Fat and glycogen are the major sources for maintaining ATP resynthesis when needed. However, carbohydrates are the only macronutrient substrate whose stored energy generates ATP anaerobically. Hence, during maximal exercise that requires rapid energy release above levels supplied by aerobic metabolism, intramuscular glycogen supplies most of the energy for ATP synthesis. During light and moderate exercise, carbohydrates supply about one third of the body’s energy requirements. Aerobic breakdown of carbohydrate for energy occurs more rapidly than energy generation from fatty acid breakdown. For some tissues (such as brain, medulla of the kidney and rapidly contracting skeletal muscle), glucose is the only source of metabolic energy. In the muscle, complete glucose breakdown generates 36 moles of ATP, or 224 kCal conserved per mole of glucose consumed.

While carbohydrate may be the preferred macronutrient for immediate energy release, short-lived exercise and for some tissues, carbohydrate reserves generally mount to less than 2000 kCal, while stored fat provides almost unlimited energy: 60,000 and 100,000 kCal from triacylglycerol in adipocytes and about 3,000 kCal from intramuscular triacylglycerol. While 1 molecule of glucose produced 36 ATP, each molecule of catabolized triacylglycerol produces 460 molecules of ATP. Fat constitutes the second line of energy source after glycogen and is the main source of energy for high-intensity, long-duration exercise. Continual carbohydrate metabolism is thought to be needed for fat oxidation to occur. For example, the intermediate substrate oxaloacetate is regenerated from pyruvate (a product of glycolysis), and is required for the entry of fatty acids into the citric acid cycle. Fat oxidation is also slower than catabolism of glycogen or glucose. Therefore depletion of glycogen in the muscle leads to slower generation of energy to power muscles and also fatigue. Proteins play only a contributory role as energy substrate during endurance activities and intense training and are rarely used as a primary source of energy.

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