Meeting the Metabolic Demand during 24h of Cold Survival Simulation: A New Perspective on Energy Needs, Fuel Selection and Muscle Recruitment

Abstract

Cold environments have been an ongoing challenge for humans over the ages. To date, however, knowledge on the capacity to sustain the metabolic demand of shivering is mostly limited to modeling efforts. Actual models of shivering endurance, by a presumed analogy with exercise, rely on the assumptions that carbohydrate stores are used at a constant rate and essential to maintain heat production. Therefore, my thesis firstly aimed to characterize a hypothesized capacity to adjust the fuel mixture oxidized in order to maintain heat production during 24h of cold exposure. An accidental cold exposure with food scarcity was simulated. Following a 5 km walk, lightly clothed individuals were exposed to an environmental temperature of 7.5 ○C for 24h. Results show that thermogenesis increased to 8 kJ per minute, a metabolic demand that was sustainable for 24h since lipid oxidation substituted a decreased carbohydrate oxidation. Secondly, this thesis aimed at characterizing the roles of muscle glycogen and plasma glucose in these changes in carbohydrate metabolism. It was found, as hypothesized, that such changes result from a reduction in muscle glycogen utilization. Electromyographic activity of shivering muscles was also measured to address the third objective of the thesis: determining whether shivering contributes similarly to heat production during 24h and whether muscles contribute in similar proportions. Results show that total shivering intensity was constant during 24h. However, different muscles contributed differently over time, enlightening a potential mechanism for changes in fuel selection. Questions remain concerning the origin of this shift in fuel use towards lipid dominance during this prolonged cold exposure. Blood metabolites and glucose tracer data suggest that a high availability of circulating fatty acids is more likely in cause than a depletion of glycogen reserves while the implication of inadequate feeding needs to be clarified.