Tuesday, December 13, 2011

Factors that Influence the Selection of Fuel Substrates During Exercise

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Factors that Influence the Selection of Fuel Substrates During Exercise


Fuel substrates for producing energy in the body are carbohydrates, fats and proteins. Carbohydrates and fats are the primary fuel substrates in the selection for producing energy. Only % and no more than 15% during the final minutes of a prolonged exercise are contributed by proteins. Several factors influence the selection of fuel substrates which includes the type of diet of the individual, the intensity of the exercise, and the duration of the exercise.


Individuals who eat a high-fat and low-carbohydrate diet will have a higher rate of fat metabolism. A high-carbohydrate and low-fat diet will produce a higher rate of carbohydrate metabolism in the individual. A high-protein no-carbohydrate diet can cause health problems because essential amino acids are broken down as fuel.


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Fats are the primary fuel source for muscle during low-intensity exercises. An example would be an exercise of walking using less than 0% VO max. As the exercise increases in intensity, there is a steady increase in carbohydrate metabolism and a decrease in fat metabolism. Eventually there is a crossover to carbohydrates for use as the primary fuel substrate instead of fats. This crossover occurs somewhere around an exercise with a Vo max of 5%.


Two main factors that cause this shift from using fats as a primary fuel source to carbohydrates are the recruitment of fast twitch fibers in the muscle and increasing levels of epinephrine in the blood. The fast fibers in the muscle have more glycolytic enzymes than lipolytic enzymes. Because glycolysis requires carbohydrates as the fuel source and lipolysis uses fats, the fast fibers are better suited to break down carbohydrates. Higher levels of epinephrine stimulate the enzyme phosphorylase which causes an increase in glycogenolysis and glycolysis. An increase in glycolysis raises lactate levels which inhibits lypolysis. Since there are more carbohydrates, there is less fat metabolism.


During prolonged exercises approximately greater than 0-0 minutes, there is a gradual shift from carbohydrate metabolism to an increased rate of fat metabolism. Lipolysis is the process that breaks down fat molecules or triglycerides into free fatty acids and glycerol by the enzyme lipase. These lipase enzymes are inactive until there is an increase in certain hormones such as epinephrine, norepinephrine, and glucagon. These hormones stimulate the lipase enzymes which increases lipolysis. Lipolysis increases free fatty acid levels in the blood and muscles which promotes fat metabolism. Since lipolysis is a slow process, this will only occur after several minutes of a prolonged low-intensity exercise. There is also a decrease in insulin which inhibits mobilization of FFA. By decreasing the level of insulin, lipolysis can speed up and FFA can increase.


In conclusion, the body contains proteins, carbohydrates, and fats which are used as fuel substrates for producing energy. Proteins contribute a small percentage of the fuel supply, approximately %, and only 15% during the final minutes of a prolonged exercise. Primary fuel substrates are carbohydrates and fats. Factors influencing fuel selection include the type of diet of the individual, the intensity of the exercise, and the duration of the exercise.





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