QUESTION: I have been reading about glycogen but am confused about what it is and why it is important for people who exercise. Would you please say something about this topic in your column sometime? Thank you.
ANSWER: Glycogen is simply a group of glucose molecules that are hooked together chemically so that they can be stored more effectively in the body.When we eat, the carbohydrates in the diet are broken down by the digestive processes into small, simple sugars, mostly glucose, that are carried through the liver and then to all of the cells in the body. When glucose enters either a liver or muscle cell, it is "energized" and processed in such a way that it can become a small piece of the larger glycogen molecule to decrease the osmotic pressure that would occur if the sugar molecule were stored separately. If sugar were not stored as glycogen, the cell would absorb water through the osmotic process until it actually burst.
Glycogen stored in the liver can be released into the blood a few hours after a meal to help maintain normal blood sugar levels. However, muscle glycogen is never released back into the blood but is broken down to be used for energy in the muscle where it is stored.
Glycogen normally makes up about 1 to 2 percent of the wet weight of muscle. In other words, every 100 grams of wet muscle contains about 1 to 2 grams of glycogen. Since muscle makes up about 40 percent of the total body weight in an average person, a 70 kilogram (154 pounds) person would have about 28 kilograms of muscle (62 pounds) and about a pound (420 grams) of glycogen stored in the muscle. Each gram of glycogen has about 4 calories of energy, so the total energy stored in the form of glycogen for this person would be about 1,700 calories.
For long races, this energy could easily be used up, so many athletes "load" their glycogen stores before a race to increase the amount of energy they have. Carbohydrate loading can more than double the amount of energy stored in the muscles, and this can have significant effect on performance in long-distance events.
Several studies have shown that glycogen levels decrease over time with hard training. This means that if you exercise intensely all week, you may end up on Saturday with a low level of muscle glycogen right when you need it most to run a race or accomplish some other endurance task. This is one reason why most athletes "taper" (decrease the total work they do) three or four days prior to competition - to allow the glycogen stores to build to normal or higher than normal levels.
In addition, one research study ("Journal of Applied Physiology," Volume 64:4) showed that consuming carbohydrates within 15 minutes of an intense exercise bout helped restore the glycogen levels much more rapidly than waiting two hours before replacing these stores.
This article recommended consuming 100 grams of carbohydrates (equal to a bowl of cereal or a carbohydrate drink) immediately after endurance exercise and another 100 grams about two hours later, then to eat a well-balanced diet containing carbohydrates such as pasta, potatoes, fruits, vegetables and whole-grained foods.