Multi-frequency bioelectrical impedance for assessing total body water and extracellular water in elderly subjects.

OBJECTIVE

The objective of this study was to investigate the utility of multifrequency bioelectrical impedance for the estimation of total body water (TBW) and extracellular water (ECW) in an elderly population.

DESIGN

Body impedance at 8 frequencies (1-1350 kHz) was measured in duplicate within 1 week. TBW and ECW were determined using deuterium oxide and potassium bromide dilution respectively.

SETTING

At the Department of Human Nutrition.

SUBJECTS

81 women and 36 men, aged 63-87 years, volunteered as subjects.

RESULTS

Application of prediction equations from the literature, which are mostly based on impedance measurements in young and middle-aged subjects, resulted in large prediction errors of TBW and ECW which were related to the water distribution of the body. New sex-specific prediction equations for the estimation of TBW and ECW were developed for the elderly population and internally validated in random sub-groups. TBW and ECW were best predicted using impedance at frequencies of 5 and 50 kHz respectively, and by using body weight. Prediction errors for TBW were 3.1 kg (7.3%) and 2.7 kg (8.5%) and for ECW 2.2 kg (12.3%) and 1.0 kg (7.4%) for men and women respectively.

CONCLUSIONS

Multi-frequency impedance measurements are useful to assess TBW and ECW in groups of elderly subjects. However, the prediction errors are larger compared to young and middle-aged subjects, and are related to body water distribution. Individual errors are sometimes unacceptably large.