How Bioelectrical Impedance Analysis Works
Bioelectrical impedance analysis (BIA) operates by passing a small alternating electrical current through the body and measuring the opposition to current flow, exploiting the principle that tissues conduct electricity based on their water and electrolyte content, with fluids providing the main source of resistance (R) and cellular membranes acting as capacitive elements that generate reactance (Xc). 1
Fundamental Electrical Principles
BIA leverages the distinct conductive properties of different body tissues:
- Body fluids (which contain dissolved electrolytes) act as the primary conductors and represent the main source of electrical resistance 1
- Cellular membranes and tissue interfaces act as capacitors, creating reactance that opposes the alternating current 1
- Fat and bone are relatively nonconductive compared to lean tissue 1
The method uses electrodes applied to body extremities (typically wrists and ankles) to pass a small, alternating current and measure impedance between contact points 1
Key Measured Parameters
Primary Measurements
- Resistance (R): The opposition to current flow through body fluids 1
- Reactance (Xc): The opposition created by cellular membranes and tissue interfaces 1
- Phase Angle (PhA): Calculated as (Xc/R)×(180/π), representing the arctangent between resistance and reactance 1
Clinical Significance
The phase angle serves as a biomarker positively associated with the intracellular-to-extracellular water (ICW/ECW) ratio and reflects cell membrane integrity 1
Body Composition Estimation Process
Traditional Single-Frequency Approach
The classic methodology follows this sequence:
- Total body water (TBW) estimation is derived from the measured impedance 1
- Fat-free mass (FFM) calculation assumes that 73% of FFM is water 1
- Fat mass (FM) determination is calculated as the difference between total body mass and FFM 1
This approach initially defined BIA as a hydration-dependent method, particularly for estimating fat mass 1
Advanced Multifrequency Techniques
Multifrequency BIA (bioimpedance spectroscopy) differentiates intracellular water from extracellular water compartments, which is particularly valuable for evaluating hydration variations 1
This advancement allows for:
- Assessment of fluid distribution between compartments 1
- Evaluation of skeletal muscle mass 1
- More accurate body composition estimates independent of hydration status 1
Segmental Analysis
A multisegmental BIA approach treats the body as groups of cylinders (arms, legs, trunk) to provide estimates of regional fat-free mass distribution and body fat distribution 1
Evolution Beyond Prediction Equations
Bioelectrical Impedance Vector Analysis (BIVA)
Introduced in 1994, BIVA analyzes raw bioelectrical data without relying on prediction equations 1:
- Resistance and reactance are standardized by the subject's height 1
- Values are plotted on a Cartesian plane (R on x-axis, Xc on y-axis) 1
- Vector length is inversely associated with total body water 1
- Vector direction graphically represents the phase angle 1
This approach provides semi-quantitative body composition information from tissue hydration and soft-tissue mass data 2
Critical Limitations and Caveats
Population-Specific Accuracy
The validity of BIA measurements is significantly influenced by sex, age, disease state, race/ethnicity, level of fatness, environment, menstrual cycle phase, and underlying medical conditions. 1
The American Heart Association emphasizes that BIA measurements validated for specific ethnic, racial, and clinical populations can provide accurate body fat measures in those groups but not others, highlighting the critical importance of using population-specific calibration equations 1
Hydration Dependency
Despite advances with multicomponent models, BIA accuracy remains affected by hydration status, particularly when using traditional single-frequency approaches 1
Contraindications
BIA is not recommended for patients with pacemakers due to the electrical current used 1
Practical Advantages
The method offers several clinical benefits: