The selection of metabolic fuel depends on both the intensity of work being performed and also whether the individual is in the fed or fasting state.
10.6.1 The effect of work intensity on muscle fuel selection
Intense physical activity requires rapid generation of ATP, usually for a relatively short time. Under these conditions substrates and oxygen cannot enter the muscle at an adequate rate to meet the demand, and muscle depends on anaerobic glycolysis of its glycogen reserves. As discussed in section 18.104.22.168, this leads to the release of lactate into the bloodstream, which is used as a substrate for gluconeogenesis in the liver after the exercise has finished.
Less intense physical activity is often referred to as aerobic exercise, because it involves mainly red muscle fibres (and type IIA white fibres) and there is less accumulation of lactate.
The increased rate of glycolysis for exercise is achieved in three ways:
• As ADP begins to accumulate in muscle, it undergoes a reaction catalysed by adenylate kinase: 2 X ADP ^ ATP + AMP As discussed in section 10.2.2.1, AMP is a potent activator of phosphofructokinase, reversing the inhibition of this key regulatory enzyme by ATP, and so increasing the rate of glycolysis.
moderate exercise fatty acids fatty acids
intense exercise fatty acids
In prolonged aerobic exercise at a relatively high intensity (e.g. cross-country or marathon running), muscle glycogen and endogenous triacylglycerol are the major fuels, with a modest contribution from plasma non-esterified fatty acids and glucose (see Figure 10.12). As the exercise continues, and muscle glycogen and triacylglycerol begin to be depleted, so plasma non-esterified fatty acids become more important.
At more moderate levels of exercise (e.g. gentle jogging or walking briskly), plasma non-esterified fatty acids provide the major fuel. This means that, for weight reduction, when the aim is to reduce adipose tissue reserves (section 6.3), relatively prolonged exercise of moderate intensity is more desirable than shorter periods of more intense fatty acids
Figure 10.12 Utilization of different metabolic fuels in muscle in moderate and intense exercise.
exercise. More importantly for overweight people, most of the non-esterified fatty acids that are metabolized in moderate exercise are derived from abdominal rather than subcutaneous adipose tissue (section 6.2.3).
At rest, triacylglycerol from plasma lipoproteins is a significant fuel for muscle, providing 5—10% of the fatty acids for oxidation, but non-esterified fatty acids are more important in exercise.
10.6.2 muscle fuel utilization in the fed and fasting states
Glucose is the main fuel for muscle in the fed state, but in the fasting state glucose is spared for use by the brain and red blood cells; glycogen, fatty acids and ketone bodies are now the main fuels for muscle.
As shown in Figure 10.13, there are five mechanisms involved in the control of glucose utilization by muscle:
10.6.2.1 Regulation of fatty acid metabolism in muscle
P-Oxidation of fatty acids is controlled by the uptake of fatty acids into the mitochondria — as discussed in section 5.5.1, this is controlled by the activity of carnitine acyl transferase on the outer mitochondrial membrane, and by the countertransport of acyl-carnitine and free carnitine across the inner mitochondrial membrane.
Carnitine acyl transferase activity is controlled by malonyl CoA. As discussed in section 10.5.2, in liver and adipose tissue this serves to inhibit mitochondrial uptake and P-oxidation of fatty acids when fatty acids are being synthesized in the cytosol. Muscle also has an active acetyl CoA carboxylase, and synthesizes malonyl CoA, although it does not synthesize fatty acids, and muscle carnitine acyl transferase is more sensitive to inhibition by malonyl CoA than is the enzyme in liver and adipose tissue.
Muscle also has malonyl CoA decarboxylase, which acts to decarboxylate malonyl CoA back to acetyl CoA. Acetyl CoA carboxylase and malonyl CoA decarboxylase are regulated in opposite directions by phosphorylation catalysed by a 5'-AMP-dependent protein kinase (which thus reflects the state of ATP reserves in the cell; section 10.2.2.1). Phosphorylation in response to an increase in intracellular 5'-AMP results in:
This results in a rapid fall in the concentration of malonyl CoA, so relieving the inhibition of carnitine palmitoyl transferase and permitting mitochondrial uptake and ß-oxidation of fatty acids in response to a fall in ATP, and hence a need for increased energy-yielding metabolism.
In the fed state, there is decreased oxidation of fatty acids in muscle as a result of increased activity of acetyl CoA carboxylase in response to insulin action.
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