Bioelectrical impedance analysis

Authors: Eva Heinen, graduate sports scientist (Univ), Prof. Dr. med. Edgar Heinen

Bioelectrical impedance analysis (BIA) is a method for the indirect determination of body composition. It is based on the physical principles of electrical conductivity and the application of these to the complex system of the human body. The electrical resistance is measured between at least 2 and usually 4 electrodes. The resistance depends on the electrolyte/water content and the diameters.

The fat-free mass is calculated on the basis of correlations between the determined resistances and the body composition, which was determined in a control collective using a different method (e.g. the DEXA method). Since the electrical resistance of a conductor is inversely proportional to the smallest diameter of a cylinder, the result is mainly determined by the water/electrolyte content at the wrists and ankles. Minor edema after prolonged standing simulates a massive decrease in fatty tissue. Large changes in the water content in the abdomen, on the other hand, do not lead to any significant change in resistance. For example, the removal of a large ascites of several liters results in almost no change in resistance. This would mean that draining the ascites would not only reduce the FFM but also the FM of the patient.

When measuring with a dumbbell, only the upper extremity and the shoulder girdle are affected by the current, the rest of the body is irrelevant. Conversely, with the fat scale, only the lower extremity with the pelvis is included in the measurement; from the pelvic blade upwards at the latest, everything is disregarded!

The BIA is widely used due to its simple handling, cost-effectiveness, speed and low stress. Standardized measurement conditions must be adhered to, as the factors influencing the measurement are manifold. Numerous publications point out the difficulties of the BIA method for use in weight reduction, in older people, in obese people and in patients with oedema.