What Is Glycogen?

Glycogen is the main way the body stores glucose for later use. Since most of the carbohydrate we eat ends up as glucose, it's important to be able to store some of it to control blood glucose levels and provide glucose to the parts of the body that need it. Glycogen molecules are that storage. Glycogen in animals, including humans, has been compared to starch in plants, as starch molecules are the main glucose storage in plants.

Confusion Alert: Glycogen is sometimes confused with the hormone glucagon, which is also important in carbohydrate metabolism and blood glucose control.

More About Glycogen

Glycogen is a large molecule produced in the liver and stored mainly in the liver and muscle cells. After we eat more carbohydrate than our bodies can use at the moment, glycogen is made from the leftover glucose. Later, when blood sugar levels fall, the glycogen is broken down to release more glucose into the blood. Low-carb diets initially deplete glycogen storage, although to some extent any weight loss diet has a similar effect.

Since glycogen molecules have quite a bit of water attached (three to four times the weight of the glucose), some "water weight" is lost at the beginning of a weight loss diet, and this is particularly true on a low-carb diet. The glycogen stores are partially replaced subsequently, which means some of the "water weight" also returns.

This results in a temporary weight loss stall (but not a fat loss stall).

Glycogen and Exercise

The body can store about 2000 calories of glucose as glycogen. This becomes an issue for endurance athletes (e.g. marathon runners and long-distance cyclists) who can burn that many calories in a couple of hours.

When athletes run out of glycogen, they experience a very uncomfortable state commonly called "hitting the wall" where they lack the energy to continue exercising. Two common strategies to avoid this are:

1. Carbo-loading: eating a lot of extra carbohydrate before an endurance event. This method has largely fallen out of favor.
2. Consuming glucose gels and other carbohydrates that are easy to swallow and digest during the event.

There is a third way that some athletes and coaches are experimenting with, which is to follow a low-carb ketogenic diet until the body reaches a state called keto-adaptation. In this state, the body is able to access stored fat for energy much more readily, and since the body can store very large amounts of calories as fat, glucose becomes much less of a factor in fueling the activity. There have now been many reports of athletes going long periods without large amounts of carbohydrates when they are keto-adapted, and there is some preliminary research on the phenomenon. One example of this is shown in the movie "Run on Fat", which follows a couple who rowed from California to Hawaii on a diet that was 9 percent carbohydrate. It may turn out that glucose storage doesn't have to be the limiting factor it was once thought to be through keto-adaptation.

Sources:

Eberle SG. Endurance Sports Nutrition. Champaign, IL: Human Kinetics; 2014. 

Kreitzman SN, Coxon AY, and Szaz KF. Glycogen storage: illusions of easy weight loss, excessive weight regain, and distortions in estimates of body composition American Journal of Clinical Nutrition. Vol 56, 292S-293S.

Volek JS, Phinney SD. The Art and Science of Low Carbohydrate Performance. Berlín: Beyond Obesity LLC; 2012.