Impact of extreme cold temperature on acute metabolic response in humans

Our ancestors survived in an unforgiving environment dominated by ice ages. Nevertheless, because climate change is considered to be one of the largest threats to humanity in the future, how to survive the next ice age is an important topic for study. Humans have evolved the function of sustaining constant temperature in a variety of circumstances. To survive in extreme environments, the body has to adapt physiologically. When the body is repeatedly exposed to different environments, resistance to new stress is increased. In cold-exposed adult humans, significant decreases in body temperature are delayed by reducing rates of heat loss via peripheral vasoconstriction and by increasing rates of heat production via shivering and non-shivering thermogenesis. Shivering is elicited by exposure to cold air and this can increase the resting metabolic rate. Fuel selection mechanisms are responsible for sustaining shivering thermogenesis. It has been reported that over a 3-month trip in Antarctica, subjects lost more than 25% of body weight, despite an average energy intake of 21.3MJ/day (Stroud et al. 1997). However, the adaptation process of the biological systems of humans to Antarctic and other cold environments is not well understood. Cold environments have many different effects on the body, such as pronounced metabolic changes, and we are especially interested in physiological adaptations under those conditions. We have studied how acute cold stress affects human metabolism, carried out in a -15°C cold room. The results of this study showed that an acute cold environment at -15° increased resting metabolism and fat oxidation, but carbohydrate oxidation was not influenced. Extreme shivering appeared in some subjects during cold exposure at -15°, but not in all subjects. For psychological stress activities, the levels of s-amylase activity and cortisol showed a significant increase at -15°. Further investigations are needed to optimize nutrients and energy balance for extreme cold environments and to understand the mechanism underlying the combined effects of physiological and neurological responses to extreme cold stress. This study was supported partly by the Grant for the Polar Research Phase VIII Project of the National Institute of Polar Research and the Joint Research Program of the Institute of Low Temperature Science, Hokkaido University.