case the body, is simply a path in which the electrons from a voltage or current supply flow. The voltage is often considered to be the cause, whilst the current is the effect. In other words, the voltage is a measure of energy carried by the charge and the current is the rate of flow of the charge.
There are various types of electrical current (shown above). Bioelctrical Impedance Analysis uses an alternating current (AC), this means that an AC also flows in a reverse direction, while, for example a direct current (DC) only flows in one direction. The data received is measured in the form of impedance. Impedance consists of resistance, capacitive reactance (reflective of capacitors in the circuit) and inductive reactance (reflective of inductors). BIA is concerned with capacitive resistance which reflects how the electrical charge builds up and how it is subsequently discharged. This information is then used to understand the type of cellular opposition that the current encounters. Resistance is an imaginary number derived from a mathematical equation, whereas reactance, the ‘real’ element of capacitance, reflects the cells opposition to the flow of the electrical current and is only present with AC type devices.
TERMINOLOGY
Body Cell Mass (BCM): Metabolic active tissue - intracellular fluid and metabolic tissue.
Demodulators: This is an electronic circuit that is used to translate the information from a carrier wave, i.e. the sinusoidal wave from the voltage and current flow.
Extracellular Mass (ECM): Metabolic Inactive tissue - extracellular fluid, fat, skeletal mass (bone and tendons).
Viscosity: a measure of resistance in fluid, the denser the fluid the higher the viscosity.
Dielectrics: An electrical insulator, therefore a poor conductor of electricity.
Clinical Nutrition 31 (2012) 854-861
"Bioelectrical phase angle and impedance vector analysis – Clinical relevance and applicability of impedance parameters"
"PhA, a superior prognostic marker, should be considered as a screening tool for the identification of risk patients with impaired nutritional and functional status, BIVA is recommended for further nutritional assessment and monitoring, in particular when calculation of body composition is not feasible."
Anja Bosy-Westphal, et al.
Institut für Humanernährung und Lebensmittelkunde, Christian-Albrechts-Universität Kiel, Germany; Institut für Ernährungsmedizin, Universität Hohenheim, Stuttgart, Germany.
Clinical Nutrition 32 (2013) 294-299
"Low Phase Angle determined by bioelectrical impedance analysis is associated with malnutrition and nutritional risk at hospital admission"
"There is a significant association between low PhA and nutritional risk, LOS and non-survival. PhA is helpful to identify patients who are at nutritional risk at hospital admission in order to limit the number of in-depth nutritional assessments."
Claude Pichard, et al.
Clinical Nutrition Department, Geneva University Hospital, Geneva, Switzerland.
European Journal of Clinical Nutrition 68, 683-689 (June 2014)
"A low bioimpedance phase angle predicts a higher mortality and lower nutritional status in chronic dialysis patients"
"These results suggest that bioelectrical impedance analysis and phase angle measurements are a useful tool for identifying dialysis patients at high risk for malnutrition and/or increased mortality."
Francis Dumler MD ,
Division of Nephrology, William Beaumont Hospital, Royal Oak, MI, USA.
, 24) and in a hemodialysis population (25). Buffa et al (26) also showed a significant decrease in phase angle with age in healthy elderly subjects, and Kyle et al (27) found the same age and sex differences in 2740 healthy adults. Introduction
Bioelectrical Impedance Analysis (BIA) measures body composition by running a series of alternating electrical currents (AC Circuit) ranging from 5 kHz to 1 mHz, through the body.
These signals then interact differently with body cells and fluids, transmitting the potential difference (voltage) back to the BIA device. The resulting data is given as impedance (Ohms) which is a combination of resistance and capacitance data. Data obtained can be used either in its raw form or via a series of body composition regression equations. The raw data may be used in a number of ways, by studying capacitance, monitoring the impedance ratio (or Prediction Marker) or utilising Phase Angle. This brochure examines Phase Angle, how it is derived and the medical applications.
Phase Angle in brief:
Impedance is a ratio of the magnitude of potential difference to the magnitude of current. The phase shift is given by the time difference between voltage and current signal waves. From this we obtain the phase angle. phase angle is the time relationship between the electrical current passing through the body and the potential difference invoked by this current across body tissue. Bodystat is unique in that it MEASURES phase shift (as opposed to calculating), which is important in clinical practice.
How does this apply in clinical practice? The phase angle reflects the relative contribution of body fluid (resistance) and cellular membrane integrity (capacitance). Malnutrition reduces cellular membrane mass and integrity and promotes shifts in fluid balance. As a consequence of these changes the phase angle decreases. Conversely, a higher phase angle implies larger body cell mass and preserved membrane integrity.
WHAT IS PHASE ANGLE?Laws of physics dictate the calculation of phase angle. In a capacitor AC (impedance) circuit, reactance of the current through cells causes the current to lag behind the voltage; consequently, current (blue) and voltage (red) do not meet at the same time. The time difference in the period between this voltage peak and current peak is called the phase shift.
Current flows by the movement of ions; movement impeded by the viscosity of the medium is known as resistance and measured in Ohms. It should be noted that the human body does not adhere to Ohm’s Law (Current = Voltage / Resistance) due to the presence of dielectrics (or insulators) in the body. Groups of cells perform specialised functions and form part of a complex communicative network, sending signals throughout the body via an ion concentration or gradient. These electromagnetic gradients or dielectrics absorb some of the current causing the cells to become electronically charged, an essential function for cellular survival. It is these cells that store electricity that act as capacitors (measured in Farad F).
The impedance obtained by the BIA device is a combination of capacitive and resistive type elements, the calculation of which is shown opposite. Consequently, BIA measures resistance (R) and capacitance in combination. Reactance (Xc) is subsequently calculated from capacitance and representative of the opposition to the electrical current or voltage. Furthermore, the phase shift is always relative
to the resistance line as the resistance line is always in phase with the voltage. When the device receives this signal it is ‘decoded’ using a series of electronics called synchronous demodulators. This is why the quality of the electronics within the device is fundamental in obtaining accurate and consistent measurements.
Once the phase shift data is received, a series of complex number equations are then applied to derive the resistance and the reactance and thus the Impedance measurements (refer to figure). From this the phase angle (φ) is calculated.
φ = Arctangent x (Reactance/ Resistance )
Bioelectrical Impedance Analysis calculates body composition by measuring resistance (R) and reactance (Xc) (through capacitance). By recording the voltage drop between the applied current and the two output sites, the phase shift is measured and the phase angle calculated.
|  | ||||||||||||||||||||||||||||||||||||||||
| SO HOW DOES PHASE ANGLE RELATE TO CELLULAR HEALTH AND HOW DOES THIS HELP THE MEDICAL PROFESSION? Consider the diagram. The current enters the ‘circuit’ (in this case the body), and faces a number of different obstacles called capacitors (C) (in this example body cells). This either will allow the current to pass through (R) or will impede the flow of the current (Xc). In a healthy cell, as the cellular gradient improves, more nutrients and salts enter the cell therefore intracellular fluid, increases and cells become healthier. The healthier the cell the stronger the cellular membrane (i.e. cellular insulation), with an improved ability to retain fluids, nutrition and communication properties. Healthy cell membranes are poor conductors but good capacitors (Remember: ability to retain current) at the frequency range used by bioelectrical impedance analysis. KEY POINT A low phase angle is indicative of diminished cellular integrity and thus a reduced survival time. Equally, a higher phase angle suggests larger quantities of intact cell membranes and thriving health. Illustration of current pathways KEY POINT The greater the cell’s capacitance, the greater the difference in phase shift between voltage and the current. Consequently the higher the phase angle. | |||||||||||||||||||||||||||||||||||||||||
The greater the cell’s capacitance, the greater the difference in phase shift between voltage and current. Consequently, the higher the phase angle. Equally, when the cell has less ability to store electrical current, the capacitance of the cell membrane is lower and the phase shift lower. The degradation of the cellular membrane is due to shifts in the gradient, that is, cellular losses of essential nutrients. This protein leakage is commonly associated with illness and muscle wasting. Conversely, the integrity of the cell membrane can be greatly improved with nutritional intervention and resistance-type training programmes.
TO SUMMARISE: Phase angle is positively associated with capacitance and negatively associated with resistance. That is, phase angle reflects the relative contribution of fluid (resistance) and cellular membranes (capacitance) of the human body.
A PROGNOSTIC INDICATOR
Phase Angle is commonly accepted as a prognostic indicator of morbidity and mortality. As health status changes, so does the relationship between Intra/extracellular water. With bioelectrical impedance analysis measurements, volume of TBW can be ascertained from the resistance, in turn reactance reflects the ability of the cell membrane to act as an imperfect capacitor. Therefore the phase angle is an indicator of the intra and extra cellular area. This relationship is represented by changes in the phase angle. A low phase angle is associated with a reduced survival time, conversely a higher phase angle is associated with improved cellular health.
This has been demonstrated in various population groups including oncology, HIV, liver cirrhosis, COPD, heart failure, haemodialysis and sepsis.
Additionally, muscle strength and phase angle correlate which is suggestive of a lower phase angle being associated with decreasing functioning status.
POINTSTO CONSIDER
+ Phase Angle is independant of age, weight or body fat, given the characteristics of cellular capacitance (therefore phase angle)
+ It is also widely accepted that male phase angle tends to be significantly higher than females, due to differing physiology (i.e. fat to lean ratios).
NUTRITIONALSTATS
Many research papers have examined the relationship between phase angle and malnutrition and have found a correlation between low phase angle and higher nutritional risk. Population groups used in both research and clinical practice include nephrology, HIV, oncology and surgical patients.
Bioelectrical Impedance Analysis is becoming a preferred method to establish and monitor malnutrition. Alternative methods, such as blood tests, arm circumferences and skin-fold tests are time consuming, require training and maybe affected by other nutritional changes. Traditional methods may also miss subtle changes in body cell mass (intracellular water and metabolic tissue).
Malnutrition is characterised by changes in the integrity of the cellular membrane, marked by fluid shifts. Study of phase angle, as a reflection of water distribution between ICW/ECW water, is an easy, quick, non-invasive way to ascertain nutritional status.
"Phase Angle, BIVA and the Prediction MarkerTM are obtained directly from resistance, reactance or impedence, and evidence in the literature indicated that they could be ued as prognostic or nutritional markers."
ESPEN, "Blue" Book, Basics in Clinical Nutrition Fourth Edition Page 20 (2011)
AN ALTERNATIVE METHOD THE PREDICTIONMARKER
The Prediction Marker or impedance ratio, is simply the ratio of raw impedance values. We know that at a low frequency this is not strong enough to penetrate the cellular membrane and therefore gives the extracellular water. Higher frequencies allow penetration of the cellular membrane and therefore give total body water. The Prediction Marker is simply the ratio between the high and low frequencies. This is considered a more ’pure’ form of data, and as a direct measurement, i.e. devoid of complex numbers and equations. Many research papers have recognised this as a substitute for phase angle and it is thought to be more accurate in oedematous patients as phase angle only uses 50 kHz.
Recent research papers on the Prediction Marker suggest that not only is the correlation between the Prediction Marker and phase angle ‘very tight’ but that it is also easier to use in a clinical situations as well as easier to understand and explain the origins of the data. As with phase angle, the Prediction Marker is a non-invasive, objective, direct, quick method to determine nutritional status and morbidity risks in patients. It can also be used on any population, age or gender and is independent of weight or height.
GLOSSARY
Pulsating
Direct
Alternating
Variable
Illustration of different types of current
ELECTRICAL CIRCUITS 101An electric circuit, in this case the body, is simply a path in which the electrons from a voltage or current supply flow. The voltage is often considered to be the cause, whilst the current is the effect. In other words, the voltage is a measure of energy carried by the charge and the current is the rate of flow of the charge.
There are various types of electrical current (shown above). Bioelctrical Impedance Analysis uses an alternating current (AC), this means that an AC also flows in a reverse direction, while, for example a direct current (DC) only flows in one direction. The data received is measured in the form of impedance. Impedance consists of resistance, capacitive reactance (reflective of capacitors in the circuit) and inductive reactance (reflective of inductors). BIA is concerned with capacitive resistance which reflects how the electrical charge builds up and how it is subsequently discharged. This information is then used to understand the type of cellular opposition that the current encounters. Resistance is an imaginary number derived from a mathematical equation, whereas reactance, the ‘real’ element of capacitance, reflects the cells opposition to the flow of the electrical current and is only present with AC type devices.
TERMINOLOGY
Body Cell Mass (BCM): Metabolic active tissue - intracellular fluid and metabolic tissue.
Demodulators: This is an electronic circuit that is used to translate the information from a carrier wave, i.e. the sinusoidal wave from the voltage and current flow.
Extracellular Mass (ECM): Metabolic Inactive tissue - extracellular fluid, fat, skeletal mass (bone and tendons).
Viscosity: a measure of resistance in fluid, the denser the fluid the higher the viscosity.
Dielectrics: An electrical insulator, therefore a poor conductor of electricity.
Clinical Nutrition 31 (2012) 854-861
"Bioelectrical phase angle and impedance vector analysis – Clinical relevance and applicability of impedance parameters"
"PhA, a superior prognostic marker, should be considered as a screening tool for the identification of risk patients with impaired nutritional and functional status, BIVA is recommended for further nutritional assessment and monitoring, in particular when calculation of body composition is not feasible."
Anja Bosy-Westphal, et al.
Institut für Humanernährung und Lebensmittelkunde, Christian-Albrechts-Universität Kiel, Germany; Institut für Ernährungsmedizin, Universität Hohenheim, Stuttgart, Germany.
Clinical Nutrition 32 (2013) 294-299
"Low Phase Angle determined by bioelectrical impedance analysis is associated with malnutrition and nutritional risk at hospital admission"
"There is a significant association between low PhA and nutritional risk, LOS and non-survival. PhA is helpful to identify patients who are at nutritional risk at hospital admission in order to limit the number of in-depth nutritional assessments."
Claude Pichard, et al.
Clinical Nutrition Department, Geneva University Hospital, Geneva, Switzerland.
European Journal of Clinical Nutrition 68, 683-689 (June 2014)
"A low bioimpedance phase angle predicts a higher mortality and lower nutritional status in chronic dialysis patients"
"These results suggest that bioelectrical impedance analysis and phase angle measurements are a useful tool for identifying dialysis patients at high risk for malnutrition and/or increased mortality."
Francis Dumler MD ,
Division of Nephrology, William Beaumont Hospital, Royal Oak, MI, USA.
Bioelectrical impedance analysis: population reference values for phase angle by age and sex1,2,3
- Maria Cristina G Barbosa-Silva,
- Aluísio JD Barros,
- Jack Wang,
- Steven B Heymsfield, and
- Richard N Pierson Jr
+ Author Affiliations
Abstract
Background: Phase angle is an indicator based on reactance and resistance obtained from bioelectrical impedance analysis (BIA). Although its biological meaning is still not clear, phase angle appears to have an important prognostic role.
Objective: The aim of this study was to estimate population averages and SDs of phase angle that can be used as reference values.
Design: BIA and other methods used to evaluate body composition, including hydrodensitometry and total body water, were completed in 1967 healthy adults aged 18–94 y. Phase angle was calculated directly from body resistance and reactance, and fat mass (FM) was estimated from the combination of weight, hydrodensitometry, and total body water by using the 3-compartment Siri equation. Phase angle values were compared across categories of sex, age, body mass index (BMI), and percentage FM.
Results: Phase angle was significantly (P < 0.001) smaller in women than in men and was lower with greater age (P < 0.001). Phase angle increased with an increase in BMI and was significantly inversely associated with percentage fat in men. Phase angle was significantly predicted from sex, age, BMI, and percentage FM in multiple regression models.
Conclusions: Phase angle differs across categories of sex, age, BMI, and percentage fat. These reference values can serve as a basis for phase angle evaluations in the clinical setting.
- Bioelectrical impedance analysis
- phase angle
- nutritional assessment
- body composition
- diagnostic methods
INTRODUCTION
Bioelectrical impedance analysis (BIA) is a noninvasive, inexpensive, and portable method that has been used mainly for body-composition analysis over the past decade. However, BIA does not measure body composition directly. It measures 2 bioelectrical parameters: body resistance and reactance. Resistance is the opposition offered by the body to the flow of an alternating electrical current, and it is inversely related to the water and electrolyte content of tissue. Reactance is related to the capacitance properties of the cell membrane, and variations can occur depending on its integrity, function, and composition (1).
BIA is considered to be a statistically derived fat-estimation method, because it depends on a regression analysis between impedance and a reference method for the development of a prediction formula (2). Many prediction equations are available to estimate body compartments as a function of resistance, reactance, anthropometric variables (weight and height), sex, and age. Prediction equations are only valid for the specific population they are developed for, which makes these equations inappropriate in clinical situations. Patients who are malnourished, who are critically ill, and who have eating disorders have a fluid imbalance; therefore, the constant hydration of lean body mass may not be acceptable (3).
Phase angle is a derived measure obtained from the relation between the direct measures of resistance and reactance (4). Phase angle is calculated directly from reactance and resistance: 
(1) Its biological meaning and pathogenic effects are not completely understood. Phase angle has been interpreted as an indicator of membrane integrity and water distribution between the intra- and extracellular spaces (4). Phase angle has also been used to predict body cell mass (5, 6); for this reason, it has also been used as a nutritional indicator in adults and children (6, 7).
(1) Its biological meaning and pathogenic effects are not completely understood. Phase angle has been interpreted as an indicator of membrane integrity and water distribution between the intra- and extracellular spaces (4). Phase angle has also been used to predict body cell mass (5, 6); for this reason, it has also been used as a nutritional indicator in adults and children (6, 7).
Some authors have studied the role of phase angle as a prognostic indicator. A positive association was shown between phase angle and survival in patients with HIV-positive AIDS (4, 8), with lung cancer (9), undergoing hemodialysis (5, 7), and who are critically ill (10, 11). These authors suggested that phase angle could be an important tool for evaluating clinical outcome or for monitoring disease progression and may be superior to other serum or anthropometric nutritional indicators.
The lack of phase angle reference values has limited its use in clinical and epidemiologic situations. Such values are needed to properly assess individual deviations in relation to the population average and to analyze the influence of phase angle on various outcomes within epidemiologic studies. We conducted the present study to understand the relation between phase angle and such variables as sex, age, race, and body-composition indicators [eg, body mass index (BMI) and percentage fat]. We also estimated population averages and SDs for phase angle to serve as reference values. With these reference values, it is possible to standardize individual values and to make comparisons between different age and sex groups in clinical or population studies.
SUBJECTS AND METHODS
Between 1986 and 1999, a study to evaluate body composition was performed at the St Luke’s–Roosevelt Hospital Center in 1967 healthy adults aged 18–94 y, who were recruited from hospital staff and the local area. All subjects were fully informed about the study objectives and methods and were asked to sign a written consent form. The Institutional Review Board of St Luke’s–Roosevelt Hospital approved the study.
The subjects were studied after fasting for ≥8 h. Several body-composition tests were performed, such as hydrodensitometry and total body water (TBW). Of direct interest to the present analysis, body weight (BW) was measured to the nearest 0.1 kg with a Weight-Tronix Scale (Scale Electronics Development, New York, NY) while each subject was wearing a hospital gown, underwear, and no shoes; height was measured to the nearest 0.1 cm with a wall-mounted stadiometer (Holtain Ltd, Crosswell, United Kingdom). BMI (in kg/m2) was calculated as body weight/height squared. BIA was performed with the use of an RJL instrument (model 101; RJL Systems, Mt Clemens, MI), which applies an 800-μA current at a frequency of 50 KHz. The measurements were performed under a strict standardization of the procedure, according to the National Institutes of Health (12). The subjects were in a supine position 5 min before the measurement, which was performed under a thermoneutral environment of 25°C. Phase angle was calculated as previously described (1). Fat mass (FM) was estimated by using the three-compartment Siri equation: 
(2) and %FM was estimated as 
(3) Total body water was obtained from tritium space (3H2O; in L) and corrected for nonaqueous hydrogen exchange. The details about these body-composition methods (TBW and hydrodensitometry) are described in detail elsewhere (2).
(2) and %FM was estimated as (3) Total body water was obtained from tritium space (3H2O; in L) and corrected for nonaqueous hydrogen exchange. The details about these body-composition methods (TBW and hydrodensitometry) are described in detail elsewhere (2).
The statistical analyses were performed by using STATA 6.0 (Stata Corporation, College Station, TX) (13). The correlations between phase angle and the other variables were estimated. The crude effect of sex, race, age, BMI, and %FM on phase angle was assessed by comparing the means of the first 2 variables (t test and ANOVA, respectively) and by using the correlation coefficients for the last 3 variables. A multiple linear regression analysis was used to adjust the effects of the variables and to identify those variables that were independently associated with phase angle. On the basis of these results, we could identify the smallest set of variables that explained most of the observed variability, so that reference values could be calculated for the smallest number of subgroups. The usual significance level of 5% was used for all tests.
RESULTS
The age, weight, height, and BMI of the 1967 study subjects are presented in Table 1⇓; 46% of the subjects were white, 22% were African American, 14% were Asian, and 18% were Hispanic or of another race. The women (58%) were significantly older than the men. The mean BMI was 25.9; and no significant difference was found between the women and the men.
View this table:
TABLE 1
Age, weight, height, and BMI of the study subjects1
Phase angle was significantly larger in the men than in the women (7.48 ± 1.10° and 6.53°± 1.01°, respectively; P < 0.001). A comparison of phase angle by race showed a significant difference in crude analysis (P < 0.001): 6.55 ± 1.10° for Asians, 6.82 ± 1.13° for whites, 7.00 ± 1.01° for multiracial subjects, 7.21 ± 1.19° for African Americans, 7.33 ± 1.13° for Hispanics, and 7.45 ± 0.98° for other races.
Phase angle showed a positive correlation with BMI (R2 = 0.17) and a negative correlation with age and %FM (R2 = −0.49 and −0.32, respectively); all correlations were significant (P < 0.001).
The final regression model obtained was rather complex and explained almost one-half of the observed variance in phase angle (R2 = 0.49). After age and sex were controlled for, race was no longer significant, which suggested that the crude association was due to confounding. Sex, age, BMI, and %FM remained associated with phase angle, including the interactions of sex with age and of BMI with %FM. However, for sex and age it was possible to achieve 82% of the variability explained by the full model (0.40 out of 0.49).
Because BMI was significantly associated with phase angle in the previous analysis, it was important to check whether the distribution of this variable in our sample was similar to its distribution in the population. We thus compared the mean BMIs, by sex and age, with the mean BMIs published by Flegal and Triano (14) with the use of population-based data from the third National Health and Nutrition Examination Survey (NHANES III). Some differences were found: men and women from the study conducted in New York had a BMI lower than that of the NHANES III value, especially those aged >50 y. (The largest mean differences in BMI were 1.7 in men and 2.3 in women.) To correct for this difference, phase angle values were adjusted by NHANES III BMI means for each age and sex category. Mean differences of 0.03 and 0.04° were found between the original and adjusted values for women and men, respectively. The largest differences were found in persons aged >70 y: –0.09° (−1.5%) in women and −0.07° (−1.1%) in men. The corrections were of no clinical relevance, and the adjustment for BMI was abandoned.
Given that sex and age accounted for most of the phase angle variability explained by available variables and that BMI and %FM are not always available in clinical situations (eg, for bedridden patients), phase angle reference values were estimated for the subgroups generated by sex and age only.
The distribution of phase angle was fairly normal in our data. Mean (±SD) phase angles, and 5th and 95th percentiles, are shown by age and sex in Table 2⇓. The overall mean phase angle mean was 6.93 ± 1.15°; 7.48 ± 1.10° for men, and 6.53 ± 1.01° for women. Phase angle was significantly greater in the men than in the women in all age categories. There was a significant and decreasing linear trend in phase angle with age, in both sexes. Phase angle decreased from 7.90° (youngest group) to 6.19° (oldest group) in men and from 7.04° (youngest group) to 5.64° (oldest group) in women.
View this table:
TABLE 2
Phase angles according to age group and sex1
DISCUSSION
Phase angle has been reported to be a prognostic tool in various clinical situations, such as HIV (4, 8), bacteremia (15), cirrhosis of the liver (16), renal disease (5, 17-19), pulmonary tuberculosis (20), and cancer (9, 21). Despite this, relatively little is known about reference values for phase angle in healthy populations. The objective of this study was to obtain phase angle values in a sample of healthy subjects who were volunteers in other body-composition studies. This fact enabled us to study not only phase angles but also the relation of phase angles to other characteristics of body composition, such as body fat measured by using reference methods.
Phase angle can be calculated as the arc-tangent of the ratio of reactance to resistance and then converted to degrees. Some authors have used a simplified equation (phase angle = reactance/resistance; converted to degrees) to obtain its value. Although not strictly correct, the simplified equation gives similar results because the ratio between reactance and resistance results in very small values (from 0.06 to 0.2 in our sample). In this situation, the arc-tangent returned a similar value, but this would not have happened if the values were larger.
The high inverse correlation with age and positive correlation with BMI were also found by Dittmar (22). The finding of a higher phase angle in persons with a higher BMI is not surprising. Phase angle is directly related to cell membranes (amount and functional status), which are what reactance stands for. Persons with higher BMIs have more cells (fat or muscle cells), and this results in higher phase angle values.
The age- and sex-related differences found in our study were not found in some previous studies. Baumgartner et al (1), in the first study of phase angle and body composition, found no significant difference in phase angle values between sex and age groups. Selberg and Selberg (16) also found no significant difference in phase angle values by sex in healthy subjects, probably because of their very small sample size (74 adults and 48 subjects aged <18 50="" a="" adults="" al="" and="" baumgartner="" class="xref-bibr" consequent="" difference="" et="" found="" healthy="" however="" href="http://ajcn.nutrition.org/content/82/1/49.full#ref-23" id="xref-ref-23-1" in="" lack="" larger="" of="" power.="" s="" selberg="" studies="" study="" subjects="" this="" was="" y="">23
The decrease in phase angle values with increasing age may suggest that phase angle is an indicator of function and general health, not only an indicator of body composition or nutritional status. The phase angle values found in a hemodialysis population were clearly smaller than those found in our healthy sample (median: 5.16° in men and 4.01° in women) (25). In the same study, the presence of diabetes resulted in phase angle values that were even smaller. A mean phase angle of 4.57° was found in lung cancer patients, and the survival of patients with a phase angle smaller than this value was significantly shorter (9). The use of standardized values found in our study makes possible the individual comparison of healthy and sick people with its age- and sex-specific phase angle mean. This approach is more likely to indicate a high-risk situation than is the comparison of individual values with the overall mean phase angle.
A study conducted in a Swiss population of healthy subjects was designed to determine reference values for fat-free mass, FM, and %FM obtained from BIA (23). In the Swiss population, phase angle values were smaller than those found in the present study (10.5% in men and 7.7% in women). Although the prevalences of overweight and obesity were lower in the Swiss study than in the US population in the present study, phase angle values remained smaller even after adjustment for BMI and %FM. This may suggest that phase angle, as other anthropometric variables, may have reference values that are specific to each population. Further studies are necessary to show how phase angle differs between different populations and whether they vary with the bioimpedance device used.
Once the sample was obtained from the subjects, we needed to know whether it could be considered representative of the US population. The adjustment for differences in the BMI distribution in the NHANES III data presented no clinically relevant effect on age- and sex-specific phase angle values. We are confident that our results can be used as reference values for the US population and possibly for other populations with similar body composition. However, the reference values for the youngest group (18–20 y of age) in our study should be used with caution because of the small sample size of each sex in this group.
Because phase angles differ by age and sex, it becomes difficult to compare values across populations of different sexes and of different age groups. One way to make such values comparable, regardless of age and sex, is to standardize them, as is commonly done with nutritional status (eg, weight is standardized for age and sex and transformed into a z score). Standardized phase angles for specific age and sex groups can be obtained by dividing mean age- and sex-specific phase angles by their SDs. Standardized phase angles have a mean of 0 and an SD of 1 for everyone and are comparable regardless of age and sex.
The prognostic role of phase angle is easier to assess if standardized values are used. Standardized phase angles on the positive side of the scale (ie, values greater than the mean) are expected for healthy subjects. Sick individuals (eg, cancer patients) are expected to have negative standardized phase angles (ie, values lower than the mean), which become increasingly lower with a worsening prognosis. The use of standardized phase angles are likely to produce better results than is the use of a single population reference value for identifying high-risk persons.
In summary, we showed that phase angle changes with sex and age. Its dependence on body composition is complex, being determined by BMI, %FM, and their interaction. The age- and sex-specific means and SDs presented in this study make it possible to calculate standardized phase angle values that make comparisons across subjects possible, even when the age and sex of the population vary widely. Also, studies of the prognostic value of phase angle in various subject groups—such as surgical, cancer, and intensive care patients—will now have access to a single set of reference values. Furthermore, cutoffs determined to identify high-risk subjects, based on standardized phase angles, will not depend on the age and sex structure of the studied samples.
Acknowledgments
We thank the Obesity Research Center (St Luke’s–Roosevelt Hospital Center), especially Frederick Rubiano (Human Body Composition Laboratory Supervisor) and Dana Kotler.
MCGB-S proposed the idea of the paper and had primary responsibility for the data analysis and the writing of the manuscript. AJDB helped choose the methodologic approach and helped with the data analysis and the writing of the manuscript. JW, SBH, and RNP Jr designed the experiment, conducted the study from which the data originated (body-composition studies), actively participated in the data analysis, and reviewed the manuscript. No conflicts of interest were declared.
Footnotes
- ↵2 Supported by CAPES, Ministry of Education, Brazil, who partially funded this research through its scholarship program.
- ↵3 Address reprint requests to MCG Barbosa-Silva, R Ariano de Carvalho, 304, 96055-800 Pelotas, RS, Brazil. E-mail: cristinagbs@hotmail.com.
- Received November 9, 2004.
- Accepted February 11, 2005.
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