Field use of D2 18O to measure energy expenditure of soldiers at different energy intakes.

To test the application of doubly labeled water under adverse field conditions, energy expenditures of 16 special operations soldiers were measured during a 28-day field training exercise. Subjects were matched by fat-free mass and divided equally between an ad libitum ready-to-eat meal diet and a 2,000 kcal/day lightweight ration. Subjects recorded intakes daily, and body composition was measured before and after the exercise. At the beginning of the study, subjects moved to a new northerly location and, therefore, a new water supply. To compensate for this, a group of soldiers who did not receive heavy water was followed to measure isotopic base-line changes. Energy expenditure by doubly labeled water was in agreement with intake/balance (3,400 +/- 260 vs. 3,230 +/- 520 kcal/day). The overall coefficient of variation of energy expenditure by doubly labeled water was half that of intake/balance (7.6 vs. 16.1%). The coefficient of variation of repeat measures with doubly labeled water was 7.3%. Energy expenditure of the ready-to-eat meal group, 3,540 +/- 180 kcal/day, was not significantly different from the lightweight ration group, 3,330 +/- 301 kcal/day. Doubly labeled water was valid under field conditions.