Why is Glycogen Stored More Effectively Immediately After Exercise?
Glycogen is known to be stored quickly immediately post-exercise, while rates of synthesis are slower further away from exercise.
Glycogen is the storage form of glucose in humans, found in muscle and liver tissue, which can be depleted by prolonged exercise. If there is no need to restore glycogen quickly, 24 hours of a high carbohydrate diet is generally enough to restore glycogen levels to normal. However, restoring glycogen quickly is a priority for recovery in the hours after exercise if exercise needs to be performed again soon. Glycogen is known to be stored quickly in the immediate post-exercise time period, while rates of resynthesis are slower further away from exercise.
What factors determine increased glycogen storage in muscles?
Before we dive into the question of this article in more detail, it is important to understand a bit more about the process itself and the factors that are important. Of course, in order to store glucose as glycogen it must be available in the muscle and actually at the site of storage inside the muscle. This means we need to supply it through proper nutrition, we need to make sure it is absorbed into the muscle. Then it needs to be converted to glycogen. Assuming the supply is optimized, we will look at the uptake into the muscle and then the actual formation of glycogen.
What process permits absorption of glucose into muscle cells?
Glucose does not simply diffuse into a muscle cell. The muscle cells have a membrane that glucose cannot cross without a transporter. This transporter is GLUT4. The transporter is normally inactive and not located at the membrane but there are 2 signals that will make sure the transporters move to the membrane to transport glucose. The first is insulin. Insulin (which will be higher after a meal) will increase GLUT4 at the cell membrane. The second factor is exercise. Exercise will also move GLUT4 transporters to the cell membrane. When insulin is lower and exercise is stopped, the transporters return to their homes, away from the membrane.
How is Glucose Stored?
Glycogen is large chain of glucose molecules and it is heavily branched. In order to add one more glucose to these branches there are a number of biochemical process that need to take place. The most important one is ‘gluing’ one more glucose molecule to the existing large glycogen molecule. We have an enzyme for this, and this enzyme is called glycogen synthase. The enzyme is inactivated by exercise but stimulated by insulin. Glycogen synthase is also activated by low levels of glycogen. This makes sense because during exercise you want to mobilize glucose, not store it. And when stores are low (after exercise), you want to make sure they get filled up again.
‘Fast’ Glycogen Resynthesis – Insulin Independent - When Should You Replenish your Glycogen Stores After Exercise?
Glycogen resynthesis is fastest in the 30 to 60 minutes after exercise, and this period is often known as the ‘insulin independent’ phase. This means that the synthesis of glycogen does not require the action of the hormone insulin, which at other times is the primary stimulus for glycogen synthesis. After exercise many GLUT4 transporters will still be at the cell membrane and it takes 30 to 60 minutes for them to return to their homes in the muscle cell. This means that the uptake of glucose in the cell is the highest immediately after exercise and then gradually declines. But of course, we can also influence glucose transport by eating. So, if we start to eat after exercise, we also elevate insulin, and this will stimulate glucose transport as well. Carbohydrate must be ingested soon after exercise to use the effects of the insulin independent phase. In addition, this ingested carbohydrate will increase insulin which will help to maintain GLUT4 transporters at the membrane and this will keep glucose transport high. It will also further activate glycogen synthase, which may already have a high activity if glycogen concentrations in the muscle are low. Therefore, the ‘fast’ glycogen synthesis that occurs immediately after exercise is partly due to the increase in the activity of glycogen synthase (caused by muscle contraction and low glycogen stores), as well as the increased transport of glucose into the muscle cell by increased glucose transporter availability.
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References
- Robergs R. Nutrition and Exercise Determinants of Postexercise Glycogen Synthesis. Int J Sport Nutr. 1(4):307-37, 1991
- Jentjens RL, Jeukendrup AE. Determinants of Post-Exercise Glycogen Synthesis During Short-Term Recovery. Sports Med. 33(2):117-44, 2003
- Price T, Rothman D, Taylor R, Avison M, Shulman G, Shulman R. Human muscle glycogen resynthesis after exercise: insulin-dependent and -independent phases. J Appl Physiol. 76(1):104-11, 1994
- Ivy JL, Kuo C. Regulation of GLUT4 protein and glycogen synthase during muscle glycogen synthesis after exercise. Acta Physiol Scand. 162(3):295-304, 1998
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