Eighty-four hours of sustained operations alter thermoregulation during cold exposure

Purpose: This study examined the effects of short-term (3.5 d) sustained military operations (SUSOPS) on thermoregulatory responses to cold stress. Methods: Ten men (22.8 +/- 1.4 yr) were assessed during a cold-air test (CAT) after a control week (control) and again after an 84-h SUSOPS (sleep = 2 h(.)d(-1), energy intake = similar to1650 kcal(.)d(-1), and energy expenditure = similar to4500 kcal(.)d(-1)). CAT consisted of a resting subject (seminude) being exposed to an ambient temperature ramp from 25degreesC to 10degreesC during the initial 30 min of CAT, with the ambient temperature then remaining at 10degreesC for an additional 150 min. Results: SUSOPS decreased (P < 0.05) body weight, % body fat, and fat-free mass by 3.9 kg, 1.6%, and 1.8 kg, respectively. During CAT, rectal temperature decreased to a greater extent (P < 0.05) after SUSOPS (0.52 +/- 0.09degreesC) versus control (0.45 +/- 0.12degreesC). Metabolic heat production was lower (P < 0.05) after SUSOPS at min 30 (55.4 +/- 3.3 W(.)m(-2)) versus control (66.9 +/- 4.4 W(.)m(-2)). Examination of the mean body temperature-metabolic heat production relationship indicated that the threshold for shivering was lower (P < 0.05) after SUSOPS (34.8 +/- 0.2degreesC) versus control (35.8 +/- 0.2degreesC). Mean weighted skin temperatures (degreesC) were lower during the initial 1.5 h of CAT in SUSOPS versus control. Heat debt was similar between trials. Conclusion: These results indicate that sustained (84-h) military operations leads to greater declines in core temperature, due to either a lag in the initial shivering response or heat redistribution secondary to an insulative acclimation.