What Is Glycogen?

Glycogen is a branched polysaccharide (also called a polycarbohydrate) composed of many glucose molecules linked together. It is the primary storage form of carbohydrates in the body and is mainly stored in the liver and skeletal muscle.

Your body can break down glycogen to release glucose into the bloodstream when you need a quick energy boost or when blood glucose levels drop.

The amount of glycogen stored in the body can depend on several factors, including diet, fitness level, and body mass.

This article will explore what glycogen is and the important roles it has in the body. It will also discuss how to maintain glycogen levels.

Glycogen Structure

Glycogen is a highly branched glucose polymer. A polymer is a substance that consists of very large molecules. Glycogen is a polysaccharide, meaning it is made up of long chains of monosaccharides (simple sugars) connected by two glycosidic bonds: an alpha-1,4-glycosidic bond and an alpha-1,6-glycosidic bond.

Most of the glucose units are linked linearly by the alpha-1,4-glycosidic bonds. Branch points occur at roughly every eight to 10 glucose units via alpha-1,6 glycosidic linkage.

A fully formed glycogen molecule is believed to contain around 55,000 glucose units. Every glycogen molecule also has glycogenin, a protein involved in glycogen synthesis.

Glycogen is stored in the liver, fat cells, and muscle in a hydrated form that consists of three to four parts water and 0.45 millimoles of potassium per gram of glycogen.

Glycogen Function

The carbohydrates you eat are digested by the body and broken down into simple sugars (glucose molecules) that can be absorbed into the bloodstream. Glucose is the primary energy source for cells, tissue, and organs in the body.

Excess glucose gets stored short term in the liver and muscles as glycogen or long term as fat.

Liver glycogen acts as a reservoir for your body's glucose to help maintain normal blood glucose levels.

When you eat a meal containing carbohydrates, glucose levels in the blood rise, triggering the pancreas to release insulin, a hormone that pulls glucose into cells to be used for fuel or stored for future use. Insulin also acts on liver cells to activate enzymes, including glycogen synthase, which links chains of glucose units together.

When we need additional energy, enzymes break down glycogen to supply the body with glucose.

Glucose broken down from liver glycogen is the body's main source of energy. Unlike glycogen stored in the liver that can be distributed throughout the body, glycogen stored in the muscles is only used to fuel the muscles themselves. This is because muscle cells do not produce the enzyme glucose-6-phosphatase, which is needed to release glucose into the bloodstream.

Glycogen has also been identified in other tissues in the body, including the heart, brain, fat tissue, red blood cells, and kidney. However, its functions in these areas are largely unknown.

How Much Glycogen Do I Have?

The average total body glycogen content is 600 grams, with about 500 grams being stored in muscle and 80 grams in the liver.

Glycogen Metabolism

The two main metabolic pathways of glycogen are glycogenesis and glycogenolysis. In short, glycogenesis is the formation of glycogen from glucose molecules. Glycogenolysis, on the other hand, is the breakdown of glycogen into glucose molecules.

Glycogenesis

Glycogenesis is the process of forming glycogen from glucose. Glycogen is produced depending on the body's demand for energy and glucose. The synthesis of glycogen requires energy from the high-energy nucleotide uridine triphosphate (UTP).

The first step of glycogenesis is the conversion of glucose into glucose-6-Phosphate. This reaction is catalyzed by the enzyme hexokinases.

Next, phosphoglucomutase triggers the phosphate group transfer from carbon 6 to carbon 1, which converts glucose-6-phosphate into glucose-1-phosphate.

Glucose-1-phosphate then reacts with UTP to form UDP (uridine diphosphate)-glucose. This reaction is catalyzed by UDP-glucose pyrophosphorylase.

Glycogen is first synthesized by the protein glycogenin, which acts as a primer to initiate glycogen synthesis. This is because it has the nonessential amino acid tyrosine, which serves as an attachment point for the initial UDP-glucose unit.

Additional glucose molecules are then added to the reducing end of the previous glucose molecule to create a linear chain of around eight to ten glucose molecules. Next, the enzyme glycogen synthase extends the chain by adding glucose through alpha-1,4 glycosidic linkages.

After a chain of at least 10 glucose molecules has been formed, the branching enzyme transfers a terminal segment of at least six glucose molecules from a nonreducing end of the glycogen chain to another glucose molecule, where it is joined with an alpha-1,6 bond.

Glycogenolysis

Glycogenolysis is the enzymatic process that occurs when glycogen breaks down into glucose to maintain blood glucose levels during fasting and fuel muscle contractions. It primarily takes place in liver and muscle cells.

The enzyme glycogen phosphorylase aids in the break down of glycogen into glucose by removing glucose-1-phosphate from the nonreducing end of a glycogen chain.

Next, glucose-1-phosphate is converted to glucose-6-phosphate by the enzyme phosphoglucomutase.

From there, glucose-1-phosphate can enter the glycolytic pathway. If glucose-6-phosphatase is present, free glucose can be formed and released into the bloodstream. This process can happen in the liver but not in the muscle in which glucose-6-phosphatase is absent.

Because glycogen phosphorylase cannot cleave glucose from branches, a debranching enzyme is required to transfer glucose to the linear chain to degrade the branches.

Glycogen and Exercise

Glycogen stores in the muscle and liver decline during physical activity. Athletes who participate in activities that burn many calories, such as long-distance running or cycling, may experience a depletion of glycogen if they do not consume enough carbohydrates. As a result, a person may experience fatigue, weakness, and decreased performance.

Maintaining Glycogen Levels

Some studies suggest that a person may be able to increase their glycogen stores by improving their fitness level and participating in endurance training.

There are also additional strategies to help maintain glycogen levels, including:

• Aim to consume 30–60 grams of carbohydrates per hour following the first hour of exercise.
• Try carbohydrate loading, which is consuming excess amounts of carbohydrates, before endurance exercise.
• Meet the daily carbohydrate intake of 8–12 grams per kilogram of body weight recommended for athletes to replenish glycogen stores.
• Consume 1–4 grams of carbohydrate per kilogram of body weight one to four hours before exercise.

What Happens When Your Body Runs Out of Glycogen?

When glycogen stores become depleted, your body will begin to metabolize fat for energy.

Glycogen Storage Diseases

Glycogen storage disease (GSD) is a condition that happens when a person can not break down or store glycogen properly. It is often caused by a genetic enzyme defect passed down to children from their parents. However, some forms can also appear later in life.

Depending on the type, it can affect the liver or muscles, or other parts of the body. Several different enzymes are utilized by the body to store glycogen. As such, there are many different types of GSD.

The most common types are:

• Type 1 (von Gierke disease)
• Type 2 (Pompe disease)
• Type 3 (Cori disease or Forbes disease)
• Type 4 (Andersen's disease)

Symptoms of GSD include:

• Delayed growth
• Low blood sugar (hypoglycemia)
• A swollen abdomen
• Muscle weakness
• Heat intolerance
• Muscle cramps or pain
  

Summary

Glycogen is a multibranched polysaccharide that is the stored form of glucose in the body. It is mainly synthesized in the liver and muscle cells. Glycogen is a readily available form of glucose and can provide rapid energy when needed. It also plays a role in maintaining our blood glucose concentration.

The two main metabolic pathways of glycogen are glycogenesis (the formation of glycogen from glucose molecules) and glycogenolysis (the breakdown of glycogen into glucose molecules).

Physical activity can lower glycogen stores resulting in fatigue and decreased performance. The best way to maintain glycogen levels in the body is by consuming enough carbohydrates.

A Word From Verywell

Glycogen plays an important role in keeping your body fueled. Glycogen levels vary depending on your diet, body mass, and exercise frequency. If you find yourself exhausted during a high-intensity workout or endurance training, your glycogen levels may be depleted. It's important to consume sufficient amounts of carbohydrates to replenish your glycogen stores to maintain training intensity and energy levels.

Frequently Asked Questions

The best foods to build up glycogen levels are starchy vegetables, fruit, and whole-grains such as oatmeal, quinoa, and brown rice.

Glycogen plays a key role in keeping our muscles fueled during exercise. It also helps maintain healthy blood sugar levels.

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

1. Adeva-Andany MM, González-Lucán M, Donapetry-García C, Fernández-Fernández C, Ameneiros-Rodríguez E. Glycogen metabolism in humans. BBA Clinical. 2016;5:85-100. doi:10.1016/j.bbacli.2016.02.001
2. Nemours Children's Health. Glycogen.
3. Murray B, Rosenbloom C. Fundamentals of glycogen metabolism for coaches and athletes. Nutr Rev. 2018;76(4):243-259. doi:10.1093/nutrit/nuy001
4. Science Direct. Glycosidic Bond.
5. Bezborodkina NN, Chestnova AYu, Vorobev ML, Kudryavtsev BN. Spatial structure of glycogen molecules in cells. Biochemistry Moscow. 2018;83(5):467-482. doi:10.1134/S0006297918050012
6. Kreitzman SN, Coxon AY, Szaz KF. Glycogen storage: illusions of easy weight loss, excessive weight regain, and distortions in estimates of body composition. Am J Clin Nutr. 1992;56(1 Suppl):292S-293S. doi:10.1093/ajcn/56.1.292S
7. MedlinePlus. Carbohydrates.
8. Science Direct. Glucose.
9. University of California. Diabetes Education Online.
10. Colorado State University. Physiologic Effects of Insulin.
11. Science Direct. Glycogen.
12. Adeva-Andany MM, González-Lucán M, Donapetry-García C, Fernández-Fernández C, Ameneiros-Rodríguez E. Glycogen metabolism in humans. BBA Clin. 2016;5:85-100. doi:10.1016/j.bbacli.2016.02.001
13. Science Direct. Glycogen Synthesis.
14. Science Direct. Glycogenin.
15. Science Direct. Glycogenolysis.
16. Science Direct. Glucose-6-Phosphatase.
17. Murray B, Rosenbloom C. Fundamentals of glycogen metabolism for coaches and athletes. Nutr Rev. 2018;76(4):243-259. doi:10.1093/nutrit/nuy001
18. Jeukendrup A. A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Med. 2014;44(Suppl 1):25-33. doi:10.1007/s40279-014-0148-z
19. Kerksick CM, Arent S, Schoenfeld BJ, et al. International society of sports nutrition position stand: nutrient timing.J. Int. Soc. Sports Nutr. 2017;14(1):33. doi:https://doi.org/10.1186/s12970-017-0189-4
20. Carlsohn A. Aktuelle Ernährungsempfehlungen für Ausdauersportler. Dtsch Z Sportmed. 2016;2016(01):7-12. doi:10.5960/dzsm.2015.193
21. Johns Hopkins. Glycogen Storage Disease.
22. Children's Hospital of Philadelphia. Glycogen Storage Disease.

By Lindsey DeSoto, RD, LD
Lindsey DeSoto, RD, is a registered dietitian specializing in nutrition and health and wellness content.

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