Muscles fuels used

The primary fuel used to support muscle contraction depends on the magnitude and duration of exercise as well as the major fibers involved. Skeletal muscle has stores of both glycogen and some triglycerides. Blood glucose and free fatty acids also may be used. 

A fuel is a compound that leads to generation of ATP from ADP and P, (Figure 13.17). The fuels used by muscle differ according to fibre, the type of activity and the conditions under which the activity takes place. For example, in athletics, the fuel used varies according to the particular event, from the 100 m sprint to the marathon. 

Figure 13.20 The use of glycogen and/or fatty acids during a prolonged running event (an ultramarathon). The distance of an ultramarathon is usually >50 miles. In the early part of the run, both glycogen and fatty acids are the fuels oxidised by the muscle. After several hours, glycogen is exhausted and fatty acids are the only fuel used. As fatty acid oxidation cannot provide more than about 60% of the ATP required for maximum power output, if the athlete is running at about 70 or 80% of the maximum, the output (i.e. the pace) must slow. Hence the rate of oxygen consumption (VO2) falls to about 60% of maximum (V02 ax), as shown in the Figure. The data on which the plot is based are from Davies Thompson (1979).

Skeletal muscle can use free fatty acids, ketone bodies, or glucose as fuel, depending on the degree of muscular activity (Fig. 23-17). In resting muscle, the primary fuels are free fatty acids from adipose tissue and... 

Ketones themselves may be used as fuel. The brain, which normally prefers glucose, can use ketones in states of starvation. Cardiac muscle commonly uses ketones as fuel. 

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. 

Rates of energy fuel use can be determined in several ways. Internal energy fuel use can be measured by a tissue biopsy. A special probe is inserted into the tissue, enabling the researcher to cut out and withdraw a small sample. Biopsies arc sometimes taken from liver and muscle by exercise physiologists. Biopsies per-... 

M. A. Christmass, B. Dawson, P. Passeretto, and P. G. Arthur, A Comparison of Skeletal Muscle Oxygenation and Fuel Use in Sustained Continuous and Intermittent Exercise, Eur. J. Appl. Physiol. Occupat. Physiol., 80(5), 423-435 (1999). 

Some organs (e.g., heart and skeletal muscle) can use ketone bodies (/J-hydroxybutyrate and acetoacetate) as an energy source under normal conditions. During starvation the brain uses them as an important fuel source. Because liver does not have /J-oxoacid-CoA transferase, it cannot use ketone bodies as an energy source. These reactions are reversible. 

Muscle fuel utilization at rest is dependent on the serum levels of glucose, amino acids, and fatty acids. If blood glucose and amino acids are elevated, glucose will be converted to glycogen, and branched-chain amino acid metabolism will be high. Fatty acids will be used for acetyl CoA production and will satisfy the energy needs of the muscle under these conditions. 

---