Validation of bioimpedance spectroscopy to assess acute changes in hydration status

Abstract

In this study bioimpedance spectroscopy (BIS) was validated as a field method for measuring short-term, small changes in hydration status by comparing extracellular water change (ΔECW) estimated by BIS with a criterion method (bromide dilution), dual energy X-ray absorptiometry (DXA) and body weight (BW). A secondary aim was to compare BW to bromide dilution as a method for estimating acute ΔECW. Finally, BIS was compared to DXA and single frequency bioimpedance analysis (SF-BIA) instruments to assess acute hydration effects on body composition estimates. During dehydration, no significant differences were found between bromide and BIS measures of ΔECW. The ΔECW measured by DXA (DXA-ΔECW) and BW (BW-ΔECW) was significantly different from bromide-estimated ΔECW (Br-ΔECW), but not from BIS estimates (BIS-ΔECW). During rehydration, there were no significant differences between Br-ΔECW and the other methods. When using BW as the reference, results were more consistent in that BW-ΔECW was significantly correlated with both BIS-ΔECW and DXA-ΔECW regardless of hydration status. These findings suggest that bromide may not be an appropriate criterion method for estimating short-term changes in hydration status. Regardless of hydration status, BIS provided accurate measures of fat-free mass (BIS-FFM) and fat mass (BIS-FM) that were comparable to, or better than, estimates by SF-BIA. At baseline and after dehydration BIS-FFM had the highest correlation with DXA estimates (DXA-FFM), although two SF-BIA instruments (Bio-Resistance Body Composition Analyzer from Valhalla Scientific and The Body Comp Scale from American Weights & Measures) produced good estimates of FFM. Rehydration appeared to affect the accuracy of FFM measurements by BIS and SF-BIA as evidenced by lower, more moderate correlations to DXA-FFM. Phase-dependent effects on percentage body fat (%BF) estimates were also apparent. In contrast, all methods performed reasonably well for estimates of FM, regardless of hydration status. In summary, BIS provides accurate estimates of ΔECW compared to either bromide dilution or BW, especially in the direction of dehydration. BIS also provides accurate estimates of FFM and FM regardless of hydration status. Further study of bromide dilution as a criterion measure is needed to validate its use in measuring ΔECW during acute shifts in hydration.