The balance of carbohydrate and lipid used by an individual during exercise depends mainly on relative intensity, or level of effort as related to the individual's maximal rate of oxygen consumption (Vo2max) the greatest oxygen consumption that can be attained during an all out physical effort). In general, Vo2max is related to body muscle mass and is a relatively constant value for a given individual but it can be altered by various factors, particularly aerobic training, which will induce a change of 10 to 20 percent. Thus, on an absolute basis, bigger individuals tend to have a larger Vo2max (measured in liters of O2 consumed/minute) than do smaller individuals. However, Vo2max is also related to the size of the body and the heart. Hence, for purposes of comparison, Vo2max is frequently considered in terms of mL/kg/min. Some examples are illustrative. An unfit man of average weight (70 kg) might have an absolute Vo2max of 2.8 L/min, corresponding to 40 mL/kg/min (2.8 L/70 kg/min). If the man's resting metabolic rate (RMR) is 250 mL/min, he would be expected to be capable of 11.5 MET (40 mL/kg/min divided by 1 MET defined as 3.5 mL O2/kg/min). However, a heart disease patient of the same body size might be capable of only a Vo2max of 0.50 to 0.75 L/min, corresponding to 7 mL/kg/min (0.5 L/70 kg/min) to 10 mL/kg/min (0.75 L/70 kg/min). This would be equivalent to 2 (7 mL/kg/min divided by 3.5 mL O2/kg/min) or 3 METs (10 mL/kg/min divided by 3.5 mL O2/kg/min), while an Olympic-class middle distance runner of the same weight may be capable of achieving a
918 DIETARY REFERENCE INTAKES
Vo2max of 6 L/min, which is equivalent to 85 mL 02/kg/min (6 L/70 kg/ min), or 24 METs (85 mL 02/kg/min divided by 3.5 mL 02/kg/min).
Lipid is the main energy source in muscle and at the whole-body level during rest and mild intensity activity (Brooks and Mercier, 1994). As intensity increases, a shift from the predominant use of lipid to carbohydrate occurs. Figure 12-7 describes this crossover concept and, as can be seen in the figure, the relative use of fat is greatest at relatively low exercise intensities, particularly when individuals are fasting. Training slightly increases the relative use of fat as the energy source during low to moderate exercise intensities, particularly in the fasted state. In regard to the amount of fat oxidized, it must be considered that the energy output for a given percent of Vo2max is proportionally higher (in this case 50 percent) in trained rather than in untrained cyclists. However, at relatively high power outputs, substrate use crosses over to predominant use of carbohydrate energy sources regardless of training state or recent carbohydrate nutrition.
To be used for energy generation, protein must first be degraded to amino acids before the carbon-hydrogen-oxygen skeleton can be used as an energy source through the pathways of carbohydrate and lipid metabolism, while the amino acid nitrogen is transferred and eliminated, primarily in the form of urea. The rate at which amino acids contribute to energy generation is fairly constant and does not increase nearly as much as glucose and fatty acid oxidation during periods of physical exertion. While the rate of oxidation of particular amino acids (e.g., leucine) may rise significantly during exercise, not all amino acids respond in the same way, and amino acids diminish in relative importance as fuels when power output rises during exercise (Brooks et al., 2000), providing only a small percentage of the energy used during physical activity (Brooks, 1987). Indeed, using amino acids as a major energy source would be wasteful, since protein is the most limited energy yielding nutrient. Beyond the overriding effect of relative exercise intensity, other factors such as exercise duration, gender, training status, and dietary history play important, but secondary, roles in determining the pattern of substrate utilization (Brooks et al., 2000). Therefore, the same general relationships among relative exercise intensity, duration, and pattern of substrate utilization hold for most persons, including endurance athletes.
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