Body water distribution in severe obesity and its assessment from eight-polar bioelectrical impedance analysis

Objective: To measure body water distribution and to evaluate the accuracy of eight-polar bioelectrical impedance analysis (BIA) for the assessment of total body water (TBW) and extracellular water (ECW) in severe obesity. Design: Cross-sectional study. Setting: Obesity clinic. Subjects: In all, 75 women aged 18-66y, 25 with body mass index (BMI) between 19.1 and 29.9 kg/m2 (ie not obese), 25 with BMI between 30.0 and 39.9 kg/m2 (ie class I and II obese), and 25 with BMI between 40.0 and 48.2 kg/m2 (ie class III obese). Methods: TBW and ECW were measured by 2H20 and Br dilution. Body resistance (R) was obtained by summing the resistances of arms, trunk and legs as measured by eight-polar BIA (InBody 3.0, Biospace, Seoul, Korea). The resistance index at a frequency of x kHz (RIx) was calculated as height 2/Rx. Results: ECW : TBW was similar in women with class III (46 ± 3%, mean ± s.d.) and class I-II obesity (45 ± 3%) but higher than in nonobese women (39 ± 3%, P<0.05). In a random subsample of 37 subjects, RI500 explained 82% of TBW variance (P<0.0001) and cross-validation of the obtained algorithm in the remaining 38 subjects gave a percent root mean square error (RMSE%) of 5% and a pure error (PE) of 2.11. In the same subjects, RIs explained 87% of ECW variance (P<0.0001) and cross-validation of the obtained algorithm gave a RMSE% of 8% and a PE of 1.4 I. The contribution of weight and BMI to the prediction of TBW and ECW was nil or negligible on practical grounds. Conclusions: ECW : TBW is similar in women with class I-II and class III obesity up to BMI values of 48.2 kg/m2. Eight-polar BIA offers accurate estimates of TBW and ECW in women with a wide range of BMI (19.1-48.2 kg/m2) without the need of population-specific formulae. © 2005 Nature Publishing Group. All rights reserved.