Understanding the Role of Glycogen in Exercise and Low-Carb Diets

The Role It Plays in Low-Carb Diet and Exercise

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Glycogen is the stored form of glucose that the body warehouses for future use. It is stored mainly in the liver and the skeletal muscles. When energy is needed, glycogen is quickly mobilized to deliver the body the fuel that it needs.

The amount of glycogen stored in these cells can vary depending on how active you are, how much energy you burn at rest, and the types of food you eat. Glycogen stored in the muscle is primarily used for the muscles themselves, while those stored in the liver are distributed throughout the body but mostly to the brain and spinal cord.

Glycogen is should not be confused with the hormone glucagon, which is also important in carbohydrate metabolism and blood glucose control.

How Glycogen is Made and Stored

Most of the carbohydrate we eat is converted to glucose, our main form of energy. When the body doesn't need fuel, the glucose molecules are linked together in chain comprised about eight to 12 glucose units, resulting in the formation of the glycogen molecule.

The main trigger for this process is insulin. As soon as you have eaten a meal with carbohydrates, your blood glucose level will begin to rise. This will signal the pancreas to produce insulin, a hormone whose role it is to take glucose from the blood for energy.

Insulin does this by instructing liver cell) to produce an enzyme known as glycogen synthase. This is the enzyme that links the chains of glucose together. As long as glucose and insulin remain plentiful, this process will continue, delivering glycogen molecules to the liver, muscle, and even fat cells for storage.

Glycogen can make up to six percent of the liver's total weight. Far less is stored in the muscles (only around one to two percent), which is why we tend to run out energy quickly during strenuous exercise.

How Glycogen Is Used

At any time, there will be about four grams of glucose in your blood. When those levels begin to decline, either because you have not eaten or are burning them off, insulin levels will also drop.

When this happens, an enzyme called glycogen phosphorylase will break glycogen apart to supply the body with the glucose it needs. Thereafter, for the next eight to 12 hours, glucose derived from liver glycogen will be the primary source of energy for the body.

Of all of the body's organs, the brain will consume more than half of it blood glucose during inactivity and around 20 percent during an average day.

Glycogen and Diet

The foods that you eat and the activities you partake in can influence the production of glycogen and the way that your body functions. This is particularly true with a low-carb diet in which the primary source of glucose synthesis—carbohydrate—is suddenly restricted.

When first starting a low-carb diet, glycogen stores can be severely depleted, resulting in symptoms of fatigue and mental dullness. It is only after the body begins to adjust and renew its glycogen stores that you can expect to feel more normal again. To some extent, any weight loss effort can trigger this effect.

Another phenomenon will occur when starting a low-carb diet. On the onset, you will likely experience a rapid drop in weight which, after a period of time, will plateau and even increase. This is because glycogen is composed primarily of water (as much as three to four times the weight of glucose itself).

As such, the rapid depletion of glycogen at the onset of the diet will trigger the rapid loss of water weight. Over time, however, as glycogen stores are renewed, the water weight will return, causing the weight loss to stall. It's important to remember, however, that this is caused by a temporary gain in water weight, not fat weight. Fat loss can continue despite this short-term plateauing effect.

Glycogen and Exercise

The body can store around 2,000 calories of glucose as glycogen. This can become an impediment for endurance athletes who can burn that many calories in a couple of hours. When they run out of glycogen, they will almost immediately be unable to perform, a state commonly described as"hitting the wall."

There are several strategies that athletes use to avoid this. Among them:

  • Carbo-loading is a technique in which you eat excessive amounts of carbohydrate before an endurance event. While this may provide ample fuel, the method has largely fallen out of favor as it can lead to excess water weight and digestion problems.
  • Glucose gels can be consumed in advance of and during an endurance event to increase blood glucose levels as needed.
  • Low-carb ketogenic diet, involving the intake of high fat and low carbs, can place a person in a keto-adaptative state where the body is able to access stored fat for energy and rely less on glucose as a fuel source.
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