Lipids pack more joules per gram because they can be stored dehydrated and, to a lesser extent, because they are more chemically reduced than other fuels. 1B) and, therefore, are stored in large amounts ( Fig. Lipids show unique characteristics that make them ideally suited for long-lasting physiological work because they contain the most energy per unit mass ( Fig. 1B–E compares lipids and carbohydrates for the main parameters determining whether these fuels tend to support prolonged low-intensity tasks or intense activities of short duration. This example is used to characterize the shape of the more general relationship between metabolic rate and duration. 1A illustrates the relationship between metabolic intensity and duration by plotting record times of human athletes running over different distances vs average speed. The rate of energy expenditure necessary for a particular task determines for how long that task can be maintained. Therefore, fuel selection strategies aim to exploit the advantages of individual substrates while minimizing the impact of their disadvantages. The regulation of energy metabolism is a complex challenge because the fuels available vary widely in stored quantity, energy density, speed of conversion to ATP and water solubility. The fluxes of multiple substrates must be modulated to achieve real-time selection of mixtures able to support adequate metabolic rates for variable physiological circumstances: from years of torpor to seconds of sprinting. These dietary fatty acids become incorporated in membrane phospholipids and bind to peroxisome proliferator-activated receptors to activate membrane proteins and modify gene expression.Īnimal life depends on the capacity to match metabolic fuel supply to changing rates of energy use. Some migrant birds use dietary omega-3 fatty acids as performance-enhancing agents to boost their ability to process lipids.
These endurance athletes support record lipolytic rates in adipocytes, use lipoprotein shuttles to accelerate transport and show increased capacity for lipid oxidation in muscle mitochondria. High lipid fluxes are achieved through the coordinated upregulation of mobilization, transport and oxidation by activating enzymes, lipid-solubilizing proteins and membrane transporters.
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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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