Background – Bioimpedance spectroscopy (BIS) is recognised as a more accurate method for the assessment of body composition than single frequency bioimpedance analysis (BIA). Nevertheless the method still relies on certain assumptions, most notably the assumed values for the resistivities of intra- and extracellular water (ICW and ECW respectively). Currently used values for adults originate from a study in an Italian population using D2O and Br dilution as reference methods for total body water (TBW) and ECW respectively.
Objective – To determine resistivity constants for BIS by two methods in an Australian population.
Design – ECW was measured in 12 healthy control subjects (9 M, 3F) by Br dilution and TBW by D2O dilution or from fat-free mass measurements using DXA (Hologic Discovery) assuming an hydration constant of 0.732. Concurrently, whole body BIS measurements were performed using an SFB7 impedance instrument (Impedimed, Brisbane) and apparent resistivity constants calculated according to the mixture theory model of impedances of body fluid volumes.
Outcomes – TBWDXA and TBWD2O were highly and significantly correlated (r = 0.88, P < 0.001). The resistivity constants for ECW were 201.5 ohm.cm and 183.6 ohm.cm for males and females respectively. ICW resistivity constants differed dependent upon whether D2O or DXA was used as the reference method: 735.8 ohm.cmDXA and 770.8 ohm.cmD2O for males and 690.8 ohm.cmDXA and 757.2 ohm.cmD2O for females respectively.
Conclusions – Resistivity constants for use in BIS analysis of body composition were derived in an Australian population. The exact values of the constants depend upon the reference method used in their derivation and thus they should not be used interchangeably. It is recommended that the appropriate constants be used where body composition data, derived from BIS, are to be compared with those from either DXA or dilution studies.