Can the body break down glycogen? Understanding the process

December 5, 2025 •

3 min read

According to research, the body stores glucose as a multi-branched polysaccharide called glycogen primarily in the liver and skeletal muscles for later use. The body can then break down this stored glycogen in a process called glycogenolysis to provide a readily available source of glucose for energy. This critical metabolic process is essential for maintaining stable blood sugar levels and fueling muscle activity, especially during periods of fasting or exercise.

Quick Summary

The body breaks down stored glycogen in the liver and muscles through a process called glycogenolysis to release glucose for immediate energy. Hormones like glucagon and epinephrine regulate this catabolic pathway, ensuring a consistent fuel supply to maintain blood glucose and power muscles during physical exertion.

Key Points

What is Glycogenolysis: The breakdown of glycogen into glucose to provide immediate energy, a process primarily occurring in the liver and muscles.
Location-Specific Functions: Liver glycogen regulates overall blood glucose levels, while muscle glycogen provides fuel exclusively for muscle contraction.
Key Enzymes: Glycogen phosphorylase starts the breakdown, and a debranching enzyme handles the complex branch points in the glycogen molecule.
Hormonal Triggers: Glucagon and epinephrine regulate glycogenolysis; glucagon for low blood sugar and epinephrine for stress or exercise.
Glucose Release Varies by Tissue: The liver releases free glucose into the bloodstream, while muscles convert it to glucose-6-phosphate for internal use.
Energy Management: The process is vital for ensuring energy supply during fasting and high-intensity physical activity.

What is Glycogenolysis? The Breakdown of Glycogen

Glycogenolysis is the biochemical pathway by which the body breaks down glycogen into glucose, which can then be used for energy. This process is the opposite of glycogenesis, where glucose is converted into glycogen for storage. Glycogenolysis is triggered in response to hormonal signals that indicate a need for energy, such as during fasting, intense exercise, or stress. The stored glycogen is a highly branched molecule, with numerous non-reducing ends that serve as access points for the enzymes that facilitate its breakdown. This branched structure allows for a rapid release of glucose when energy is needed quickly.

The Location and Function of Glycogen Stores

Where glycogen is stored dictates its primary function and how it is used by the body. The liver and skeletal muscles are the two main storage sites for glycogen.

Liver Glycogen: The liver's glycogen stores are primarily used to maintain overall blood glucose homeostasis. When blood sugar levels drop, the liver breaks down its glycogen and releases glucose into the bloodstream for other organs, especially the brain, to use as fuel.
Muscle Glycogen: In contrast, the glycogen stored in skeletal muscles serves as a local, immediate fuel source for the muscle cells themselves. Muscle cells lack the necessary enzyme, glucose-6-phosphatase, to release glucose into the bloodstream, meaning their energy supply is reserved for internal use, particularly during exercise.

The Step-by-Step Process of Glycogenolysis

Glycogenolysis is a multi-step enzymatic process that involves several key players working in sequence.

Phosphorolytic Cleavage: The enzyme glycogen phosphorylase initiates the breakdown process. It cleaves the $\alpha-1,4$ glycosidic bonds that link the glucose residues together from the non-reducing ends of the glycogen branches. This reaction, known as phosphorolysis, produces glucose-1-phosphate, not free glucose. The enzyme continues to cleave residues until it reaches a point just four glucose units away from an $\alpha-1,6$ branch point.
Debranching: Glycogen phosphorylase cannot break the $\alpha-1,6$ branch points. At this stage, a bifunctional debranching enzyme takes over. First, its transferase activity moves a block of three glucose residues from the branch and attaches it to the end of another chain.
Hydrolyzing the Branch Point: The second activity of the debranching enzyme, the $\alpha-1,6$-glucosidase activity, hydrolyzes the single remaining glucose residue at the branch point, releasing one free glucose molecule.
Isomerization and Dephosphorylation: The majority of the glucose is in the form of glucose-1-phosphate. The enzyme phosphoglucomutase converts this into glucose-6-phosphate. In the liver, the enzyme glucose-6-phosphatase removes the phosphate group, allowing the free glucose to be released into the bloodstream. In muscles, lacking this enzyme, the glucose-6-phosphate enters glycolysis directly to provide energy for muscle contraction.

Hormonal Control of Glycogenolysis

Two primary hormones, glucagon and epinephrine (adrenaline), are crucial for regulating glycogenolysis.

Glucagon: Released by the pancreas when blood glucose levels are low (e.g., during fasting), glucagon acts primarily on the liver. It stimulates the liver to increase glycogen breakdown and release glucose into the bloodstream to raise blood sugar levels.
Epinephrine: Released from the adrenal glands in response to stress or a 'fight-or-flight' situation, epinephrine acts on both the liver and muscles. In the liver, it helps release glucose into the blood. In muscles, it stimulates glycogen breakdown to fuel immediate muscle contraction.

Comparison of Glycogenolysis in Liver vs. Muscle


Feature	Liver Glycogenolysis	Muscle Glycogenolysis
Primary Role	Regulates overall blood glucose levels for the body.	Provides immediate energy for the muscle cells themselves.
Hormonal Stimuli	Primarily glucagon (low blood sugar) and epinephrine (stress).	Primarily epinephrine (stress, exercise) and neural signals.
Final Product	Releases free glucose into the bloodstream.	Utilizes glucose-6-phosphate for glycolysis within the cell.
Key Enzyme Present	Contains glucose-6-phosphatase to dephosphorylate glucose.	Lacks glucose-6-phosphatase, trapping glucose-6-phosphate.
Energy Demand	Sustained need during fasting or between meals.	Immediate, high-demand energy for muscle contraction.

Conclusion: Glycogen breakdown as a survival mechanism

Ultimately, the body's ability to break down glycogen is a fundamental survival mechanism. It ensures a stable and readily available energy supply to power critical functions, especially for the brain, and to fuel muscle activity when energy is in high demand, such as during exercise. This tightly regulated process, controlled by hormones like glucagon and epinephrine, highlights the body's efficient system for managing energy reserves and responding to changing metabolic needs. Understanding glycogenolysis provides insight into how the body maintains glucose balance and supports physical performance and overall health.

For more in-depth information on metabolic pathways, explore the Glycogen Metabolism article on Biology LibreTexts.

Frequently Asked Questions

Glycogen is a complex carbohydrate and the storage form of glucose in the body. It is primarily stored in the liver, which holds a smaller total amount, and in the skeletal muscles, which hold the majority of the body's total glycogen stores.

The breakdown of glycogen is triggered by hormonal signals, primarily glucagon and epinephrine. When blood glucose levels fall (e.g., during fasting), the pancreas releases glucagon. During stress or exercise, the adrenal glands release epinephrine, signaling the body to mobilize energy stores.

The main difference is the end product and its destination. The liver releases free glucose into the bloodstream to maintain blood sugar levels for the entire body. Muscle cells break down glycogen for their own immediate energy needs and cannot release it into the bloodstream.

An inability to properly break down glycogen can lead to inherited metabolic disorders known as glycogen storage diseases (GSDs). These diseases are caused by deficiencies in the enzymes needed for glycogenolysis and can result in symptoms like low blood sugar, muscle weakness, and liver issues.

Yes, during sleep, the body continues to use energy, and as blood glucose levels naturally decrease, the liver breaks down its glycogen stores to release glucose and maintain a stable blood sugar level throughout the night.

Glycogenolysis itself is a catabolic process that releases glucose-1-phosphate, which can then enter either aerobic or anaerobic pathways for energy production. For example, during high-intensity exercise, muscle glycogen is broken down rapidly and converted into lactate via anaerobic glycolysis.

The debranching enzyme is crucial for completely breaking down the branched glycogen molecule. It first transfers three glucose units from a branch to a main chain, then hydrolyzes the final $\alpha-1,6$ bond to release the last glucose residue at the branch point.