Glycogen

What is glycogen?

Glycogen is the stored form of glucose that’s made up of many connected glucose molecules.

Glucose (sugar) is your body’s main source of energy. It comes from carbohydrates (a macronutrient) in certain foods and fluids you consume. When your body doesn’t immediately need glucose from the food you eat for energy, it stores glucose primarily in your muscles and liver as glycogen for later use.

Your body creates glycogen from glucose through a process called glycogenesis. Your body breaks down glycogen for use through a process called glycogenolysis. Several different enzymes are responsible for these two processes.

An enzyme is a type of protein in a cell that acts as a catalyst and allows certain bodily processes to happen. There are thousands of enzymes throughout your body that have important functions.

What’s the difference between glycogen, glucose and glucagon?

Glycogen, glucose and glucagon are all related to how your body uses its main source of energy from carbohydrates, but they all have different functions.

You get glucose from carbohydrates in the food you eat. Blood glucose (blood sugar) is the main sugar found in your blood. This sugar is an important source of energy and provides nutrients to your body’s organs, muscles and nervous system. Glucose is very important because it’s the primary source of energy for your brain. In fact, your brain’s constant requirement for glucose is the primary reason why the current recommended dietary allowance (RDA) for carbohydrates for all adults is at least 130 grams per day.

When your body doesn’t need glucose right away, it stores it as glycogen in your liver and muscles.

Glucagon is a hormone your pancreas makes that triggers glycogen to convert back into glucose and to enter your bloodstream so your body can use it for energy. Glucagon and insulin are the primary natural hormones that regulate your body’s blood glucose levels.

Is glycogen a carbohydrate?

Glycogen comes from carbohydrates (a macronutrient), but it’s not technically a carbohydrate. When you eat foods and drink fluids containing carbohydrates, your body digests them and turns them into glucose so it can use the glucose for fuel. Glycogen is the stored form of glucose. It’s made of many connected glucose molecules.

Where is glycogen stored?

Your body mainly stores glycogen in your liver and skeletal muscles (the muscles attached to your bones and tendons), with small amounts in your brain.

Although your liver stores a greater ratio of glycogen than your skeletal muscle, since your total muscle mass is greater than that of your liver, about three-quarters of your body’s total glycogen is in your muscles.

During intense and prolonged exercise, the glycogen in your active muscle cells can substantially reduce. The amount of glycogen in your liver cells varies throughout each day depending on certain factors, including:

• The number of carbohydrates you consume.
• The length of time between your meals.
• The intensity and duration of recent physical activity.

After 12 hours to 24 hours of fasting, liver glycogen is almost totally used up.

What is the function of glycogen?

Glycogen has different functions and uses depending on where it’s stored: your muscles or your liver.

Function of liver glycogen stores

Your body mainly uses the store of glycogen in your liver to help regulate your blood glucose (sugar) levels.

Your body normally carefully regulates your blood glucose primarily with the hormones glucagon and insulin. When your blood glucose levels fall too low (hypoglycemia), your pancreas releases more glucagon. Glucagon, in part, triggers glycogen in your liver to convert back to glucose so it can enter your bloodstream. This process is called glycogenolysis. When glucose is in your bloodstream, cells throughout your body can use it for energy.

The glycogen stores in your liver also partially help with muscle activity and exercise. At the start of exercise, your liver begins breaking down glycogen to maintain blood glucose levels as your working muscles use it for energy. However, your muscles primarily use their own glycogen stores to function.

Function of muscle glycogen stores

Muscle glycogen serves mainly as a source of metabolic fuel for your muscles.

Your muscles need lots of energy to function in order for you to move. If your muscles relied on glucose from your bloodstream for this energy, your body would quickly run out of glucose.

Because of this, your body stores three-quarters of your total glycogen in all of your skeletal muscles so they have a consistent supply of energy, especially during exercise, without dramatically affecting the levels of your blood glucose.

The rate at which your muscle glycogen reduces is primarily related to the intensity of physical activity — the greater the exercise intensity, the greater the rate at which muscle glycogen runs out. As a result, high-intensity activity, such as repeated sprinting, can quickly lower glycogen stores in active muscle cells, even though the total time of activity might be relatively brief.

Your muscles restore with glycogen when you consume enough carbohydrates.

What is glycogen storage disease (GSD)?

Glycogen storage disease (GSD) is a rare inherited (passed down from parent to child) condition in which a person is born without certain enzymes that are necessary for your body to make and/or break down glycogen. As your body uses many different enzymes to process glycogen, there are several types of GSD. GSD often results in liver damage and muscle weakness. With many types of GSD, symptoms first appear in infants or very young children.

What tests check glycogen levels?

There’s no specific test that just measures glycogen levels, and your glycogen levels are constantly fluctuating based on your level of activity and how many carbohydrates you eat throughout the day.

Instead, healthcare providers use other tests to see if there could be an issue with how your body makes and breaks down glycogen (glycogen storage disease, GSD) if you’re having certain symptoms. These tests include:

• Blood tests: A liver function panel and kidney function panel can check how well these organs are working. A blood glucose level test can help detect if your body is using glycogen properly. People with GSD typically have low blood sugar levels.
• Abdominal ultrasound: This imaging test can help your provider see if your liver is enlarged.
• Tissue biopsy: Your provider may take a sample of tissue from a muscle or your liver to measure the level of glycogen or enzymes present.
• Genetic testing: Your provider might recommend genetic testing to see if you carry a gene for glycogen storage disease.

A note from Cleveland Clinic

Glycogen is essential for helping regulate your blood sugar levels and providing energy for exercise. Fortunately, issues related to your body’s ability to make and use glycogen are rare. The best thing you can do for your glycogen levels, especially if you’re an athlete, is to make sure you’re consuming enough carbohydrates every day. Talk to your healthcare provider or a registered dietitian or nutritionist if you have questions about your diet and exercise goals.