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

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 (?ECW), measured by bioimpedance spectroscopy (BIS-?ECW), was compared with ?ECW, measured by bromide dilution (Br-?ECW), and change in body weight (?BW). Results: Regardless of hydration status, BIS-?ECW and Br-?ECW 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 ?ECW = ?BW, ?ECW as estimated from ?BW differed significantly from Br-?ECW, but not from BIS-?ECW. In addition, BIS-?ECW correlated significantly with ?BW (r = 0.57 and 0.65 during dehydration and rehydration, respectively). However, the magnitude of ?ECW impacted the accuracy of BIS-?ECW because BIS measures tended to overestimate ?ECW 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 ?ECW compared with bromide dilution during short-term changes in hydration