The Role of Glycogen in Exercise and Low-Carb Diets

Glycogen is a form of glucose that the body warehouses for future use. It is stored mainly in the liver and the muscles. When energy is needed, glycogen is quickly mobilized to deliver the fuel that the body needs. Low-carb and ketogenic diets work by depleting glycogen, causing the body to burn fat for energy. Strenuous exercise also depletes glycogen stores and burns fat.

Most of the carbohydrates we eat are converted to glucose, our main source of energy. When the body doesn't need fuel, the glucose molecules are linked together in chains comprised of 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 cells 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 6 percent of the liver's total weight. Far less is stored in the muscles (only around 1 to 2 percent), which is why we tend to run out of energy quickly during strenuous exercise.

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 by the muscles themselves, while those stored in the liver are distributed throughout the body, but mainly to the brain and spinal cord.

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

At any given time, there are about 4 grams of glucose in your blood. When the level begins to decline—either because you have not eaten or are burning glucose during exercise—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 blood glucose during inactivity and around 20 percent during an average day.

The foods you eat and the activities you do 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's 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. Initially, you will likely experience a rapid drop in weight which, after a period of time, will plateau and possibly even increase. This is because glycogen is composed primarily of water, which is 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 weight loss to stall. It's important to remember that this is caused by a temporary gain in water weight, not fat. Fat loss can continue despite this short-term plateau effect.

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: This 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.
• Consuming glucose gels: Energy gels containing glycogen can be consumed in advance of and during an endurance event to increase blood glucose levels as needed.
• Eating a low-carb ketogenic diet: These types of diets, which are high in fat and low in 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.

It's a good idea to talk to your healthcare provider before starting a ketogenic diet or a strenuous exercise program, particularly if you have any chronic health conditions.