Precise measurement of total body water using trace quantities of deuterium oxide

Abstract

This study was undertaken to investigate the possibility of measuring total body water in human subjects to better than ±0.5%. Accurate serial estimates of total body water were required to complement densitometric and anthropometric measurements used to monitor body compositional changes in obese patients undergoing dietary or surgical weight reduction therapy. The method required the oral administration of 1-2 g of deuterium oxide and the analysis of pre-dose and respective equilibrated samples of urine, plasma or saliva. The sample size required for analysis was 5 μl and the conversion of gaseous phase was accomplished using a uranium reduction furnace. Isotopic erichment of samples was measured using a mass spectrometer incorporating several features designed to cope with problems inherent in H₂/H²H isootopic analysis. Reproducibility of sample preparation and accuracy of the mass spectrometer were tested using international standards and shown to give an overall sensitivity of 2 parts in 10⁷ for the determination of deuterium in H₂O/H²HO mixtures. This precision has enabled us to demonstrate that isotopic fractionation of deuterium with respect to hydrogen occurs within the body and expands the potential use of this isotope for quantitative biochemical studies in the human subject.