What is Glycogen?

Glycogen is the main source of carbohydrate that is stored in the human body. It is found mainly in muscle and the liver and is the major energy source for moderate and high intensity exercise.

What is the Structure, Function and Role of Glycogen?

Much like plants store starches, humans also store complex carbohydrates. Glycogen is a glucose polysaccharide, meaning it is made of many glucose molecules bound together. While glycogen is made of glucose and has many branches in its structure, starches are made of amylose or amylopectin, which are less branched and therefore harder to break down. Glycogen is built and broken down by different enzymes, which are available wherever glycogen is stored. The vast majority of glycogen is found in muscles and in the liver, although very small amounts are also found in the kidneys, the brain and some other tissues. The amount of glycogen stored in the muscle depends on how much muscle someone has and what he or she has eaten. But typically, there are 400-900 grams in muscle in total when carbohydrate stores are full. A much smaller amount is stored in the liver, probably around 80g. This total amount of carbohydrate stored would be enough for 1.5 to 2 hours of intense exercise if this was the only energy source that would be used. Athletes who are highly aerobically trained can store more glycogen than those who aren’t. Muscle glycogen can be increased beyond normal amounts by ‘carb loading’, which can maximise the energy available for exercise.

Carbohydrate Digestion and Storage

When carbohydrates are ingested, they are digested and absorbed as individual glucose molecules (or fructose or galactose, depending on the carbohydrate source. (check the articles on Not all carbohydrates are equal and What is a carbohydrate?) Once they are absorbed, they enter the blood and can be used in a variety of tissues, including the heart and the brain. However, when large amounts of carbohydrate are consumed, the majority of carbohydrate goes to forming glycogen

Glycogen is Stored with Water

When glycogen is stored in muscle tissue, water is stored with it. Roughly in a ratio of 3:1; meaning for every 1g of glycogen, 3g of water is also stored. This is why glycogen loading programs can lead to relatively large gains in body weight, despite comparatively small amounts of carbohydrate being stored. This water will become available once glycogen is broken down and can help with hydration of the body. If an athlete stores and additional 400 grams of glycogen as a result of glycogen loading, this will mean a weight gain of roughly 1.6 kg (3.5 lb).

How does Glycogen Act as a Source of Energy?

Glycogen is fundamentally an energy source; it is broken down to provide glucose very quickly when it is needed as fuel, such as during exercise. Carbohydrate is an important energy source during exercise, especially high intensity exercise. If no carbohydrate is being ingested during exercise, glycogen is pretty much the sole provider of energy. This is true in moderate and high intensity exercise, which not only includes traditional endurance exercise like marathons, but also many intermittent sports like soccer or rugby. Glycogen stores can effectively run out or become severely depleted as exercise progresses. When glycogen stores fall below a critical level, this is often known as ‘hitting the wall’ or ‘bonking’. We have plenty of examples in professional sport where athletes lose races because of this.

Liver Glycogen vs. Muscle Glycogen

The glycogen stored in muscle and liver, although the same in terms of structure, have different roles. Muscle glycogen is broken down when energy is needed, and used by the muscle that it is stored in. All muscles can store glycogen, and exercise that uses those muscles will use the glycogen from those muscles. For example, glycogen stored in the biceps and triceps muscles of the upper arm will not be used up at the same rates during running as the glycogen stored in the thigh and calf muscles. Liver glycogen is used to control blood sugar , which can be used by any tissue that needs it, most notably the brain and heart. This means that even when resting, small amounts of liver glycogen are being broken down to keep blood glucose levels constant. Blood glucose is tightly controlled, which is essential for good health. If blood glucose levels are too low (hypoglycemia) this can cause trembling, dizziness or even seizures. If they are too high for long periods (hyperglycemia), this can lead to cardiovascular disease, nerve or kidney damage. The worst effects of poor blood glucose control are limited to people with diabetes, but hypoglycemia can easily happen during long or fasted exercise or in long periods of fasting without food.

Glycogen is a Sensor of Energy Availability

Because thousands of calories can be stored in the body as glycogen, the amount stored is monitored by the body. The amount of glycogen stored can act as an ‘energy sensor’, which can inform other aspects of the body’s metabolism. Low glycogen stores can act as a signal to increase the ability to utilize fat for energy, which is part of the theory behind ‘training low’, or fasted training, to improve aerobic metabolism.

Why is Glycogen Stored More Effectively Immediately After Exercise?

What is a Carbohydrate?

References

Kreitzman S, Coxon A, Szaz K. Glycogen storage: illusions of easy weight loss, excessive weight regain, and distortions in estimates of body composition. Am J Clin Nutr. 56:292S-3S, 1992
Fernández-Elías V, Ortega J, Nelson R, Mora-Rodriguez R. Relationship between muscle water and glycogen recovery after prolonged exercise in the heat in humans. Eur J Appl Physiol. 115(9):1919-26, 2015
Jensen J, Rustad PI, Kolnes AJ, Lai YC. The role of skeletal muscle glycogen breakdown for regulation of insulin sensitivity by exercise. Front Physiol. 2:112, 2011
Taylor R, Magnusson I, Rothman D, Cline G, Caumo A, Cobelli C, Shulman G. Direct Assessment of Liver Glycogen Storage by 13C Nuclear Magnetic Resonance Spectroscopy and Regulation of Glucose Homeostasis after a Mixed Meal in Normal Subjects. J Clin Invest. 97:126-32, 1996
Shao D, Tian R. Glucose Transporters in Cardiac Metabolism and Hypertrophy. Compr Physiol. 6(1):331-51, 2015
Impey SG, Hammond KM, Shepherd SO, Sharples AP, Stewart C, Limb M, Smith K, Philp A, Jeromson S, Hamilton DL, Close GL, Morton JP. Fuel for the work required: a practical approach to amalgamating train-low paradigms for endurance athletes. Physiological Reports. 4(10). 2016