Glycogen Depletion
Significant glycogen depletion occurs during endurance exercise that exceeds 90 minutes (such as marathon running). When glycogen stores drop to critically low levels, high-intensity exercise cannot be maintained. In practical terms, the athlete is exhausted and must either stop exercising or drastically reduce the pace.
However, glycogen depletion may also be a gradual process, occurring over repeated days of heavy training, in which muscle glycogen breakdown exceeds its replacement. The same process may occur during high-intensity exercise that is repeated several times during competition or training. How long does it take to replenish glycogen sores? It depends on the amount of carbohydrates in our diet, and on the timing of their consumption. Generally, it takes 24-48 hours to fully replenish glycogen, which is quite a bit longer than it takes to use it up.
Chart 1: The influence of dietary carbohydrates on muscle glycogen stores during repeated days of training. In a classic study, glycogen synthesis was compared on a 40% vs. 70% carbohydrate diet during repeated days of 2-hour workouts (Chart 1).
On the high carbohydrate diet: the athletes enjoyed nearly maximal repletion of the muscle glycogen stores after the strenuous training, allowing them to continue the heavy training.
This study and others, suggest that athletes who fail to consume enough carbohydrates on a daily basis while training, will likely decrease endurance as well as exercise performance.
Other studies showed dose-related glycogen storage (i.e. a direct relation between the amount of carbohydrates consumed, and the amount of glycogen stored in the muscle). Three diets were consumed by athletes: 15% carbs, 55% carbs and 55-65% carbs. The highest carb-diet yielded 4 times more accumulated glycogen compared to the lowest carb-diet. When these athletes were subjected to “all-out” exercise, their exercise times were proportional to the amount of glycogen stored before the test (Chart 2). 
Remember, carbohydrates are important for not only endurance athletes but also those who train hard day after day and want to maintain high energy. If you eat a low-carbohydrate diet, your muscles will feel chronically fatigued. You will train, but not at your best.
However, glycogen depletion may also be a gradual process, occurring over repeated days of heavy training, in which muscle glycogen breakdown exceeds its replacement. The same process may occur during high-intensity exercise that is repeated several times during competition or training. How long does it take to replenish glycogen sores? It depends on the amount of carbohydrates in our diet, and on the timing of their consumption. Generally, it takes 24-48 hours to fully replenish glycogen, which is quite a bit longer than it takes to use it up.
Chart 1: The influence of dietary carbohydrates on muscle glycogen stores during repeated days of training. In a classic study, glycogen synthesis was compared on a 40% vs. 70% carbohydrate diet during repeated days of 2-hour workouts (Chart 1).
On the low carbohydrate diet: the muscle glycogen stores dropped lower with each successive day of training. After several days of the diet and exercise regimen, the athletes had low muscle glycogen stores, and could not exercise at even a moderate intensity.
On the high carbohydrate diet: the athletes enjoyed nearly maximal repletion of the muscle glycogen stores after the strenuous training, allowing them to continue the heavy training.
This study and others, suggest that athletes who fail to consume enough carbohydrates on a daily basis while training, will likely decrease endurance as well as exercise performance.
Other studies showed dose-related glycogen storage (i.e. a direct relation between the amount of carbohydrates consumed, and the amount of glycogen stored in the muscle). Three diets were consumed by athletes: 15% carbs, 55% carbs and 55-65% carbs. The highest carb-diet yielded 4 times more accumulated glycogen compared to the lowest carb-diet. When these athletes were subjected to “all-out” exercise, their exercise times were proportional to the amount of glycogen stored before the test (Chart 2).
It is suggested that athletes in heavy training should consume a carbohydrate intake of 7 to 10 g/kg bodyweight/day, to help prevent daily glycogen depletion. For example, a person weighing 70kg (154 lb) would need between 490 g a day (70*7), and 700 g a day (70*10), with consideration of total energy needs (Burke LM, 2004).
Remember, carbohydrates are important for not only endurance athletes but also those who train hard day after day and want to maintain high energy. If you eat a low-carbohydrate diet, your muscles will feel chronically fatigued. You will train, but not at your best.
posted by MedionMan at 11:34 AM
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