The energy required for physical activity requiring a low amount of effort that can be maintained for hours is provided primarily by the metabolism of fat. The energy required for physical activity requiring a moderately high amount of effort that can be maintained for 90-120 minutes is provided by the metabolism of both fat and carbohydrate. The energy required for physical activity requiring intense effort lasting 60 seconds is provided by the anaerobic metabolism of muscle glycogen and blood glucose.
Muscle glycogen can be reduced and fatigue can occur while one is engaging in 30-75 minutes of intermittent physical activity requiring very intense effort. Also, fatigue will occur for moderately high physical activity lasting longer than 90 minutes when muscle glycogen and blood glucose reach low levels, are lowered by some amount, or reach some critical threshold. Physical activity requiring a low effort can last for hours, and fatigue is more likely to be related to dehydration, hyperthermia, orthopedic problems, or boredom.
Under normal conditions, protein is metabolized in very small amounts during exercise and the capacity for the oxidation of protein will rarely determine performance capability. Additionally, under normal circumstances, body fat stores are almost always adequate to support long-duration exercise of moderate and high intensities; thus, fat availability rarely determines performance capability. On the other hand, the levels of bodily carbohydrates play an important role in the level of physical activity that can be maintained at moderate and moderately high intensities. Thus, it is important to replenish bodily carbohydrate reserves to maintain their concentration at “optimal” levels.
Normalization of bodily carbohydrate stores between sessions of physical activity is at minimum, essential to “optimally” influence training and performance capabilities. Bodily carbohydrate reserves can most likely be recovered in 24 hours after moderately high-intensity physical activity provided between 500-600 g of carbohydrate are consumed over the 24 hours following the activity (i.e., 7-10 g carbohydrate/kg body weight). On the other hand, it is likely that bodily carbohydrate reserves can be recovered in 24 hours after moderate-intensity physical activity, provided that about 300-350 g of carbohydrate are consumed over the 24 hours after the activity (i.e., 5 g carbohydrate/kg body weight).
Recovery of bodily carbohydrate reserves is apparently affected by the glycemic index of the consumed carbohydrate. It appears that carbohydrates that produce a high glycemic index and insulin response promote more rapid recovery of carbohydrate reserves than do those carbohydrates that produce a low glycemic index and insulin response.
Adapted in part from: Coyle, EF. Substrate utilization in active persons. In: New dimensions in carbohydrates. A symposium sponsored by the American Society for Clinical Nutrition, Inc., and the Sugar Association, Washington, DC, December 1993.
Carbohydrate that is ingested before and during exercise provides an alternate source of muscle fuel that can support moderate and moderately high-intensity physical activity. Concurrently, the ingestion of carbohydrate at these times reduces the mobilization and oxidation of fats. Importantly, this reduction in the metabolism of fats does not impair performance; rather, the provision of the ingested carbohydrate usually improves performance.
Although the training endurance athlete and the athlete who trains intensely using intermittent exercise will necessarily rely on adequate carbohydrate consumption to optimize training and thereby performance capability, the recreational athlete can most likely undertake leisure-time physical activity by consuming a healthy diet (less than 30 percent of energy from fat, 12-15 percent of energy from protein, and the balance of energy from carbohydrate). Nevertheless, both types of sports persons must assure adequate hydration and minimize environmental stress during physical activity. Table I provides more specific information on carbohydrate supplementation and physical activity.
|Table 1. Summary of Perspectives on Carbohydrates Supplementation and Physical Activity|
|For Recreational Athletes|
|For Endurance Athletes|
|For Sprinting Athletes|
The above perspectives probably apply to all sports persons regardless of age, gender, or
ethnicity. There is the suggestion that women have a lower capability to increase muscle glycogen
compared with men and that phases of the menstrual cycle influence carbohydrate and fat metabolism; however, these perspectives require much additional research before they can be accepted as principles.
Very little research exists on the applicability of these findings to wheel-chair bound individuals. Such individuals will use a much smaller muscle mass to produce movement; consequently, it is likely that these recommendations are especially applicable to these individuals.
*Adapted in part from: Coyle, EF. Substrate utilization in active persons. In: New dimensions in carbohydrates. A symposium sponsored by the American Society for Clinical Nutrition, Inc., and the Sugar Association, Washington DC, December 1993.
Return to Contents