Rates of energy substrates utilization during human cold exposure

Although it is well established in animals that acute cold exposure markedly increases the oxidation of energy substrates, the absolute quality and quantity of substrate oxidation is poorly understood in humans. This study compared the rates of substrate utilization in seven healthy young men exposed to both the warm (control exposure at 29 degrees C; semi-nude, 14 h fasted) and to the cold for 2 h (10 degrees C, 1 m.s-1 wind velocity). Substrate utilization was calculated using indirect calorimetry and the nonprotein respiratory exchange ratio, which was derived from the urinary urea nitrogen output. Cold exposure induced a 3.1 +/- 0.2 degrees C drop in mean body temperature and a body heat debt of 825.9 +/- 63.3 kJ (p less than 0.01). These parameters remained essentially unchanged in the warm. Cold exposure elevated the 2 h energy expenditure 2.46-fold in comparison to the warm (p less than 0.01). This cold-induced thermogenesis was accompanied by increases of 588% in carbohydrate oxidation (p less than 0.01) and 63% in fat oxidation (p less than 0.05), whereas protein oxidation remained unchanged. Although the greatest proportion of the energy expenditure in the warm was derived from lipid (59%), carbohydrate oxidation represented the major fuel for thermogenesis in the cold, since it accounted for 51% of the corresponding total energy expenditure. The results demonstrate that cold exposure causes a much greater increase in the utilization of carbohydrate than lipid. It is suggested that these substrates are directly utilized for thermogenesis in the shivering skeletal muscles.