Validation of bioimpedance spectroscopy to assess acute changes in hydration status.

Abstract

To validate bioimpedance spectroscopy (BIS) as a field method for measuring small, acute changes in extracellular water (ECW) during a fluid fast/rehydration manipulation. Seventeen young adults (15 female, 2 male) participated in a 4-d fluid fast/rehydration protocol designed to induce acute changes in ECW. ECW change (DeltaECW), measured by bioimpedance spectroscopy (BIS-DeltaECW), was compared with DeltaECW, measured by bromide dilution (Br-DeltaECW), and change in body weight (DeltaBW). Regardless of hydration status, BIS-DeltaECW and Br-DeltaECW were similar; during dehydration, the mean difference between these two methods was 0.16 kg (P=0.61), and during rehydration, the mean difference was 0.58 kg (P=0.06). Assuming that DeltaECW=DeltaBW, DeltaECW as estimated from DeltaBW differed significantly from Br-DeltaECW, but not from BIS-DeltaECW. In addition, BIS-DeltaECW correlated significantly with DeltaBW (r=0.57 and 0.65 during dehydration and rehydration, respectively). However, the magnitude of DeltaECW impacted the accuracy of BIS-DeltaECW because BIS measures tended to overestimate DeltaECW at values between 0.0 and 1.0 kg and to underestimate changes at values above 1.0 kg. These findings suggest that BIS provides an accurate estimate of DeltaECW compared with bromide dilution during short-term changes in hydration.