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.
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 molecule. Also, fats may be metabolized to a greater extent during longer aerobic activity. During constant or intermittent running activities over longer periods of time aerobic glycolysis would contribute most significantly to energy production. Athletes playing soccer, basketball, hockey, and distance runners would be the examples.
Carbohydrates, proteins, and fats are the three basic sources of calories in the human diet (Figure 6-2).2 Carbohydrates represent a readily usable energy source. Fats represent a calorie dense source of energy. A significant number of calories are released in the body as these longer chain molecules are broken down. As carbohydrates and fats as energy sources in the body are fully consumed, proteins within the body will be metabolized as a final source of energy. Children and adolescents may have an increased requirement for calorie intake compared to adults. Increased calories are required for growth and development. Since children and adolescents have increased energy consumption for physical activity compared to adults, the young athletes may have a slightly greater calorie requirement.3
Carbohydrates are the primary energy source for physical activity of the body. For this reason, 50% to 60% of the calories in the athlete’s diet should be carbohydrates. Consumed carbohydrates may be broken down into glucose circulating in the blood. Blood glucose serves as a ready source of energy in the cells. Consumed carbohydrates may be mobilized to the liver for glycogen storage or finally mobilized in the muscle for storage as muscle glycogen. Liver glycogen can be mobilized to maintain blood glucose to other tissue. Muscle glycogen stores can be metabolized within the muscle during prolonged physical activity as an energy source.
Blood glucose is regulated by insulin and exercise. In the resting state, insulin facilitates the uptake of glucose by the cell. Exercise is associated with an upregulation of membrane bound glucose receptors, which results in an increase uptake of glucose by the active cell. Thus, exercise will increase the effect of insulin.
Carbohydrates can be classified based on a glycemic index. Carbohydrate rich foods will affect blood sugars based on the complexity of the carbohydrate. Foods with a higher percentage of complex sugars will be digested and absorbed more slowly. These foods will have a low-glycemic index and contribute to less elevation in blood glucose. Examples would be dairy foods, fruits, pasta, dried beans, and nuts. Foods with a higher percentage of simple sugars would be digested and absorbed more rapidly and result in greater increase in blood sugars. Examples of these high-glycemic index foods would be carrots, white potatoes, honey, white bread, corn chips, and sports drinks. Other foods such as rice cakes, crackers, soda, wheat bread, ice cream, sweet potatoes, and potato chips would have a moderate glycemic index and a mix of complex and simple sugars.
Athletes competing in endurance activity may experience an increase in blood glucose by consuming high-glycemic index foods just prior to activity. Some athletes, who consume high-glycemic index foods may experience hypoglycemia because of overproduction and effect of insulin. This may actually decrease performance because of rebound insulin overload. Consumption of low-glycemic index foods several hours before exercise for some athletes can help to maintain a more even blood glucose level.
Timing of the carbohydrate consumption in relationship to exercise may affect performance. Generally, the pre-event consumption of carbohydrate should satisfy hunger and elevate blood glucose without causing insulin rebound phenomenon. This may be achieved by consuming most carbohydrates several hours prior to the event (Table 6-1).4
When | How Much (g) | How |
|---|---|---|
Before activity | 300 | Several hours before the activity |
During activity | 30 | Every hour during the activity |
After activity | 75 | Within 30 min after cessation of activity |
| 100 | Every 60 min for 2 h after cessation of activity |
Carbohydrate loading may improve performance in long-endurance events. This technique should only be repeated twice during a season. Loading begins 1 week prior to the event with the depletion of glycogen stores. During the first 3 days, the athlete increases activity while decreasing the percentage of carbohydrate consumed at each meal. After the glycogen depletion phase, the athlete replenishes glycogen stores slowly by gradually decreasing workout bouts and increasing the percentage of carbohydrates consumed on the days leading up to the event. While this technique is effective in adults, it has not been studied in children.5
Proteins are an alternate energy source when stored glycogen and fat are depleted during endurance exercise. More importantly, proteins provide the building blocks for muscle development and repair. Increased protein intake has been suggested for active individuals as well children and adolescents who are experiencing growth and development (Table 6-2).6
Not meeting the body’s protein requirements can result in muscle mass wasting, lowered immunity, decreased injury repair, and fatigue. This is common in athletes who are attempting to lose weight by restricting calories. Repeated injury with poor recovery can be the hallmark of protein deficiency in athletes. Exceeding the protein requirements can result in increased body fat, dehydration by nitrogen loss, and calcium loss. Increased protein consumption is often associated with carbohydrate deficiency. The end result is poor athletic performance.
The final energy substrate macronutrient in the body is fat. Fats serve as a high-caloric storage source of energy. Not more than 30% of dietary calories should be from fats. Limiting fat intake can limit energy storage as well as essential fat deficiencies. Significantly, limiting fat intake can result in deficiencies in the fat-soluble vitamins A, D, E, and K. Children with developing nervous tissue are at risk with fat deficiency. Fat consumption should be evenly divided between saturated, polyunsaturated, and monounsaturated fat sources. Saturated fats should not account for greater than 10% of total dietary caloric intake.
