![]() ![]() Long-distance migrants provide an excellent model to characterize how to increase maximal substrate fluxes. The selection patterns of shivering and exercise are different: at the same, a muscle producing only heat (shivering) or significant movement (exercise) strikes a different balance between lipid and carbohydrate oxidation. Electromyographic analyses show that shivering humans can modulate carbohydrate oxidation either through the selective recruitment of type II fibers within the same muscles or by regulating pathway recruitment within type I fibers. Fuel selection is performed by recruiting different muscles, different fibers within the same muscles or different pathways within the same fibers. However, highly aerobic species rely more on intramuscular fuels because energy supply from the circulation is constrained by trans-sarcolemmal transfer. All exercising mammals share a general pattern of fuel selection: at the same they oxidize the same ratio of lipids to carbohydrates. The aim of fuel selection strategies is to exploit the advantages of individual substrates while minimizing the impact of disadvantages. Animals must regulate the fluxes of multiple fuels to support changing metabolic rates that result from variation in physiological circumstances.
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