Carbohydrate Maintenance During Exercise

Avoiding low blood glucose and avoiding depletion of muscle glycogen stores are both critical for maintaining exercise performance. Consumption of carbohydrate-containing beverages (e.g., sports beverages) and food during exercise delays fatigue and improves performance, even if the consumption occurs late in the exercise session. This strategy delays fatigue through the following mechanisms:

1. Maintains blood glucose, which preserves liver glycogen
2. Maintains branched-chain amino acid (BCAA) levels, which avoids central fatigue by maintaining the ratio of tryptop an and BCAAs
3. Inhibits the production of cortisol, which is catabolic to muscle tissue
4. Reduces the usage of muscle glycogen by providing a constant source of glucose from the blood to working muscle cells

During exercise, carbohydrate is best obtained through a 6 to 7 percent carbohydrate solution, with 4 to 8 ounces (120 to 240 milliliters) taken every 10 to 20 minutes (the amount to consume depends on sweat rate). A number of different carbohydrate solutions are available to athletes, each with a different concentration and composition of carbohydrate. Issues of gastrointestinal distress and osmolarity should be considered. Of equal 6 percent concentrations of glucose, fructose, or sucrose, the fructose has been shown to cause more gastrointestinal distress. Therefore, athletes should carefully check their tolerance of fructose-only beverages before consuming them in critical situations. (Most sports beverages contain multiple carbohydrate types.)

Few sports beverages are milk based or use lactose as a predominant form of carbohydrate because of the relatively common problem of lactose intolerance. This condition, caused by an inadequate production of the enzyme lactase, results in diarrhea, gas, and abdominal pain. Given the possibility of lactose intolerance in some athletes, it is prudent for athletes to avoid lactose-containing products immediately before and during physical activity.³⁰

Glucose polymers have the advantage of being able to deliver more carbohydrate in a lower osmolar solution.³¹ Isocaloric with simple sugars, glucose polymers improve gastric emptying and enhance absorption. Athletes involved in extremely high-intensity activity for a long duration may need a high volume of carbohydrate calories during physical activity, and glucose polymers may provide a good solution for these athletes.