Glycogen: The Polysaccharide Stored Form of Glucose in the Liver and Skeletal Muscles

January 12, 2026 •

2 min read

Glycogen, often referred to as 'animal starch,' serves as the primary storage form of glucose in humans and other vertebrates, with the liver and skeletal muscles being its main depots. Its branched structure allows for rapid breakdown, providing a quick energy source when blood glucose levels drop or during periods of intense physical activity.

Quick Summary

Glycogen is the key polysaccharide used for storing glucose in liver and muscle cells. Liver glycogen helps maintain stable blood glucose levels, while muscle glycogen provides a fuel reserve exclusively for muscle contraction.

Key Points

Glycogen is a Polysaccharide: This complex carbohydrate is the stored form of glucose in animals, composed of multi-branched chains of glucose molecules.
Storage Locations: The primary sites for glycogen storage are the liver and skeletal muscle cells.
Distinct Functions: Liver glycogen regulates blood glucose levels for the whole body, while muscle glycogen provides energy exclusively for the muscle's own use.
Enzyme Difference is Key: The liver contains the enzyme glucose-6-phosphatase, allowing it to release glucose into the bloodstream; muscle cells lack this enzyme and therefore cannot share their glucose stores.
Glycogen Metabolism is Regulated: The processes of building (glycogenesis) and breaking down (glycogenolysis) glycogen are controlled by hormones like insulin and glucagon to manage the body's energy needs.
Branched Structure for Rapid Mobilization: The branched nature of the glycogen molecule allows for rapid synthesis and breakdown by providing numerous access points for enzymes.

What is Glycogen?

Glycogen is a complex, multi-branched polysaccharide composed of glucose units linked by $\alpha$(1$\to$4) and $\alpha$(1$\to$6) glycosidic bonds, giving it a tree-like structure. This highly branched arrangement is crucial as it provides numerous terminal glucose residues for rapid breakdown when the body needs energy. It serves as a readily available, short-term energy supply, unlike fat which is for long-term storage. While the liver has a higher concentration, skeletal muscle holds the majority of the body's total glycogen due to its mass.

The Two Storage Depots: Liver vs. Muscle Glycogen

Glycogen's function varies based on its location in the liver or muscles, due to enzymatic differences.

Liver Glycogen: The Body's Glucose Buffer The liver regulates blood glucose. After eating, insulin promotes glucose uptake by liver cells to form glycogen (glycogenesis). When blood glucose drops, glucagon prompts the liver to break down glycogen (glycogenolysis). Liver cells have glucose-6-phosphatase, enabling the release of free glucose into the blood to maintain levels, crucial for the brain.

Muscle Glycogen: Fuel for Contraction Muscle glycogen acts as an internal energy source for muscle contraction during exercise. Muscle cells lack glucose-6-phosphatase, trapping glucose-6-phosphate from glycogen breakdown within the cell for ATP production via glycolysis. This localized fuel is vital for intense activity.

The Metabolism of Glycogen

Glycogen metabolism involves synthesis (glycogenesis) and breakdown (glycogenolysis).

Glycogenesis: Stimulated by insulin when glucose is high, glucose units are added to a glycogen chain with the help of enzymes like glycogen synthase and branching enzyme.
Glycogenolysis: Triggered by glucagon (liver) and epinephrine (muscle/liver) during low blood glucose or stress, glycogen phosphorylase breaks down glycogen, and a debranching enzyme addresses branch points.

Regulation and Clinical Significance

Glycogen metabolism is tightly regulated by hormones and allosteric mechanisms. Imbalances can lead to conditions like diabetes and glycogen storage diseases (GSDs), which are inherited disorders affecting glycogen synthesis or breakdown.

Comparison of Liver and Muscle Glycogen


Feature	Liver Glycogen	Muscle Glycogen
Primary Function	Maintains blood glucose homeostasis for the entire body.	Provides energy for the muscle cells' own use.
Hormonal Regulation	Insulin (storage), Glucagon (release).	Insulin (storage), Epinephrine (release).
Glucose-6-Phosphatase	Present.	Absent.
Glucose Release	Can release free glucose into the bloodstream.	Cannot release free glucose into the bloodstream.
Total Amount in Body	Smaller amount (approx. 100g), higher concentration.	Larger amount (approx. 400g), majority of total body glycogen.

Conclusion: The Vital Role of Glycogen

Glycogen is the critical polysaccharide serving as the body's immediate energy reserve, with liver and muscle glycogen having distinct roles. Liver glycogen regulates systemic blood glucose, while muscle glycogen fuels local cellular activity. This system ensures energy supply for vital organs and muscles. Maintaining glycogen stores through diet is important, especially for those with high energy demands.

NIH Bookshelf: Glycogen Storage Disorders

Frequently Asked Questions

The primary function of liver glycogen is to maintain stable blood glucose levels for the entire body. When blood sugar drops, the liver breaks down its glycogen stores and releases glucose into the bloodstream to supply energy to other organs, especially the brain.

Muscle cells lack the enzyme glucose-6-phosphatase, which is necessary to convert glucose-6-phosphate back into free glucose. As a result, the glucose from muscle glycogen is trapped within the muscle cell and used solely for its own energy needs during activity.

Hormones like insulin, glucagon, and epinephrine regulate glycogen metabolism. Insulin promotes glycogenesis (storage) when blood glucose is high, while glucagon (in the liver) and epinephrine (in muscle and liver) trigger glycogenolysis (breakdown) when blood glucose is low or during stress/exercise.

When glycogen stores are depleted, such as during prolonged fasting or endurance exercise, the body must turn to other energy sources. This includes gluconeogenesis (creating glucose from non-carbohydrate sources like amino acids) and breaking down fats for fuel.

Both glycogen and starch are glucose polymers, but they differ in their structure and function. Glycogen, found in animals, is more highly branched and compact than starch, which is the energy storage form in plants. The higher branching density allows for faster mobilization of glucose from glycogen.

Glycogen storage diseases (GSDs) are a group of inherited disorders caused by enzyme deficiencies involved in glycogen metabolism. This can lead to abnormal accumulation of glycogen in certain tissues, resulting in symptoms such as liver enlargement or muscle weakness.

During exercise, particularly high-intensity activity, muscle glycogen stores are broken down to provide the necessary energy for muscle contraction. The rate of depletion is proportional to the intensity of the exercise. Athletes often consume carbohydrates after a workout to replenish these stores.