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 ([U-¹³C]glucose ingestion), we have determined the specific contributions of plasma glucose, muscle glycogen, proteins, and lipids to total heat production (Ḣ_prod) 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 Ḣ_prod 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% Ḣ_prod for lipids, CHO, and proteins, respectively) to CHO-based metabolism for Hi (23, 65, and 12% Ḣ_prod for lipids, CHO, and proteins, respectively) was observed. Plasma glucose oxidation remains a minor fuel under all conditions (<13% Ḣ_prod), falling to 7% Ḣ_prod 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.