A rapid analytical technique for the determination of energy expenditure by the doubly labelled water method

The doubly labelled water method involves the administration of water enriched in 2H and 18O followed by determination of the turnover rates of these isotopes. Since 18O is eliminated from the body as both CO2 and water, while 2H leaves only as water, the difference between the two turnover rates provides a measure of CO2 production and hence energy expenditure. Isotopic analysis by conventional stable isotope ratio analysis (SIRA) is labour intensive and time consuming, as it requires off-line conversion of water samples to gases (H2 and CO2) followed by sequential analysis for each of the two isotopes using the mass spectrometer. Lack of suitable automated instrumentation with the ability to process large numbers of samples has prevented routine application of the method. We describe here an automated technique in which body water samples (urine, saliva, breath water or milk) are analysed simultaneously for 2H and 18O. The single bench system comprises two mass spectrometer analysers, one for measuring 2H from H2 gas, the other for measuring 18O from the water vapour (masses 18, 20). Both analysers share a common heated inlet system into which microlitre quantities of the body fluids are injected from an autosampler (102 samples). The water vapour flows both directly to one analyser for 18O measurement and into a uranium reduction furnace for conversion to H2, prior to 2H measurement by the second analyser. Both analysers also share vacuum and electronic components, enabling savings in both space and cost. In this paper we present results illustrating performance characteristics and procedures for routine application to human subjects.(ABSTRACT TRUNCATED AT 250 WORDS)