ADAPTATION OF THE DOUBLY LABELED WATER METHOD FOR SUBJECTS CONSUMING ISOTOPICALLY ENRICHED WATER

The use of doubly labeled water (DLW) to measure energy expenditure is subject to error if the background abundance of the oxygen and hydrog en isotope tracers changes during the test period. This study evaluate d the accuracy and precision of different methods by which such backgr ound isotope changes can be corrected, including a modified method tha t allows prediction of the baseline that would be achieved if subjects were to consume water from a given source indefinitely. Subjects in t his study were eight women (4 test subjects and 4 control subjects) wh o consumed for 28 days water enriched to resemble drinking water aboar d the United States space shuttle. Test subjects and control subjects were given a DLW dose on days 1 and 15, respectively. The change to an enriched water source produced a bias in expenditure calculations tha t exceeded 2.9 MJ/day (35%), relative to calculations from intake-bala nce. The proposed correction based on the predicted final abundance of O-18 and deuterium after equilibration to the new water source elimin ated this bias, as did the traditional use of a control group. This ne w modified correction method is advantageous under field conditions wh en subject numbers are limited.