Comparative study of the relationship between multi-frequency impedence and body water compartments in two European populations.

To investigate possible differences in the relationship between multi-frequency impedance and body-water compartments (total body water (TBW) and extracellular water (ECW)) measured by dilution techniques in two European populations, we studied forty Italian (twenty male and twenty female) and forty-three Dutch (twenty-three male and twenty female) healthy subjects aged 19-41 years. The main differences in body build between the two groups were height, trunk length and the two ratios TBW/height and ECW/height. Population-specific prediction formulas for ECW (at 1 kHz) and TBW (at 100 kHz) were developed. The prediction errors for ECW and TBW were about 0.6 and 1.5 kg respectively, (CV 4%) in both groups. Cross-validation analysis showed no significant error in the prediction of TBW but a slight error (range -4.9 to +2.8%) in the ECW prediction. The biases in both TBW and ECW were correlated with ECW/TBW (r -0.44, P < 0.0005 and r +0.52, P < 0.0005 respectively) in the two groups; the biases in ECW were also related to ECW/height (r 0.51, P < 0.001), TBW/height (r 0.25, P < 0.05), trunk length (r 0.36, P < 0.001) and Z1/Z100 (r 0.32, P < 0.01). In conclusion, the water distribution between the extra- and intracellular compartments emerged in the present study as the major cause of error in the prediction of body water, and in particular of ECW from impedance measurements with a population-specific equation. Moreover, body build, expressed as TBW/height and ECW/height, had an impact on the bias.