Worth the wait? Time course of supine shifts in body water compartments on variables of bioelectrical impedance analysis

Bioelectrical impedance analysis (BIA) reference values are based on supine assessments. Little is known regarding the effects of time course shifts in body water compartments after assuming a supine position. The aim of this study was to characterize these effects and provide recommendations regarding the optimal waiting time to perform BIA. Thirty-eight healthy adults underwent BIA via the RJL Quantum Legacy analyzer immediately upon lying down and every 5 minutes for 15 minutes. Differences in resistance (R), reactance (Xc), intracellular (ICW), extracellular (ECW), total body water (TBW), body fat percentage (%BF), and phase angle (PhA) were assessed. There were small but significant increases in R, Xc, and %BF (all p<0.001), as well as small but significant decreases in ICW, ECW, and TBW (all p<0.001) over 15 minutes. No difference was observed for PhA (p=0.065). Average values changed over 15 minutes by +7.14Ω, +1.36Ω, -0.2L, -0.2L, -0.4L, +0.05° and +0.1% for R, Xc, ICW, ECW, TBW, PhA and %BF, respectively. BIA measurements are affected by shifts in body water compartments after assuming a supine position, but these differences lack clinical significance in healthy adults. Technicians working with healthy adults can perform BIA within 15 minutes after participants assume a supine position.