CORRECTION APPROACHES FOR DOUBLY LABELED WATER IN SITUATIONS OF CHANGING BACKGROUND WATER ABUNDANCE

Doubly labeled water (DLW) is an accurate, portable method for measuri ng free-living energy expenditure. However, under certain conditions s hifts in baseline abundance of deuterium and oxygen-18 tracers used in the method may produce errors in derivation of both turnover (k) rate s and calculated energy expenditure. Present objectives were to examin e during what experimental situations baseline errors arise and to add ress means of correcting for such baseline shifts so that consequent e rrors in energy expenditure calculations are minimized. Under conditio ns where shifts in baseline abundance for deuterium and oxygen-18 para llel abundances corresponding to the natural meteoric water ratio, sel f-compensating changes in k values for both deuterium and oxygen will result in minimal error to the DLW energy expenditure calculations, pr ovided that the dose ratio of isotopes also mimics the meteoric water line. However, in situations where relative shifts in abundance of eac h isotope across the measurement period are not in parallel relative t o the natural meteoric water line, then the potential for larger DLW e rrors exists. Optimally, subjects should equilibrate with the new wate r source. Failing this, correction for shifting baseline can be accomp lished by measuring isotopic abundance changes in a control group of s ubjects not given the DLW dose, but performing similar tasks and consu ming the same diet as the group given DLW. Alternatively, theoreticall y based correction values can be calculated given knowledge of the abu ndances of the final drinking water and the interval time that subject s consumed the new fluid.