Glycogen fuel during exercise

The different fuels used by exercising muscle are discussed in subsequent sections. These fuels may be arranged in the following "hierarchy," where the order of appearance approximates relative importance during exercise (1) creatine phosphate, (2) muscle and liver glycogen, (3) gluconeogenesis, and (4) fatty acids. 

Blood-bome fuels are glucose, which is derived from liver glycogen, and fatty acids derived from adipose tissue. Uptake depends on the flow of blood through the muscle, the concentration of the fuel in the blood and the demand for ATP within the muscle. During sustained exercise the flow of blood to the muscle can increase up to 50-fold and the rate of utilisation of the fuel can increase to a similar extent, yet the concentration of the fuels in blood remains remarkably constant (Table 13.5). 

During the early minutes of exercise, carbohydrate (plasma glucose and muscle glycogen) is the predominant fuel for the working muscles. When the exercise is prolonged and intensive, carbohydrate remains a predominant fuel with lipids (plasma free fatty acids and muscle triglycerides) being of lesser importance. When the exercise is of moderate intensity, lipids eventually become the primary fuel as carbohydrate stores are reduced. 

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