How muscles use fuel

Article Abstract:

The biochemical process by which muscles transform chemical energy into mechanical energy is described. Muscles use energy faster than the energy sources can be metabolized, and this factor limits how much one can exercise. The fuels that muscles use are derived from both carbohydrates, such as sugar and starch, and lipids, or fats. The main carbohydrate fuel is glycogen stored in the cells. The main lipid fuel is plasma free fatty acids, with a small amount from fat stored in cells. The proportions in which these two fuels are used depend on a number of factors. During rest and light exercise, lipids are the predominant fuel (unless one has recently eaten a high-carbohydrate meal, which forces the muscles to burn more carbohydrate). As exercise becomes more intense, the proportion of carbohydrate to lipid fuel increases. Therefore, glycogen stores are more important for endurance exercise, and can be increased by exercising to exhaustion and then eating a high-carbohydrate meal. This can increase tremendously the amount of time an athlete can exercise. Certain metabolic disorders can cause patients to depend entirely on carbohydrate fuels, leading to muscle injury and pain, weakness, and other problems. This can be prevented by eating regular, high-carbohydrate meals and avoiding prolonged exercise. McArdle's syndrome, a deficiency of the muscle enzyme phosphorylase, prevents carbohydrate assimilation, and intense or anaerobic exercise quickly leads to cramps and muscle damage. In an article in the February 7, 1991 issue of the New England Journal of Medicine, Haller and Lewis describe the role of plasma free fatty acids in sustaining exercise among patients with a deficiency of the muscle enzyme phosphofructokinase, and the role of the ''second wind.'' Carbohydrates make these patients worse by lowering their plasma fatty acid levels, but this does not happen to patients with McArdle's syndrome. The study of these rare conditions that limit exercise and cause exercise-induced fatigue, muscle contracture and pain, rapid heartbeat, hyperventilation, and muscle wasting will not only illuminate the mechanisms of disease, but will also help to explain the symptoms that limit exercise in healthy people. (Consumer Summary produced by Reliance Medical Information, Inc.)

Muscles, editorial

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Glucose-induced exertional fatigue in muscle phosphofructokinase deficiency

Article Abstract:

Phosphofructokinase is an enzyme necessary to the process that breaks down sugars and releases energy in the form of adenosine triphosphate (glycolysis). An inherited deficiency of this enzyme stops the process of manufacturing usable energy, and leads to premature fatigue, cramping, and sometimes injury to the muscles during exercise. During maximal aerobic exercise, the body must use alternative fuels such as free fatty acids to nourish the muscles, a factor in producing the ''second wind'' that many people experience during prolonged exertion. Even though the second wind is usually absent in patients with muscle phosphofructokinase deficiency, exercise tolerance does vary among them. Patients with phosphofructokinase deficiency often report that fatigue is greater after a high-carbohydrate meal. In this study five patients were observed to determine the effect of carbohydrate on exercise performance. The patients exercised on a stationary bicycle after an overnight fast, and during an infusion of glucose or triglyceride with heparin, again after an overnight fast. It was found that the capacity to engage in aerobic exercise varied according to the availability of blood-borne fuels. Glucose infusion lowered exercise tolerance by slowing conversion of fats, and thereby preventing the muscles from obtaining energy from free fatty acids in the blood and ketones, the products of fat metabolism. Under these conditions, muscles are not able to obtain oxygen, and maximal oxygen consumption is lowered. (Consumer Summary produced by Reliance Medical Information, Inc.)

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The effect of oral sucrose on exercise tolerance in patients with McArdle's disease

Article Abstract:

Patients with a muscle disease called McArdle's disease can improve their ability to exercise by drinking a sucrose drink before exercising, according to a study of 12 patients. McArdle's disease prevents muscles from converting glycogen into blood sugar. Therefore, the muscles have no energy source and cannot function properly.

Raw cane sugar, Sucrose, Sugarcane Mills, Exercise, Nutritional aspects, Diet therapy, Glycogenosis, Glycogen storage diseases

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