A teen’s fitness program should include aerobic exercise such as brisk walking, basketball, bicycling, swimming, in-line skating, soccer, jogging—any continuous activity that increases heart rate and breathing. Regular workouts improve the efficiency of the cardiorespiratory system, so that the heart and lungs don’t have to work as hard to meet the body’s increased demands for freshly oxygenated blood.
Aerobic exercise also affects body weight composition, by burning excess calories that would otherwise get converted to fat. In general, the more aerobic an activity, the more calories are expended. For instance, if a teenager weighing 132 pounds walks at a moderate pace for ten minutes, he burns forty-three calories. Running instead of walking more than doubles the amount of energy spent, to ninety calories.
Low-intensity workouts burn a higher percentage of calories from fat than high-intensity workouts do. However, the more taxing aerobic exercises ultimately burn more fat calories overall. One study compared the burn rates for a thirty-minute walk at three and a half miles per hour and a thirty-minute run at seven miles per hour. The walking group expended an average of 240 calories. Two-fifths came from fat, and three-fifths came from carbohydrates, for a total of ninety-six fat calories. In the running group, the ratio of fat energy burned versus carbohydrate energy burned was significantly less: one to four. Yet overall, the runners consumed 450 calories. Total number of fat calories burned: 108.