Comparative measurements of total body water in healthy volunteers by online breath deuterium measurement and other near-subject methods1–3

Background: We developed a new near-subject approach, using flowing afterglow–mass spectrometry (FA-MS) and deuterium dilution, which enables the immediate measurement of total body water (TBW) from single exhalations. Objectives: The objectives were to show the efficacy of the new FA-MS method in measuring TBW in healthy subjects and to compare these measurements with values derived from multifrequency bioelectrical impedance analysis, skinfold-thickness (SFT) measurements, and both recent and historical published regression equations. Design: After baseline measurement of breath deuterium abundance, 24 healthy subjects ingested 0.3 g D2O/kg body wt. A second breath sample was taken after 3 h to measure the increase in deuterium, from which TBW was calculated. Bioelectrical impedance analysis was carried out with a multifrequency analyzer, and SFT was measured by a single trained observer. Methods were compared with the use of Pearson’s correlation coefficient and Bland-Altman analyses. Results: TBW measures obtained by all methods were highly correlated (r = 0.95–0.98, P < 0.001), especially those between FA-MS, SFT measurement, and recent regression equations. The mean values obtained were within 2% of those published for agematched control subjects and varied by 1–6% when all methods were compared. Systematic bias was greatest when FA-MS was compared with bioelectrical impedance analysis, which tended to underestimate TBW in smaller, female subjects. No bias related to subject size was observed in a comparison of FA-MS with SFT measurement or with more recent regression equations. Conclusions: FA-MS is a simple and effective new approach to TBW measurement in healthy subjects. The difficulty of using population-derived equations to estimate TBW in individual subjects is emphasized. Am J Clin Nutr 2002;76:1295–301.