What Is Bioimpedance and How Does It Work?

Bioelectrical Impedance Analysis (BIA) is a non-invasive technique widely used in health and fitness settings to estimate various biological properties. It works by introducing a tiny, safe electrical current into the body and measuring the opposition, or impedance, that the tissues present to its flow. This opposition reflects the body’s composition, as different types of tissue have varying abilities to conduct electricity. The resistance encountered allows the analyzing device to make calculations about the body’s makeup.

The Physics Behind Bioimpedance Measurement

The human body functions as a complex electrical circuit, which is the foundation of bioimpedance measurement. Tissues containing large amounts of water and dissolved electrolytes, such as muscle and blood, act as good electrical conductors, while fat tissue, which contains little water, acts more like an insulator. The BIA device measures two primary components of the body’s electrical impedance: resistance and reactance.

Resistance

Resistance is the direct opposition to the flow of the electrical current, primarily driven by the amount of water in the body. Since most of the body’s water is contained within muscle and lean tissues, a lower resistance reading indicates a higher proportion of Total Body Water and, by extension, Fat-Free Mass. This measurement is fundamental because water is the medium through which the current travels.

Reactance

Reactance represents the temporary storage of electrical energy by cell membranes, which act like tiny capacitors. This is a measure of the integrity and quality of the cell membranes. Healthy, intact cells are better at storing this charge, resulting in a higher reactance reading.

Phase Angle

The relationship between resistance and reactance is mathematically expressed as the Phase Angle. This angle is derived from the time delay between the introduction of the electrical current (voltage) and its return (amperage). A higher phase angle is associated with greater cell membrane integrity and a larger body cell mass, often serving as an indicator of cellular health and nutritional status.

Metrics Derived from Bioimpedance Analysis

The raw measurements of resistance and reactance serve as the foundation for calculation. The device converts these electrical readings into understandable body composition metrics using prediction equations. These equations combine the impedance data with personal variables like height, weight, age, and sex to produce estimates.

Total Body Water (TBW)

Total Body Water (TBW) is the most direct biological quantity estimated from the resistance measurement. Because water and electrolytes are excellent conductors, the measured resistance is inversely proportional to the volume of water in the body. This TBW measurement is the starting point for calculating all other body composition metrics.

Fat-Free Mass (FFM)

The estimate for Fat-Free Mass (FFM), also called Lean Body Mass (LBM), is derived directly from the calculated TBW. This is possible because water is a relatively constant fraction of the fat-free mass, typically around 73%. FFM includes muscle, bone, organs, and all body water.

Body Fat Mass

Body Fat Mass is the final metric calculated by subtracting the estimated Fat-Free Mass from the person’s total body weight. The result is expressed as a percentage of total body weight. BIA does not measure fat directly but estimates it based on the measured electrical properties of the water-rich lean tissues.

Key Differences in Measurement Devices

BIA devices vary significantly in their complexity and the specificity of their measurements, mainly depending on their intended use.

Consumer Devices

Consumer-grade devices, such as smart scales and handheld analyzers, typically use a simple two-electrode configuration. These devices often measure only the impedance across the feet (foot-to-foot) or the hands (hand-to-hand). This simple configuration means the current takes the path of least resistance, often only measuring a segment of the body, like the legs or arms. Smart scales primarily measure the lower half of the body, which can make the whole-body composition estimate less precise. They usually employ a single-frequency current, which measures the total body water but is less effective at differentiating between water inside and outside the cells.

Professional Devices

Clinical and professional devices, in contrast, often utilize a four-electrode (tetrapolar) or eight-electrode (octopolar) setup that measures the impedance of the entire body from hand to foot. These advanced analyzers frequently use Multi-Frequency BIA (MF-BIA), sending multiple electrical frequencies through the body. The use of different frequencies allows for a more accurate distinction between the water within the cells and the water outside the cells.

This multi-frequency, multi-electrode approach enables segmental analysis, providing separate body composition estimates for each limb and the trunk. This level of detail offers greater accuracy for evaluating fluid distribution and lean mass, making these devices the preferred choice for clinical and research applications. Professional devices focus on precision and the ability to access raw impedance data for robust analysis.