Purpose: To validate bioimpedance spectroscopy (BIS) as a field method for measuring small, acute changes in extracellular water (ECW) during a fluid fast/rehydration manipulation.
Methods: 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 (ΔECW), measured by bioimpedance spectroscopy (BIS-ΔECW), was compared with ΔECW, measured by bromide dilution (Br-ΔECW), and change in body weight (ΔBW).
Results: Regardless of hydration status, BIS-ΔECW and Br-ΔECW 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 ΔECW = ΔBW, ΔECW as estimated from ΔBW differed significantly from Br-ΔECW, but not from BIS-ΔECW. In addition, BIS-ΔECW correlated significantly with ΔBW (r = 0.57 and 0.65 during dehydration and rehydration, respectively). However, the magnitude of ΔECW impacted the accuracy of BIS-ΔECW because BIS measures tended to overestimate ΔECW at values between 0.0 and 1.0 kg and to underestimate changes at values above 1.0 kg.
Conclusion: These findings suggest that BIS provides an accurate estimate of ΔECW compared with bromide dilution during short-term changes in hydration.