Measurement of fractionated water loss and CO2 production using triply labelled water.

The doubly labelled water (DLW) method for measuring CO2 production has recently been the subject of much interest since no other technique gives integrated values for CO2 production over long periods by free-living subjects. The importance of evaporative water loss and fractionation factors to the calculation of CO2 production using this technique is described. Present methods of estimating evaporative water loss and the use of fractionation factors are summarized together with a discussion of their limitations. A novel technique is proposed whereby water labelled with three isotopes can be used to measure evaporative water loss and CO2 production in completely free-living subjects, and the feasibility of the method is tested in simulations using experimental data. This technique has three advantages over existing methods of estimating evaporative water loss: (1) it can be used in completely free-living subjects without any additional experimental procedures (e.g. water-balance studies or physical trapping of water vapour); (2) it gives a direct estimate of fractionated evaporative water loss in each subject, since non-fractionated water lost as vapour is automatically compensated for; and (3) the routes of water loss do not have to be known. The appropriate calculations are presented together with a discussion of the difficulties of measuring oxygen-17 by mass spectrometry. It is estimated that the maximum theoretical error on calculated CO2 production is +/- 0.3%. Practical ways of achieving this theoretical level of accuracy are suggested. We conclude that the proposed technique will allow correction for evaporative water loss to be made more exactly, thereby increasing the accuracy of the heavy water technique for measuring CO2 production in free-living subjects.