Muscle glycogen supercompensation

During intense exercise for 90-120 min at 70% V02max (e.g. marathon), muscle glycogen stores gradually decrease. When they reach a critical level (glycogen depletion point), high-intensity exercise should not be continued because the athlete is exhausted and must either stop training or drastically reduce its intensity. Depletion of muscle glycogen is a recognized limitation of endurance. Athletes using the glycogen supercompensation technique (carbohydrate loading) can almost double their muscle glycogen stores.

The carbohydrate loading method was initially a weekly regimen that began with a series of grueling workouts one week before the competition. For the next three days, the athlete was on a low-carbohydrate diet but continued to train, further reducing muscle glycogen levels. For the three days before the competition, the athlete significantly reduced the volume of training loads and was on a high-carbohydrate diet, promoting glycogen supercompensation. This regimen had many drawbacks. Reduced carbohydrate intake often caused hypoglycemia, ketosis and associated nausea, fatigue and irritability. Diet manipulation proved to be burdensome for athletes.

The revised carbohydrate loading method proposed by Sherman et al. has eliminated many of the problems. Six days before the competition, the athlete trains for 90 min at 70% V02 max, days 5 and 4 train for 40 min at 70% V02 max, days 3 and 2 train for 20 min at 70% V02 max, and rests the day before the competition. During the first three days, the athlete eats a normal diet, providing 5 g carbohydrate/kg body weight per day. During the last three days, he uses a high-carbohydrate diet, providing 10 g carbohydrate/kg body weight per day. The last three days, when the athlete consumes the high-carbohydrate diet, are the true "loading" phase of the regimen. As a result of the modified regimen, muscle glycogen stores become equal to those provided by the classic carbohydrate loading regimen.

In a field study by Karlsson and Saltin, runners competed in a 30 km race after consuming a normal and a high-carbohydrate diet. The high-carbohydrate diet resulted in muscle glycogen levels of 193 mmol kg compared to 94 mmol kg with the normal diet. All runners completed the race faster (by about 8 min) if they started the race with high muscle glycogen levels. The carbohydrate load allows the athlete to sustain intense exercise for longer, but does not affect speed in the first hour of the race.

Endurance training promotes supercompensation of muscle glycogen by increasing the activity of glycogen synthase, the enzyme responsible for glycogen storage. The athlete must be trained for endurance, otherwise the regimen will not be effective. Since glycogen stores are specific to the muscle groups being worked, the exercises that deplete these stores must be the same as those in the competition in which the athlete is participating.

Commercially available high-carbohydrate liquid supplements may be given to athletes who have difficulty consuming enough carbohydrates in their diet. Athletes with diabetes or hypertriglyceridemia may have complications with carbohydrate loading and should seek medical clearance before beginning a loading regimen.

For every gram of stored glycogen, additional water is required. Occasionally, some athletes experience stiffness and heaviness associated with increased glycogen stores, but these sensations usually disappear with exercise.

Carbohydrate loading will only benefit athletes engaged in intense endurance exercise lasting longer than 90 minutes. Excessive glycogen stores will prevent an athlete from exercising more intensely for a shorter period of time. The stiffness and heaviness associated with increased glycogen stores may impair performance in shorter events such as 5K and 10K races.

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