Energy metabolism during cold exposure

Recent advances on the influence of cold exposure on energy metabolism in animals and humans are summarized. Although the cold-induced enhancements in carbohydrate metabolism have been the focus of numerous studies, it was only recently that pieces of evidence from animal studies have suggested that cold exposure exerts an insulin-like effect on peripheral tissue glucose uptake, which appears to proceed primarily via insulin-independent pathways. Interestingly, this phenomenon was observed in insulin-sensitive tissues of warm- a well as cold-adapted rats. Whereas previous human studies have concentrated on the cold-induced changes in basal levels of hormones and metabolic substrates, recent work from our laboratory has demonstrated that exposure to cold at rest shifts substrate utilization from mainly lipids at thermal neutrality to carbohydrates, representing the main fuel for shivering thermogenesis. Further investigation has revealed that the marked increment in carbohydrate oxidation in cold-exposed humans is derived from a greater utilization of both circulating glucose and intramuscular glycogen. With respect to lipid metabolism, recent studies have shown that the cold-induced increase in lipid oxidation in man is fuelled primarily by the fatty acids released from white adipose tissue triglycerides (TG) and possibly intramuscular TG, not plasma TG. One practical application of this work on energy metabolism in the cold resides in the pharmacological approach to improve cold tolerance, where pharmacological agents that alter energy metabolism and substrate utilization could be used to enhance cold thermogenesis and produce warmer body temperatures.