Background: Bioelectrical impedance spectroscopy (BIS) is an attractive method for measuring body composition because it is noninvasive, simple, and cheap. The effect of obesity on the accuracy of impedance measurements has been recognized for some time, but no conclusive explanations or ways to correct the measurement errors have been published. We studied the effect of the composition of weight loss on the accuracy of BIS to measure changes in body fluid volumes during severe weight loss. Within subjects the effect of variable losses of fat mass was studied.
Methods: In 10 morbidly obese female subjects who underwent gastric reduction surgery, changes in total body water (TBW) and extracellular water (ECW) were monitored for 1 year by deuterium (Deu) and bromide (Br) dilution and by BIS. Measurements were performed before the operation and after 2 weeks, 3 months, and 1 year. Extrapolated resistance values of extracellular (Recw) and intracellular water (Ricw) were used in mixture equations for calculating the corresponding fluid volumes.
Results: After 1 year, weight decreased by 53 kg, TBW (Deu) loss was 8.7 L, and ECW (Br) loss was 4.3 L. Comparison of BIS with reference methods for measuring all possible changes over 6 time-intervals revealed a mean overestimation of TBW (2.4 L, SD = 2.9) and ECW (0.74 L, SD = 2.6) losses by BIS. Overestimation increased significantly with increasing fat losses, expressed as percentage fat of the weight loss and as change in triceps skinfolds. Measured changes in Recw and Ricw were less than expected for an ideal agreement between dilution methods and mixture equations.
Conclusions: BIS with the use of mixture equations overestimates fluid losses during weight loss. The error is associated with the amount of fat loss. The large contribution of the factor weight in the mixture equations is likely to be responsible. The assumptions of mixture theory are not valid in obesity.