Carbohydrate Utilization in Exercise

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Animals have a remarkable ability to "shift gears" metabolically during periods of strenuous exercise or activity. Metabolic adaptations allow the body to draw on different sources of energy (all of which produce ATP) for different types of activity. During periods of short-term, high-intensity exercise (e.g., a 100-m dash), most of the required energy is supplied directly by existing stores of ATP and creatine phosphate (Figure, part a). Long-term, low-intensity exercise (a 10-km run or a 42.2-km marathon) is fueled almost entirely by aerobic metabolism. Between these extremes is a variety of activities (an 800-m run, for example) that rely on anaerobic glycolysis—conversion of glucose to lactate in the muscles and utilization of the Cori cycle.

For all these activities, breakdown of muscle glycogen provides much of the needed glucose. The rate of glycogen con sumption depends upon the intensity of the exercise ( b). By contrast, glucose derived from gluconeogenesi; small contributions to total glucose consumed durin During prolonged mild exercise, gluconeogenesis a only about 8% of the total glucose consumed. During cise, this percentage becomes even lower.

Choice of diet has a dramatic effect on glycogen r lowing exhaustive exercise. A diet consisting mainly and fat results in very little recovery of muscle glyc after 5 days (Figure, part c). On the other hand, a hi drate diet provides faster restoration of muscle glycog this case, however, complete recovery of glycogen about 2 days.

Carbohydrate Utilization

(a) Contributions of the various energy sources to muscle activity during mild exercise. (b) Consumption of glycogen stores in fast-twitch muscles during light, moderate, and heavy exercise. (c) Rate of glycogen replenishment following exhaustive exercise. (a and c adapted from Rhodes and Pflanzer, 1992. Human Physiology. Philadelphia: Saunders College Publishing; b adapted from Horton and Terjung 1988. Exercise, Nutrition and Energy Metabolism. New York: Macmillan.)

(a) Contributions of the various energy sources to muscle activity during mild exercise. (b) Consumption of glycogen stores in fast-twitch muscles during light, moderate, and heavy exercise. (c) Rate of glycogen replenishment following exhaustive exercise. (a and c adapted from Rhodes and Pflanzer, 1992. Human Physiology. Philadelphia: Saunders College Publishing; b adapted from Horton and Terjung 1988. Exercise, Nutrition and Energy Metabolism. New York: Macmillan.)

760 Chapter 23 • Gluconeogenesis, Glycogen Metabolism, and the Pentose Phosphate Pathway

Liver

Spleen /

Liver

Portal vein Pancreas

Pancreatic veins

FIGURE 23.21 • The portal vein system carries pancreatic secretions such as insulin and glucagon to the liver and then into the rest of the circulatory system.

Portal vein Pancreas

Pancreatic veins

FIGURE 23.21 • The portal vein system carries pancreatic secretions such as insulin and glucagon to the liver and then into the rest of the circulatory system.

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Responses

  • marilena calabrese
    How much is rhodes & pflanzer physiology?
    3 years ago

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