Muscle fuels for

A variety of fuels are available to generate ATP for muscle activity phosphocreatine glycogen (which can be converted to lactic acid or completely oxidised to CO2) glucose (from liver glycogen, transported to the muscle via the blood and completely oxidised to CO2) triacylglycerol within the muscle (completely oxidised to CO2) and fatty acids from triacylglycerol in adipose tissue (completely oxidised to CO2). 

At least six factors interplay to determine what fuel is selected  

Fuels are stored within the muscle, liver and adipose tissue (Table 13.4) and amounts vary according to the nutritional status of the subject and the previous physical activity (see also Chapter 2). Triacylglycerol that is stored within the muscle can be used but the most significant fat fuel is long-chain fatty acids, derived from triacylglycerol in adipose tissue. 

Electron micrographs kindly provided by Dr Belinda Bullard. 

Fuel Tissue Approx, content in total body tissue (g) Energy content (kj) 

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Fatty acid oxidation occurs in mitochondria and peroxisomes in most tissues but quantitatively muscle is a major consumer of fat. Although carbohydrates and fatty acids may both be used as fuels for muscle contraction, fatty acids are more calorific... 

It increases the release of fatty acids from adipose tissue which raises the plasma level of long-chain fatty acids, to provide a fuel for muscle, if it becomes physically active (Chapter 13). It also increases the cycling between triacylglycerol and fatty acids in adipose tissue. 

Fatty acids are released from chylomicrons and VLDL by the action of lipoprotein lipase (see pp. 226, 229). However, fatty acids are of secondary importance as a fuel for muscle in the well-fed state, in which glucose is the primary source of energy. 

Figure 21.14 Pathway Integration Hormonal control of glycogen breakdown. Glucagon stimulates liver glycogen breakdown when blood glucose is low. Epinephrine enhances glycogen breakdown in muscle and the liver to provide fuel for muscle contraction.

Fatty acids serve as a fuel for muscle, kidney, and most other tissues. They are oxidized to acetyl CoA, and subsequently to CO2 and H2O in the TCA cycle, producing energy in the form of adenosine triphosphate (ATP). In addition to the ATP required to maintain cellular integrity, muscle uses ATP for contraction, and the kidney uses it for urinary transport processes. 

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. 

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. 

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