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1 Biology Department, University of Ottawa, Ottawa, Ontario, Canada
2 Departement de kinesiologie, Universite de Montreal, Montreal, Quebec, Canada
3 School of Human Kinetics, University of Ottawa, Ottawa, Ontario, Canada
4 Departement de kinanthropologie, Universite du Quebec a Montreal, Montreal, Quebec, Canada
5 Departement des sciences de l activite physique, Universite du Quebec a Trois-Rivieres, Trois-Rivieres, Quebec, Canada
* To whom correspondence should be addressed. E-mail: JMWEBER{at}science.uottawa.ca.
Carbohydrates (CHO) can play an important thermogenic role during shivering, but the effect of their availability on the use of other oxidative fuels is unclear. Using indirect calorimetry and tracer methods (13C-U-glucose ingestion), we have determined the specific contributions of plasma glucose, muscle glycogen, proteins and lipids to total heat production (Hprod) in men exposed to cold for 2-h (liquid-conditioned suit perfused with 10°C water). Measurements were made after low-CHO diet and exercise (LO) and high-CHO diet without exercise (HI). The size of CHO reserves had no effect on Hprod, but a major impact on fuel selection before and during shivering. In the cold, a complete shift from lipid oxidation for LO (53%, 28% and 19% Hprod for lipids, CHO and proteins, respectively) to CHO-based metabolism for HI (23%, 65% and 12% Hprod for lipids, CHO and proteins, respectively) was observed. Plasma glucose oxidation remains a minor fuel under all conditions (<13% Hprod), falling to 7 % Hprod for LO. Therefore, adjusting plasma glucose oxidation to compensate for changes in muscle glycogen oxidation is not a strategy used for maintaining heat production. Instead, proteins and lipids share responsibility for this compensation. We conclude that humans can show remarkable flexibility in oxidative fuel selection to ensure that heat production is not compromised during sustained cold exposure.
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