Carbohydrates are a good source of fuel while lipids are a good storage fuels. Skeletal muscles and the liver store significant amount of carbohydrates. Triacylglycerols are stored in adipose tissues. At the starting of race, the body is resting. Glucose is then taken into the body, storing in the skeletal muscles and in liver as glycogen. Digested lipids from the small intestine are absorbed across the intestinal walls and converted into triacylglycerols. Triacylglycerols are incorporated with cholesterol and apolipoproteins, into chylomicrons. Triacylglycerols are hydrolysed into glycerol and fatty acids here and are taken by adipose tissues and liver.
After the race has started for five minutes, eighty-five percent of the energy is supplies by carbohydrates. This is because they are soluble due to their –OH group and can travel quickly and directly in the blood to wherever the body needs them; on the other hand, lipids cannot. For lipids to be mobilized, they need to be carried in the blood bound to proteins that counteract their insolubility and these processes are time-consuming. The fuel comes from glycogen in the muscles and liver. Glycogen in the skeletal muscles is converted to glucose-1-phosphate by the enzyme glycogen phosphorylase while in the liver, glucagon is released to signal low sugar levels and liver releases glucose by gluconeogenesis. Glucose goes through a ten reaction pathway in which glucose is degraded to pyruvate. This process known as glycolysis. Thefirst five reactions are known as the preparatory phase and consume two ATPs. The next five phases are called the payoff phases where four ATPs and 1 NADH is produced. At this stage of the race, muscles rely mainly on the anaerobic pathways. Therefore, pyruvate is converted to lactate and carbon dioxide. The net gain of ATP for this fermentation is two.
After the race has begun for forty-five minutes, most carbohydrate sources would have depleted. L

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