Fuel selection during exercise is determined by both the duration and intensity of exercise. A short sprint such as a 50-meter dash will be fueled entirely by muscle stores of glycogen and phosphocreatine. At the other extreme of a long trek, oxidation of free fatty acids is the most important fuel. Protein is not a major fuel for exercise, comprising less than 2% of the substrate during the first hour of exercise and rising to only 15% after five hours. Fats contribute about two-thirds of the energy for low levels of exercise. As the level of VO2 increases, carbohydrate becomes more important than fat as a substrate, with a crossover at about 35% of VO2 max. At the onset of moderate exercise, carbohydrates and fats contribute equally to energy production, but fat metabolism becomes relatively more important as the duration of exercise increases.

63. Exercise

The preference for carbohydrate metabolism during short, intense exercise is driven by recruitment of more fast-twitch glycolytic muscle fibers (see Chapter 7) at high levels of exercise and rapid effects of sympathetic stimulation and epinephrine on muscle glycogen breakdown and glycolysis. Lactate from glycolysis also inhibits fat metabolism. Sympathetic stimulation of the pancreas decreases insulin release, but insulin-independent glucose uptake in skeletal muscle increases during exercise. Glucose uptake is further enhanced during exercise by recruitment of more insulin-dependent glucose receptors to muscle membranes to cope with the decreased insulin levels. Replenishment of carbohydrates during exercise can improve endurance in trained athletes.

Fat is the primary metabolic fuel during prolonged exercise. Free fatty acids are generated from triglycerides by lipases, which are stimulated by epinephrine, norepi-nepherine, and glucagon. This response is slower than the sympathetic effects on muscle glycogen. Prolonged exercise generally involves more slow-twitch muscle fibers with more mitochondria and lipolytic enzymes. Also, insulin levels decrease during exercise which reduces the inhibitory effect of insulin on the mobilization of free fatty acids. However, if high levels of carbohydrates are ingested prior to exercise, this may stimulate insulin production and favor carbohydrate over fat metabolism. The hormonal control of metabolic fuels during exercise is also discussed in Chapter 42.

Appetite, or the control of metabolic fuel intake, is related to the levels of energy expended. People increase their caloric intake in direct proportion to increased energy expenditure if they are burning more than 2500 calories per day. Increased glucose uptake (insulin-independent and -dependent) persists for several hours following exercise; this increased uptake lowers blood glucose level and stimulates appetite. Other factors increasing appetite and caloric intake include more rapid emptying of the stomach, increased absorption in the small intestine, and perhaps neural reflexes involving the hypothalamus. Although exercise can increase appetite, the most effective method of weight control is increasing exercise while controlling diet.

Low Carb Diets Explained

Low Carb Diets Explained

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