Oxidative fuel selection and muscle recruitment in shivering humans

During environmental cold exposure in adult humans, a decrease in core temperature can be prevented by increasing heat production (H˙ prod) via shivering thermogenesis. The main purpose of this thesis was to determine the effects of changes in carbohydrate (CHO) availability and shivering intensity on oxidative fuel selection and muscle recruitment during cold exposure. Using a combination of metabolic and electrophysiological approaches, fuel selection and EMG activity were quantified: (i) during low-intensity shivering (LOW) in individuals with normal (N), low (LO) and high CHO availability (HI), and (ii) during high-intensity shivering in individuals with normal CHO availability (HIGH). Low-intensity shivering (2.6-fold increase in H˙prod) resulted in a stimulation of plasma glucose (+138%), muscle glycogen (+109%) and lipids (+376%) oxidation rates for N (CHAPTER 2). Despite the observed increase in plasma glucose oxidation, this fuel only supplied 10% H˙ prod (or only 25% of all the glucose oxidized). Total heat production was therefore unequally shared between lipids (50%), muscle glycogen (30%), plasma glucose (10%)and proteins (10%). The same fuel selection measurements were then performed for LO and HI (CHAPTER 3). The size of CHO reserves had no effect on H˙prod, but had a major impact on fuel selection before and during shivering. In the cold, a complete shift was observed from lipid oxidation for LO (53%, 28% and 19% H˙prod for lipids, CHO and proteins, respectively) to CHO-based metabolism for HI (23%, 65% and 12% H˙prod for lipids, CHO and proteins, respectively). As for N, plasma glucose oxidation was a minor fuel source (<13% H˙ prod), falling to 7% H˙prod for LO. Therefore, plasma glucose oxidation did not compensate for changes in muscle glycogen oxidation and thus is not a strategy used for maintaining heat production. Instead, proteins and lipids compensated for the decreased in CHO availability. Most interestingly, these drastic changes in fuel metabolism were achieved without altering the electromyographic (EMG) signature of shivering muscles (CHAPTER 4). Results demonstrate that EMG shivering intensity, pattern and spectral components of eight large muscles remains unaffected by changes in fuel selection. Therefore, humans can sustain low thermogenic rates by oxidizing widely different fuel mixtures within the same muscle fibers. Considering the low metabolic rates reached during mild shivering (LOW), oxidative fuel utilization rates were then measured at higher shivering intensity (3.5-fold rise in H˙prod) to establish whether (i) the role of plasma glucose could be increased (CHAPTER 5) (ii) modifications of fuel selection could be achieved via the recruitment of fuel specific muscle fibers. (Abstract shortened by UMI.