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There appears to be a dichotomy among children and adolescents. On the one hand is a population of youth with increasing obesity, on the other hand, active youth are involved in several sports or extensively in a single sport. Access to electronic equipments such as computers, video games, and television has lead to a more sedentary lifestyle. At the same time, ready access to high-energy food has resulted in excess caloric consumption and weight gain, whereas, athletes who train intensely may have a difficult time consuming adequate calories. More youth are participating in intense training and sports such as football, soccer, swimming, tennis, distance running, even triathlons, and marathons. These athletes may need to consume a rather large number of calories to meet the needs for intense activity on top of growth and development.

Baseline caloric demands are determined by the weight. Factors such as growth, age, and physical activity will increase metabolic demand. In children and adolescents, this can increase caloric demand greatly over the basal metabolism. Growth alone can increase basal metabolism significantly. Weight and more specifically body composition can affect metabolism. As lean body mass increases, so do energy requirements increase. Because children and adolescents are less efficient compared to adults when physically active, their energy requirements may be increased 20% to 30% for given level of activity.

Based on the length of physical activity, three different metabolic pathways may serve as the primary metabolic pathways (Figure 6-1).1 In very short burst of activity, the phosphagen pathway provides the main source of energy. And the athlete performs an activity such as an Olympic lift or 1 repetition maximal lift, phosphocreatine, a chemical present in muscles of the body, can be broken down to immediately release energy for activity. As a readily available energy source, lasting only a few seconds, this source is depleted very early in activity.

Figure 6-1

Energy sources for muscle as a function of activity duration. Schematic presentation showing how long each of the major energy systems can endure in supporting all-out-work. (From Brooks GA, Fahey TD, White TP. Exercise Physiology. New York: McGraw Hill Medical; 2005.)

As longer physical activity is performed, the anaerobic glycolysis pathway meets the body's energy demands. This pathway provides energy by the breakdown of glycogen. In this metabolic pathway, the energy is regenerated faster without breaking down glucose completely. Longer activities such as sprints, short-distance swimming, and short bouts of weightlifting would be associated with anaerobic glycolysis.

Long-duration endurance physical activity is associated with the aerobic glycolysis pathway. Ultimately, the glucose metabolized by this pathway is broken down completely to water and carbon dioxide. In the process, significantly more energy is released. Aerobic glycolysis pathway being a more complicated pathway, more metabolic resources are invested in it to supply a greater release of energy from each ...

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