What Is Glycogen?

Medically Reviewed by Mahammad Juber, MD on October 14, 2022
Written by Cheryl Whitten
5 min read

What is glycogen?

Glycogen is a form of glucose that helps regulate your blood sugar levels. Your eating and exercise habits play a role in determining your glycogen levels.  

Glycogen is the stored form of a simple sugar called glucose. Your body gets glucose from the food you eat (mostly from carbohydrates) and uses it as fuel for your cells. If you have extra glucose in your blood, your body stores it as glycogen for later use.  

The glycogen compound itself incorporates numerous glucose units packaged together as a large, complex sugar. Depending on your needs, your body does two things with glycogen. It either makes it from glucose, a process called glycogenesis, or it breaks it down to release glucose into your blood, a process called glycogenolysis. 

These processes help your body maintain a steady flow of fuel as you need it, which helps power all the activities of your body. If they are well-regulated, they also protect your body from overly high blood glucose levels. 

You store glycogen mainly in your liver but also in your skeletal muscles, brain, and other tissues. The liver stores a greater ratio in comparison to its own mass, but your muscles store more by total weight because they have a greater mass. About three-quarters of your glycogen is found in your muscles. 

The total amount in your cells, though, depends on a few factors, including:

The storage process (glycogenesis) is activated by a hormone called insulin. Your pancreas releases insulin as glucose levels rise after you eat. This insulin helps your body store unused glucose as glycogen. 

Glycogen serves as an energy reserve for your body. A sudden total loss of fuel would cause major problems for your cell activities and your brain cells, so your body keeps a backup supply. As your blood glucose levels fall, your liver converts glycogen back into glucose and releases it into your blood.  

Muscle fuel. Glycogen in your muscle cells also helps provide energy to your muscle tissue. Your muscles need a lot of fuel to help you move, especially during exercise, but taking it from the blood would cause problems for the rest of the body. So, muscles store glycogen for their own use. 

Self-protection. High blood sugar levels can cause damage, so your body tries to transfer it into your cells as quickly as possible. Making glycogen is a natural way your body protects itself. 

Glucose, glycogen, and glucagon all help power your body, but they have different roles. Whereas glucose is found in your blood, glycogen is found mainly in your liver and muscle cells. Glucose is the basic unit of fuel for your cells, while glycogen is a collection of many glucose molecules, stored for future use. 

Glucagon is the hormone responsible for glycogenolysis, which tells your body to break glycogen into glucose as your blood sugar levels fall. Proteins in your liver called enzymes help speed up this process and return glucose to your bloodstream. 

Glucagon also signals your liver to release fat stores as another form of energy. The combination of fat and glucose helps your body maintain energy and blood sugar levels. 

Glycogen breakdown in your muscles is a little different from what occurs in your liver. The liver breaks it down into glucose for all cells and tissues to use, but your muscles don’t have the same capabilities. Glycogen breakdown in your muscles releases glucose that only your muscles can use. 

Certain activities or body states can trigger glucagon and facilitate glycogen breakdown, including:

To check your glycogen levels, your doctor might perform a biopsy, taking a piece of muscle or liver tissue and looking at it under a microscope. They’ll then calculate the amount of glycogen and glycogen storage enzymes in the tissue. 

These levels can change constantly throughout the day as you eat and exercise, though, so they might first try other simple tests to see if your body has any problems relating to glucose. They might look at your hormones, how much glucose is in your blood, and your liver function. 

Possible tests include:

  • Blood tests for glucose levels and muscle and liver function
  • A liver ultrasound to see if it’s bigger than normal
  • Genetic tests to check for an inherited glycogen storage disease

It’s hard to measure your total glycogen levels because there is no specific test and because your levels constantly change. Still, reports suggest that the glycogen stores across your whole body average around 600 grams, though this will vary depending on your size, eating habits, fitness level, and whether or not you have recently exercised.

Based on biopsy tests, glycogen levels in your muscles and liver range from:

  • Muscles: An average of 500 grams with a normal range of 300 to 500 grams
  • Liver: An average of 80 grams with a normal range of 0 to 160 grams

Glycogen storage disease (GSD) is a rare inherited condition that disrupts your ability to produce or break down glycogen. Related genetic abnormalities lead to the absence of enzymes you need to use or form glycogen. As a result, glycogen builds up in your liver, or in some cases, you can’t make it at all, leading to muscle problems and other symptoms. 

There is no cure for GSD, but treatment can help you manage your symptoms. The treatment varies depending on the type of GSD and includes enzyme replacement therapy for some types and cornstarch therapy for most. Cornstarch therapy involves consuming cornstarch with meals to manage your blood sugar levels. 

You might also need to avoid certain types of carbs and sugars or eat a high-protein diet if you have GSD. Meanwhile, medications can help treat side effects and lower the acid and fat levels in your blood. 

Your body’s ability to use and store glucose is important. If you think you’re having trouble with your blood sugar, talk to your doctor.