4 - Multi-frequency impedance as a measure of body water compartments

Summary

Introduction

Following the early work of Thomasset (1962), Hoffer et al. (1969) and Nijboer (1970), and the more recent study of Lukaski et al. (1985b), the bioelectrical impedance method has become a popular way of assessing body composition. This may be due at least in part to its simplicity. Numerous studies have shown that the methodology provides valid estimates of total body water and fat-free mass (Lukaski et al., 1985; Kushner & Schoeller, 1986; Segal et al., 1988). Prediction formulae are, however, population-specific (Deurenberg et al., 1991), and the accuracy and validity of the methodology is, as some authors have pointed out, not a real improvement over other methods such as anthropometry (Diaz et al., 1989; Deurenberg et al., 1991; Kooy et al., 1992). There is disagreement as to whether the method can predict changes in body composition (Deurenberg et al., 1989; Kushner et al., 1990; de Lorenzo et al., 1991; Vasquez & Janoski, 1991; Forbes et al., 1992). It has been demonstrated that body impedance at 50 kHz depends on the distribution between extra- and intracellular water, body compartments which differ in specific resistivity (Deurenberg et al., 1989). Consequently changes in body water that alter the ratio of extra- to intracellular water are not predicted accurately. Recently a new generation of impedance instruments has become available. They differ from the first generation in that they are able to measure body impedance at more than one frequency. Modern electronics allow the quick measurement of impedance at a series of frequencies, ranging from low (about 1 kHz) to very high (> 1 MHz).