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 Dietary Supplements: Carbohydrate Supplements as Potential Modifiers of Physical Activity* 
W. Sherman Michael PhD ©
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.

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