5.6: Guest Lecturer

Guest Lecturer: Ellen Coleman, RD, MA, MPH

Glycogen Storage and the Athlete

Muscle glycogen is the preferred fuel for most types of exercise. Replenishing and maintaining muscle glycogen stores during intensive training requires a carbohydrate-rich diet. Depending on the intensity and duration of your activity, you should be consuming 6 to 10 grams of carbohydrate per kilogram of body weight daily. Adequate muscle glycogen stores allow you to exercise harder and longer with less fatigue.

Glycogen depletion can occur gradually over repeated days of heavy training when muscle glycogen breakdown exceeds its replacement. When this happens, your glycogen stores drop lower with each successive day, and your workouts become more difficult and less enjoyable. The deterioration in performance and feeling of sluggishness associated with glycogen depletion is often referred to as “staleness” and blamed on overtraining.

Glycogen depletion is often accompanied by a sudden weight loss of several pounds (due to glycogen and water loss) and you can’t maintain your usual training intensity. When you don’t consume enough carbohydrate or calories and/or don’t take days off to rest, you’re a prime candidate. Most Americans consume 5 grams of carbohydrate per kilogram of body weight – about half of their calories.

Carbohydrate Recommendations for Training

You can prevent glycogen depletion by consuming a carbohydrate-rich diet (6 to 10 grams of carbohydrate per kilogram daily) and taking periodic rest days to give your muscles time to rebuild their stores. You should consume 6 grams of carbohydrate per kilogram daily if you’re working out hard for an hour each day. Take in 8 grams of carbohydrate per kilogram if you’re working out hard two hours each day. A diet providing 10 grams of carbohydrate per kilogram is recommended when you’re training hard for three hours or more each day.

A high carbohydrate diet is even more critical for recovery from prolonged, heavy exercise. Cyclists in the grueling Tour de France consume about 12 grams of carbohydrate per kilogram and 6,000 calories each day. By keeping your carbohydrate intake high, you can minimize the chronic fatigue due to muscle glycogen depletion.

The recommendations for carbohydrate are given in grams per kilogram because this is an easy way to determine how much you need. One kilogram equals 2.2 pounds. For example, a 154-pound person (70 kg) who trains strenuously for an hour needs 420 grams of carbohydrate daily. You can determine the carbohydrate content of different foods by reading food labels.

Carbohydrate Loading

You can improve your performance when you exercise for longer than 90 minutes by maximizing your muscle glycogen stores prior to the event. During endurance exercise that exceeds 90 to 120 minutes, your muscle glycogen stores become progressively lower. When they drop to critically low levels (the point of glycogen depletion), you cannot maintain high-intensity exercise. In practical terms, you’ve “hit the wall” and must drastically reduce your pace.

Carbohydrate loading can increase your muscle glycogen stores by 50 to 100%. The greater your pre-exercise muscle glycogen content, the greater your endurance potential. You can carbohydrate load using a six-day, a three-day, or one-day regimen.

The graph below gives an overview of the six-day diet and training regimen used for carbohydrate loading. On the sixth day before the event, you exercise hard (about 70% of VO2 max) for 90 minutes. On the fifth and fourth days before the event, decrease your training to 40 minutes. During the first three days, you consume a normal diet providing about 5 grams of carbohydrate per kilogram per day. On the third and second day before the event, you reduce your training to 20 minutes. On the day before the event, you rest. During the last three days, you eat a high carbohydrate diet providing 10 grams of carbohydrate per kilogram per day.

For the three-day regimen, you exercise hard (about 70% of VO2 max) for 90 minutes three days before the event. You rest and eat a high carbohydrate diet (10 grams of carbohydrate per kilogram per day) until the event.

A one-day carbohydrate loading regimen has been proposed to minimize disruptions to your training and competition preparation. On the morning of the day before the event, skip breakfast and warm up for five minutes. Next, exercise at the highest intensity that you can maintain for 2 and ½ minutes followed by a 30 second all-out sprint. For the next 24 hours, rest and consume 10 grams of carbohydrate per kilogram. Start taking in carbohydrate within 20 minutes of completing the exercise.

You must be endurance trained or carbohydrate loading won’t work. Also, the exercise to lower glycogen stores must be the same as your competitive event because glycogen stores are specific to the muscle groups used. For example, a runner needs to decrease glycogen by running rather than cycling.

It’s essential that you decrease your training the three days prior to competition. Too much exercise during this period will use too much of your stored glycogen and defeat the purpose of the whole process. The final three days, when you taper and eat a high-carbohydrate diet, is the real loading phase of the regimen. If you have difficulty consuming enough carbohydrate from food, you can use a high-carbohydrate supplement.

For each gram of glycogen stored, additional water is stored. Some people note a feeling of stiffness and heaviness associated with the increased glycogen storage. Once you start exercising, however, these sensations will work out.

Carbohydrate loading will help only for continuous endurance exercise lasting more than 90 minutes. Greater than usual muscle glycogen stores won’t enable you to exercise harder during shorter duration exercise. In fact, the stiffness and heaviness due to increased glycogen stores can hurt your performance during shorter competitions such as 10 kilometer runs.

Carbohydrate loading enables you to maintain high intensity exercise longer, but will not affect your pace for the first hour of exercise. You won’t be able to go out faster, but you will be able to maintain your pace longer.

_References

^{Bergstrom J, et al. Diet, muscle glycogen, and physical performance. Acta Physiol Scand 71: 140, 1967.
Fairchild TJ, Fletcher S, Steele P, et al. Rapid carbohydrate loading after a short bout of near maximal-intensity exercise. Med Sci Sport Exerc 34:980-986, 2002.
Karlsson J, et al. Diet, muscle glycogen, and endurance performance. J Appl Physiol 31:203, 1971.
Sherman WM, et al. The effect of exercise and diet manipulation on muscle glycogen and its subsequent use during performance. Int J Sports Med 2: 114, 1981.}

Ellen Coleman is an exercise physiologist, registered dietitian (RD), and certified specialist in sports dietetics (CSSD) in Riverside, California. She has completed the Ironman Triathlon in Hawaii twice and numerous marathons and 200 mile bicycle races. She has consulted with the Los Angeles Lakers basketball team and Angels baseball team, and is the author of the popular books, Eating for Endurance and Ultimate Sports Nutrition.