Does multifrequency bioelectrical impedance relate to body composition?

Background: There is need for a rapid, noninvasive, inexpensive yet accurate bedside technique to measure body composition. Bioelectrical impedance analysis measures the resistance and conductance of a weak electrical current passed through the body. It has been suggested that multifrequency impedance analysis can determine the distribution of fluid between the extracellular and intracellular compartments. Materials and Methods: The correlation between the resistance signals obtained from multifrequency bioelectrical impedance measurements and body composition was determined in normal rats ranging from 150 to - 400 g in weight. Total body water, body fat, total body sodium, and total body potassium were measured using the carcass analysis technique, and extracellular water was derived from a dilutional marker using sodium bromide. Fat-free mass was calculated as the difference between body weight and body fat, and intracellular water was derived from total body water and extracellular water. Multifrequency bioelectrical impedance was measured at frequencies ranging from 3 to 300 kHz. Resistance at zero frequency and infinite frequency was calculated using the Cole and Cole equation. Results: Resistance index (i.e., length of the animal(2)/resistance) was highly correlated with all body compartments (r = 0.879-0.996) at all frequencies. There was also a high correlation among all compartments of the body (r = 0.971-0.999). Because of this high intercorrelation among the body compartments, a specific relationship between the multifrequency bioelectrical impedance signal and a specific compartment was not identified. Conclusions: Resistance index at any frequency will be correlated with any body compartment in a normal population. The utility of this technique should be determined in situations where there is a disturbance in the relationship between various compartments of the body. (C) 1996 Academic Press, Inc.