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RUBY, B. C., T. C. SHRIVER, T. W. ZDERIC, B. J. SHARKEY, C. BURKS, and S. TYSK. Total energy expenditure during arduous wildfire suppression. Med. Sci. Sports Exerc., Vol. 34, No. 6, pp. 1048–1054, 2002.The purpose of this investigation was to determine the total energy expenditure (TEE) by using the doubly labeled water (DLW) methodology during 5 d of wildfire suppression in Montana, California, Florida, Washington, and Idaho.Seventeen wildland firefighters (from three Interagency Hot Shot crews, N = 8 men, height = 177 ± 7 cm, weight = 74.6 ± 6.4 kg, age = 24.5 ± 1.8 yr;N = 9 women, height = 170 ± 7 cm, weight = 65.2 ± 8.0 kg, age = 25.0 ± 1.3 yr) served as subjects. Before wildland fire suppression, each subject was given an oral dose of 2H2O and H218O (approximately 0.23 g 2H2O·kg estimated TBW−1 and 0.39 g H218O·kg estimated TBW−1). Urine samples were collected between 0400 and 0600 daily. TEE was calculated using the two-point method for days 1–3 and 1–5, with the TEE for days 4–5 calculated by extrapolation. Urine samples from other crew members not participating in the DLW protocol were collected at the same times and used to adjust calculations of isotopic elimination for background shifts.TEE was 17.4 ± 3.7 and 17.5 ± 6.9 MJ·d−1 during days 1–3 and 4–5, respectively. The energy expenditure associated with physical activity (EEA) was 8.8 ± 3.0 and 8.9 ± 6.1 MJ·d−1 for days 1–3 and 4–5, respectively.The current data demonstrate consistently high daily energy expenditure in the wildland firefighter. These data also demonstrate that the doubly labeled water methodology is an appropriate methodology for the measure of TEE during unpredictable field operations if adjustments are made for changes in background enrichment and elevated water turnover.