Metabolic and endocrine responses to cold air in women differing in aerobic capacity.

Purpose: The purpose of this study was to measure resting metabolic rate, plasma norepinephrine, and plasma immunoreactive beta endorphin during exposures to cold air during two consecutive 5-d periods, separated by 2 weekend days, in two groups of women differing in aerobic fitness.

Methods: Plasma norepinephrine (NE), plasma immunoreactive beta-endorphin (IBE), and resting metabolic response (RMR) were measured during repeated exposures to 3.5 degrees C air in two groups of women differing in aerobic fitness. Ten women, separated into highly fit (HFW) and less fit (LFW) groups, sat in 22 degrees C air for 45 min followed by 45 min in 3.5 degrees C air each day during two consecutive 5-d periods separated by two weekend days.

Results: Norepinephrine was not different between groups during warm air exposure; however, following 45 min of cold air, NE was two times higher in HFW compared with that in LFW (P < 0.001). Plasma IBE was elevated (P < 0.02) in HFW compared with that in LFW but was not affected by exposure to cold on any test day. Warm RMR was not different between groups and remained unchanged during the study period. Cold RMR was significantly higher in LFW compared with that in HFW (P < 0.01). Resting metabolic rate peaked at 30% of VO2peak in LFW by the 5th minute of cold exposure on day 1 before declining to 21% and remaining steady. In contrast, RMR in HFW peaked at about 13% and then fell to 9.4% before slowly increasing to 14% by the end of 45 min. On other test days HFW RMR increased to 14% of VO2peak and rose slowly through 45 min of cold exposure while LFW RMR peaked at 24% of VO2peak before declining to 20% and remaining steady.

Conclusions: Our findings suggest that, in women, aerobic fitness alters the endocrine and metabolic responses to acute cold air exposure. The norepinephrine response is exaggerated in highly fit women exposed to cold but not the metabolic response. Immunoreactive beta endorphin was not affected by exposure to cold but was elevated in highly fit women. We further conclude that the temperature threshold for acclimation to cold air by women may be higher than the air temperature used in this study.