Carbohydrate fuel during exercise

Fuels for the body are limited to carbohydrates, fats, and proteins. In the American diet, these fuels are consumed in a ratio of approximately 46 42 12 with the recommended ratio being closer to 53 35 12 (.5). Thus in a nongrowing adult, these ratios provide estimates of the fuel use for daily activities. The primary fuels for exercise are carbohydrates and fats. Chapter 3 examines utilization of specific fuels during aerobic exercise. As the amount of daily exercise increases, there is an increased energy expenditure and hence increased need for energy nutrients usually reflected in increased food consumption, decreased body fat, or both (see Chapter 9). 

Inherited aldolase A deficiency and pyruvate kinase deficiency in erythrocytes cause hemolytic anemia. The exercise capacity of patients with muscle phos-phofiaictokinase deficiency is low, particularly on high-carbohydrate diets. By providing an alternative lipid fuel, eg, during starvation, when blood free fatty acids and ketone bodies are increased, work capacity is improved. 

The calorific capacity of amino acids is comparable to that of carbohydrates so despite their prime importance in maintaining structural integrity of cells as proteins, amino acids may be used as fuels especially during times when carbohydrate metabolism is compromised, for example, starvation or prolonged vigorous exercise. Muscle and liver are particularly important in the metabolism of amino acids as both have transaminase enzymes (see Figures 6.2 and 6.3 and Section 6.4.2) which convert the carbon skeletons of several different amino acids into intermediates of glycolysis (e.g. pyruvate) or the TCA cycle (e.g. oxaloacetate). Not all amino acids are catabolized to the same extent... 

The observation of possibly increased IMCL levels in highly trained athletes led to a second focus of research in the field of sports medicine. Besides carbohydrates, lipids are the major fuel of skeletal muscle during work and rest. A variety of questions have to be answered. It was examined, whether exercise of various intensities alters IMCL levels, whether those changes depend on the duration of exercise, and finally, how IMCL are replenished during post-exercise recovery. 

During the early minutes of exercise, carbohydrate (plasma glucose and muscle glycogen) is the predominant fuel for the working muscles. When the exercise is prolonged and intensive, carbohydrate remains a predominant fuel with lipids (plasma free fatty acids and muscle triglycerides) being of lesser importance. When the exercise is of moderate intensity, lipids eventually become the primary fuel as carbohydrate stores are reduced. 

As fuel for muscular exercise, the carbohydrates have been shown to be the most direct and efficient materials. The effects are especially marked during prolonged or severe exercise. Fats and proteins are used very well, but less directly and with some loss of energy in the transformations involved (25). 

Horton, T.J., Pagliassotti, M.J., Hobbs, K., and Hill, J.O., Fuel metabolism in men and women during and after long-duration exercise, J Appl Physiol, 85, 1823, 1998. Friedlander, A.L, Casazza, G.A., Homing, M.A. et al.. Training-induced alterations of carbohydrate metabolism in women women respond differently from men, J Appl Physiol, 85, 1175, 1998. 

Nevertheless, this possibility remains attractive to consumers, and there is some strong recent evidence that carbohydrates consumed together with protein hydrolysates with added leucine and phenylalanine promote higher insulin secretion than can be achieved with carbohydrate intake alone. The potential advantages of this effect are that it may (1) further promote muscle glucose uptake and stimulate muscle glycogen synthesis, and so both increase the stores of this important fuel prior to exercise and enhance its restoration in the recovery phase after exercise, and (2) stimulate muscle amino acid uptake and protein synthesis during recovery from... 

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