Evaluation of doubly labeled water for measuring energy expenditure during changing nutrition.

Change in abundance of 2H and 18O in dietary water during a doubly labeled water energy period may introduce error into the calculated carbon dioxide production rate (RCO2). To examine the accuracy of 2H2(18)O during changing nutritional regimens, we compared 2H2(18)O and periodic open-circuit respiratory gas exchange (RGE) in postsurgical infants who were changing from parenteral to alternative parenteral and/or oral nutrition. The two methods were compared before and after correction for shifts in isotopic abundance of the infant water pools during the energy-expenditure period. Baseline corrections were predicted using the difference between abundances of the initial body water and final nutrient solutions. Before isotopic correction, 2H2(18)O underestimated RCO2 in eight subjects by 11.8 +/- 20.1% (mean +/- SD). After correction, agreement between the two methods improved; the underestimate was then -8.7 +/- 12.9%. To obtain maximum precision of 2H2(18)O, subjects should be maintained on the same nutritional regimen before and during the study unless valid correction formulae are used.