What Is Glycogen? Storage, Function, and More

What is glycogen, and what is its role in the body?

Glycogen is a form of glucose stored in the liver and skeletal muscle cells. When blood glucose levels drop, these glycogen stores serve as a backup fuel source for the body. 

Discover the function of glycogen and learn why individuals who follow a low-carbohydrate diet don’t have to rely on glycogen reserves for energy.  

What is glycogen?

Glycogen is a type of polysaccharide that contains many connected glucose molecules, also known as a glycogen chain.

The body can store around 1,700 calories worth of glycogen, which is mainly found in muscle and liver cells. However, the exact amount of glycogen stored in the body depends on an individual's dietary habits, metabolic health, and muscle mass. 

According to research published in Nutrition Reviews, liver glycogen particles are significantly larger than those stored in skeletal muscles, with each glycogen chain containing more than 50,000 glucose units.

Glycogen serves as a readily available energy source when blood glucose levels drop, such as between meals or during workouts. 

While liver glycogen stores help maintain blood sugar levels within a normal range, muscle glycogen provides energy for muscle contractions during physical activity.

Watch the video below to learn more about the role of glycogen.

Glycogen, glucagon, and glucose: how are they different? 

Glycogen, glucagon, and glucose all play critical roles in blood glucose homeostasis, meaning they work together to ensure that blood sugar levels remain within a normal range.

However, each of these compounds has a unique structure and different functions.

Glucose

Glucose, also known as blood sugar, is a monosaccharide composed of a single sugar unit. It’s the body’s primary fuel source and originates from the breakdown of dietary carbohydrates and sugars. 

However, it can also be synthesized in the liver through a metabolic process known as gluconeogenesis or derived from glycogenolysis, which is the release of glucose units from glycogen chains. 

Glycogen

Glycogen is a complex carbohydrate made up of multiple glucose residues linked together in a branched structure. 

This stored form of glucose is mainly found in liver and muscle cells, providing a readily available energy source when blood sugar levels drop.

Glucagon

When blood sugar concentrations start to decline, the pancreas releases glucagon. This peptide hormone stimulates glycogenolysis, the process of breaking down glycogen into glucose molecules, which are then released into the bloodstream.

Glycogen production, storage, and function

Glycogen metabolism is predominantly regulated by glucagon and insulin. 

Insulin is released when blood sugar levels rise in response to carbohydrate and sugar intake. This hormone promotes glycogen synthesis by activating glycogen synthase and inhibiting glycogen breakdown.

Glycogen synthase is an enzyme that facilitates the formation of glycogen molecules by adding glucose units to glycogen chains. 

Glucagon, released when blood sugar levels are low, promotes glycogen breakdown by activating glycogen phosphorylase, an enzyme that cleaves glucose units from glycogen residues.   

Essentially, glycogen is a readily available energy source that can quickly be released to maintain stable blood sugar concentrations. 

Glycogen is also released during periods of acute stress. Epinephrine, a stress-related hormone, can trigger glycogenolysis to provide additional fuel to support the body’s fight-or-flight response. 

Research published in Nature Communications investigated the glycogen concentrations in different tissues and concluded, “Glycogen is stored predominantly in muscle and liver cells, and to a lesser extent in other organs and tissues including kidney, brain, fat and heart.”

It’s believed that this glycogen distribution reflects the body’s metabolic priorities, as muscles and the liver are highly active tissues with large energy demands.

Interestingly, each glycogen molecule is bound to water, and it’s believed that the storage of one gram of glycogen requires around four grams of water. This explains why glycogen depletion can trigger significant fluid loss associated with rapid weight fluctuations. 

According to a study published in Acta Physiologica, glycogen reserves can account for up to five pounds of body weight, especially in athletes or individuals with high glycogen stores.

Glycogen and the keto diet

The ketogenic diet involves limiting carbohydrates to no more than 50 grams daily while obtaining the majority of calories from fat. 

This low-carb diet plan helps maintain stable blood sugar and insulin levels, which has various health advantages, including weight loss, enhanced energy levels, and a lower risk of diabetes, obesity, and metabolic syndrome. 

Because keto limits carbohydrate intake, blood sugar levels remain low, which triggers glycogen release. However, no new glycogen is being deposited, leading to a gradual depletion of glycogen stores. 

Once no more glycogen is available to release glucose into the blood, the body switches to burning fat to generate energy. The breakdown of fat releases large quantities of ketones, a group of organic acids that act as a highly effective fuel source for your cells. 

When the body starts to utilize ketones as a primary energy source, it enters a metabolic state known as ketosis. 

Heart, liver, and muscle cells prefer ketones as an energy source during periods of carbohydrate restriction, and the notion that carbs are the most important fuel is inaccurate. 

While most tissues in the body can utilize ketones, specific cells, such as red blood cells, the cells in the retina, and certain parts of the brain, require blood glucose to function. 

However, dietary carbohydrates aren’t necessary to maintain normal blood sugar levels or satisfy the energy demands of these cells. The liver has the capacity to produce glucose from certain amino acids, glycerol, lactate, and odd-chain fatty acids, a process known as gluconeogenesis. 

This explains why a low-carb diet can still provide sufficient energy for the body, supporting overall physiological functions while meeting the specific needs of glucose-dependent cells without relying on carbohydrate or sugar intake.

Glycogen and exercise performance

Glycogen plays a crucial role in promoting optimal muscle functioning and maintaining your blood glucose concentration during prolonged or intense physical activity. 

While exercising, blood glucose is utilized at a greater rate to fuel your muscles. When blood sugar levels drop, glycogen stores are released to ensure a steady fuel supply for muscle contractions. 

As exercise continues and glycogen stores become depleted, the body may experience fatigue and decreased physical performance as limited energy is available to fuel your cells.

Consuming carbohydrate-rich foods or beverages after a workout will help restore normal blood sugar levels and replenish glycogen stores.

However, the body has a limited capacity to store glycogen, and reserves typically last only for 90 to 120 minutes of moderate physical activity, according to research published in Nutrition Reviews.

Individuals who are fat-adapted have a higher capacity to utilize stored body fat as a fuel source when glycogen reserves are depleted. This can be especially beneficial for those seeking to improve their endurance performance. 

This adaptation is commonly seen in individuals following ketogenic diets, as their bodies have become efficient at using fat for energy production.

This has been confirmed by a study published in Metabolism, which investigated the performance effects of a high-fat diet. 

The authors summarized, “Compared to a high-carbohydrate comparison group, a 12-week period of keto-adaptation and exercise training enhanced body composition, fat oxidation during exercise, and specific measures of performance relevant to competitive endurance athletes.”

Key takeaways

What is glycogen, and what is its function?

Glycogen comprises long chains of glucose molecules and is mainly stored in the liver and muscles. It serves as an easily accessible energy reserve, providing glucose when blood sugar levels decline, such as during periods of fasting or exercise.

The body's capacity to store glycogen is limited, and reserves can be depleted within a few hours. When glycogen stores are exhausted, the body switches to fat-burning to meet its energy demands.

FAQ

1. What is glycogen in simple terms?

Glycogen is the primary storage form of glucose and serves as a readily available energy reserve that can quickly be broken down when blood sugar levels are dropping. 

The liver and skeletal muscles are the main sites for glycogen storage, which helps maintain blood glucose levels and provide energy during physical activity.

2. Is glycogen fat or sugar?

Glycogen is a type of sugar, not fat. When blood sugar concentrations are elevated, excess glucose is converted into glycogen, which is stored primarily in the liver and muscles for future energy needs. 

However, the body can only store limited amounts of glycogen, and when stores are saturated, excess blood sugar is converted into fat for long-term energy storage.  

3. Is glycogen the same as blood sugar?

No, glycogen isn’t the same as blood sugar. Glycogen is a storage form of glucose that’s deposited in the liver and skeletal muscles. Blood sugar refers to the glucose in the bloodstream that provides energy to cells throughout the body. 

4. What happens if you run out of glycogen?

When glycogen stores are depleted, the body is forced to burn stored body fat to generate energy, which releases ketone bodies. Ketones are a highly efficient alternative fuel source that energizes your cells during periods of low carbohydrate availability.

5. Can you gain weight from glycogen?

Yes, glycogen storage can cause water retention, which may lead to weight gain.

It requires around four grams of water to store one gram of glycogen. Depending on an individual’s muscle mass, glycogen reserves can account for up to five pounds of body weight.

6. What happens to glycogen when you don’t eat?

During periods of fasting, blood glucose levels decrease. This signals the body to break down glycogen stores to release glucose molecules and maintain normal blood sugar levels. 

When glycogen stores are depleted, the body will start to burn stored body fats to generate energy, which releases ketones and triggers ketosis. 

7. What is glycogen storage disease? 

Glycogen storage disease (GSD) is a rare genetic defect that affects the enzymes involved in glycogen metabolism. Individuals with GSD are at risk of abnormal glycogen levels, which can lead to various health issues. 

Treatment for GSD typically involves enzyme replacement therapy and managing symptoms through dietary adjustments.

Sources

1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6019055/ 

2. https://www.nature.com/articles/s41467-022-31109-6 

3. https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1748-1716.1970.tb04764.x 

4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6019055/ 

5. https://pubmed.ncbi.nlm.nih.gov/29108901/