Blood glucose levels represent a balance between the rate of glucose production by the liver and the rate of glucose utilization by muscle and other tissues. When the rate of blood glucose utilization exceeds the rate of production by the liver, blood glucose levels fall, causing impaired function, particularly of the brain, which depends on carbohydrate for its energy. The relevance of this to fatigue in ultramarathon running is discussed subsequently.
Muscle lacks glucose 6-phosphatase and is therefore unable to produce glucose from glycogen. Instead, the glucose 6-phosphate enters another metabolic pathway known as glycolysis, the end product of which is pyruvate.
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An important feature of the glycolytic pathway is that it can proceed without the provision of an adequate oxygen supply. This is called “oxygen-independent” glycolysis. This term supercedes the old term “anaerobic glycolysis,” which suggests that glycolysis occurs only when there is anaerobiosis (i.e, an inadequate oxygen supply). This is quite incorrect. We know that glycolysis becomes very active during high-intensity exercise such as sprinting, even though there is an adequate oxygen supply to the muscles.
Most of the pyruvate produced by glycolysis will cross the mitochondrial membrane, in which the enzyme pyruvate dehydrogenase is embedded. This enzyme converts pyruvate into acetyl-CoA, which then enters the final common metabolic pathway called the citric acid or Krebs cycle. At various points in the
Krebs cycle, hydrogen is released and transferred to a third metabolic pathway, the electron transport chain, for the production of mitochondrial ATP.