Hecker, A. L., & Wheeler, K. B. (1994). Impact of hydration and energy intake on performance. The Journal of the National Athletic Trainers Association, 19, 4-9.

Carbohydrate is an important energy source. It is stored as glycogen in the muscles and liver and glucose in the circulating blood. During intense exercise, muscle glycogen is the primary energy source but circulating glucose also is used. As the duration of intense exercise increases, glucose becomes more important. Depletion of either source of fuel results in fatigue.

Athletes who burn more calories than they consume cannot produce glycogen efficiently. This occurs even if the majority of the limited diet is carbohydrates.

The form of carbohydrate for replenishment is not critical. Although complex carbohydrates are preferred, simple sugars are satisfactory if they are the only source available.

During exercise, muscle glycogen serves as a limiting energy source. Virtually no synthesis takes place during activity. Consequently, muscles in long term exercise depend upon glucose as a fuel source. Blood and liver glucose may supply as much a 30-40% of total energy needs. If blood glucose declines to half its normal value, exercising muscle fibers cannot obtain enough sugar to maintain activity. As a result, muscle glycogen is used and fatigue is accelerated.

The ingestion of CHO during exercise can delay the onset of exhaustion. A CHO replenishment system should be used that will give the athlete as much energy as possible without delaying the absorption of water (a solution of less than 2.5% of glucose). A solution of 5% glucose polymers and 2% fructose may be comparable.

The timing of feedings is critical in some persons who are exhibit reactive glycemia. CHO sources should not be delivered any closer than one hour prior to the onset of exercise. A large dose can cause an elevation in insulin which promotes glucose uptake from the blood and suppresses the mobilization of free fatty acids from adipose tissue. This is called "insulin backlash." It results in the early onset of hypoglycemia and a premature utilization of glycogen stores leading to a decreased endurance capacity. However, if CHO is ingested after exercise (i.e., vigorous warm-up), the backlash does not occur.

Return to Table of Contents for this issue.