Because glucose is such an important source of energy for muscle contraction, the body does its best to store glucose in a form that is easily accessible. Dietary carbohydrates are stored in the liver and muscle as glycogen, a polymer of glucose (see Infographic 4.2). At any one time, a body’s stores of glycogen can fuel up to 2,000 calories of activity—about 1.5 to 2.5 hours of high-intensity exercise. Athletes who run out of muscle glycogen during training or competition experience sudden, serious fatigue known as “hitting the wall.” To delay or prevent this from happening, athletes who are training on consecutive days must consume enough carbohydrates each day to allow glycogen stores to be replenished before their next training bout. If glycogen stores are not replenished at the beginning of a training session, exercise performance will suffer, while perception of the effort required—the discomfort— will increase. Therefore, exercising in a glycogen-depleted state not only feels terrible, it also decreases training effectiveness. Because exercise of longer duration and higher intensity uses more glycogen, total daily carbohydrate intake should increase as training volume (total amount of work done) and intensity increase to ensure that glycogen stores are replenished. (INFOGRAPHIC 16.8)

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Question 16.6

To increase muscle glycogen stores even more, and further delay fatigue during competition, endurance athletes often use a technique called carbohydrate loading. The resulting increase in muscle glycogen stores can increase time to exhaustion during intense exercise by 90 minutes or more. To “carbo load,” athletes will typically consume about 10 grams to 12 grams of carbohydrate per kilogram of body weight each day, for 1 to 2 days prior to competition, while reducing training volume and intensity. At 1 to 2 hours before exercise, they will often consume meals or snacks that are rich in carbohydrates to top off glycogen stores and to provide additional carbohydrates to be used during the upcoming bout of exercise. As the time between eating and beginning exercise increases so should the amount of carbohydrate consumed before an event or training bout. (INFOGRAPHIC 16.9)

Question 16.7

Eating carbs during exercise can also be helpful. During competitions, for instance, Hubbell often eats carb-heavy snack bars. Ingesting carbs during intense exercise that lasts more than an hour has consistently been shown to improve exercise performance.

For exercise sessions lasting 1 to 2 hours, experts recommend that carbohydrates should be consumed at a rate of at least 30 grams per hour. As the exercise duration increases, so also should the amount of carbohydrates ingested. When the carbohydrates being consumed comprises glucose alone (dextrose, maltodextrin, and/or starch), the maximum rate of ingestion should be limited to 72 grams per hour, because glucose cannot be absorbed from the small intestine and utilized faster than this. This rate of carbohydrate intake should be sufficient for exercise sessions lasting up to about 2.5 hours.

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High rates of carbohydrate ingestion during exercise can lead to discomfort in some people; however, research has shown that athletes can become accustomed to this practice when they regularly consume carbohydrates during training, along with a high-carbohydrate diet. This emphasizes the importance of practicing any nutrition support strategy during training, and not trying out a new strategy for the first time during competition. Carbohydrates in the form of beverages, gels, or bars are all equally good; however, when consuming gels or bars one must be mindful to consume adequate water. (INFOGRAPHIC 16.10)

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Question 16.8