Total energy expenditure in intravenously fed patients measured by the doubly labeled water technique

The potential clinical use of the doubly labeled water (DLW) technique to measure total energy expenditure (TEE) in intravenously fed patients was assessed by applying the technique to 13 stable patients receiving total parenteral nutrition (TPN). TEE was compared with resting energy expenditure (REE), which was measured using a ventilated hood (n = 13), and with 24-hour energy expenditure (24h EE) measured in a whole-body calorimeter (n = 6). Daily measurements of urinary enrichment in 2H and 18O showed predominantly small and covariant fluctuations in elimination rates of these two tracers. The standard error of the estimate (SEFCO2) for the CO2 production rate (FCO2), which includes additional errors due to variability in isotopic fractionation, was calculated to be 2.65% +/- 0.70% of FCO2. TPN-related changes in background isotopic enrichment in subjects who were not dosed with isotope virtually ceased 10 days after starting intravenous feeding. TEE (4.99 +/- 1.10 kJ/min), TEE/REE (1.21 +/- 0.16 kJ/min), TEE/basal metabolic rate (]BMR] 1.31 +/- 0.16 kJ/min), and TEE (isotopic)/24h EE (calorimetric; 1.07 +/- 0.18 kJ/min) showed wide variability. This study provides some evidence for the validity of the DLW method in selected patients receiving TPN, since significant deviations from the assumptions of the method were not found in the subjects studied. The error associated with 2H sequestration during fatty acid synthesis is likely to be small, but could not be accurately measured. The contributions of activity and thermogenesis to TEE varied greatly, but overlapped with those reported in normal subjects.