Doubly labeled water measurement of human energy expenditure during exercise at high altitude.

Estimates of total daily energy expenditure (TDEE) by the doubly labeled water (DLW, 2H(2)18O) and intake balance (I-B) methods were compared in six male soldiers studied over 6 days that included 5 days of strenuous winter exercise at 2,500- to 3,100-m elevation. Use of body energy stores [-9.54 +/- 1.54 (SD) MJ/day or -2,280 +/- 368 kcal/day] was estimated from changes in body weight, body density (hydrodensitometry), and total body water (H(2)18O dilution). The subjects wore computerized activity monitors and kept daily records of ration consumption (9.87 +/- 3.60 MJ/day or 2,359 +/- 860 kcal/day). Accuracy of individual DLW and I-B TDEE values was estimated from the correlations of TDEE with fat-free mass (FFM) or total weight (body wt + load). The DLW and I-B estimates of TDEE differed by -12.0 to 15.2% but provided comparable estimates of group mean TDEE (DLW = 19.07 +/- 2.37 MJ/day or 4,558 +/- 566 kcal/day; I-B = 19.41 +/- 3.72 MJ/day or 4,639 +/- 889 kcal/day; P > 0.05). The DLW TDEE was correlated with both FFM (r2 = 0.89, P < 0.01, power = 0.95) and total weight (r2 = 0.95, P < 0.01, power = 0.99), whereas I-B TDEE was correlated only with total weight (r2 = 0.75, P < 0.03, power = 0.81). Under adverse field conditions the DLW method provided individual TDEE estimates that were probably more accurate than those provided by the I-B method.