The Science of Bioelectrical Impedance Analysis (BIA)

What is the principle of BIA?

BIA is a method of quantifying body composition by introducing an electrical current throughout the body. Impedance can be calculated by measuring current and voltage, based on Ohm’s Law (R=V/I). Since water is considered as the only component in the body with electrical conductivity, when the electrical current passes through the body, the impedance of its water can be measured. With this impedance value, the volume of body water is calculated.

The volume V of the solution in a tube can be measured by multiplying area A and the length L of the tube. Since the height is already given, the area of the tube needs to be calculated to get the volume of the solution in the tube.

The impedance is proportional to the length or the height and inversely proportional to the area. By applying the constant of the proportion q (specific resistivity), the impedance can then be calculated as follows:

Using Formula 1, a formula to calculate the area of the tube can be produced

When substituting Formula 3 for Formula 1, the volume of the solution can be calculated from its impedance and the height of the tube as follows

Therefore, the volume of the solution in the tube can be calculated using the height of the tube and its impedance. The value calculated from L²/R represents the impedance index. L can be applied to the height of the examinee, and R to the measured impedance of the examinee. As such, the volume of body water (which is mainly composed of salt solution) can be calculated by measuring the impedance and the height of the examinee. From the volume of body water, lean body mass and fat mass can be calculated.

What are the advantages and drawbacks of BIA?

Over the past decade, a technique has been developed which analyzes body composition based on the electrical properties of biological tissues. BIA has many advantages over other methods in that it is safe, non-invasive, efficient, and highly reproducible. Thus, the technique has become widely used in hospitals, health and fitness centers, and in field studies.

Nevertheless, the clinical usefulness of conventional BIA to detect acute or chronic changes in body composition has been limited due to localized fluid accumulation or loss, and inability to accurately assess the distribution between intracellular water and extracellular water. Because of these past deficiencies, BIA experienced challenges applying to patients, the elderly, children, and athletes – many of those that could benefit most by having their body composition analyzed.

The prior limited capabilities of BIA have been overcome via technological advancements validated by clinical research studies. As the human body is not an isotropic conductor with a uniform cross-section area, it must be analyzed as five cylindrical rods (four limbs and one torso). This segmental analysis allows for the precise measurement of body water. Multi-frequency is employed to accurately measure and differentiate between intracellular water and extracellular water. Empirical estimations are not needed to compensate for inaccuracy, which have historically made the measurement insensitive to slight compositional changes.

Accurate measurement of total body water, the key to body composition analysis

How does BIA measure your body fat?

Impedance is an opposing force against the flow of electric current. Body fat, compared to body water such as blood, has much weaker conductivity and on that account, has relatively high impedance. Therefore, between two people of the same weight, the person with the greater portion of body fat will show higher impedance. By using the segmental BIA method, the impedance values for each limb and the trunk are measured separately.

Then, how is it possible to measure body fat in your body?

All body composition analyzers using BIA measure the volume of total body water. As it is inversely proportional to impedance, it is feasible to get the volume of the total body water from obtained impedance. Moreover, fat free mass (FFM) in a healthy body always consists of 73.3% water, which is constant for any race or gender. Therefore, with the volume of the total body water known, the amount of FFM can be calculated and by subtracting the calculated amount of FFM from weight, we get the amount of body fat as well.