The muscles use glucose as a fuel source during exercise. The glucose may come from the breakdown of glycogen in the muscle, or it may be delivered by the blood. The glucose delivered by the blood comes from liver glycogen stores, glucose synthesized by the liver, and carbohydrates consumed in food or drink during exercise.
How quickly glucose is used during exercise depends on the intensity of the exercise. High-intensity exercise relies on anaerobic metabolism, which uses glucose exclusively as a source of fuel. The glucose for high-intensity exercise comes mostly from muscle glycogen. Therefore, the more intense the exercise, the more glycogen used. Muscle glycogen depletion is one factor involved in the onset of fatigue during exercise.
During low- and moderate-intensity exercise, the muscles can use fat for fuel, so glycogen is depleted more slowly. But even when aerobic metabolism predominates, some glucose still is used. Some comes from muscle glycogen and some is delivered in the blood.
As muscle glycogen stores decrease, glucose delivered in the blood becomes a more important source of carbohydrates. Hormones released during exercise help ensure that blood glucose levels are maintained and can continue to supply glucose to body cells, including the muscle.
For example, within seconds of the start of exercise, the hormones epinephrine and norepinephrine are released. When blood glucose levels begin to drop, the pancreas releases the hormone glucagon. These three hormones stimulate the liver to break down glycogen and make new glucose.
When liver glycogen is broken down, glucose is released into the blood. During intense or long-lasting exercise, liver glycogen can be depleted. To ensure glucose is available, glucagon also stimulates gluconeogenesis, or the creation of new glucose. During exercise, gluconeogenesis occurs primarily in the liver. It produces glucose from three-carbon molecules, including lactic acid, alanine, and glycerol.
Lactic acid is generated by the anaerobic metabolism of glucose. Lactic acid produced in the muscle can travel to the liver and be converted back into glucose. Alanine is an amino acid generated from the release and breakdown of amino acids from the muscle. Glycerol is a product of triglyceride breakdown. When adipose tissue is broken down, each triglyceride molecule yields three fatty acids and a molecule of glycerol. The fatty acids are transported to the muscle.
They are used to make ATP through aerobic metabolism. The glycerol goes to the liver, where it can be used for gluconeogenesis. Because beta-oxidation breaks fatty acids into twocarbon molecules, fatty acids cannot be used to synthesize glucose. The amount of glucose that must be produced by gluconeogenesis during exercise depends on several things:
During prolonged exercise (three hours or more), gluconeogenesis is a major source of glucose for muscles. Carbohydrates consumed during exercise (as drinks or snacks) can provide an additional source of glucose. This is beneficial for exercise lasting an hour or more because it will spare glycogen, reduce the need for gluconeogenesis, and delay fatigue.
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