The Role of Glycogen in Diet and Exercise

When your body needs energy, it can draw on its glycogen stores. The molecules, made from glucose in the food you eat, are mainly stored in your liver and muscles. From these storage sites, your body can quickly mobilize glycogen when it needs fuel.

What you eat, how often you eat, and your activity level all influence how your body stores and uses glycogen. Low-carb and ketogenic diets, as well as strenuous exercise, deplete glycogen stores, causing the body to burn fat for energy.

Most of the carbohydrates we eat are converted to glucose, our main source of energy. When the body doesn't need fuel, the glucose molecules are linked together in chains of eight to 12 glucose units which form a glycogen molecule.

The main trigger for this process is insulin. When you eat a carbohydrate-containing meal, your blood glucose level will rise in response. Increased glucose signals the pancreas to produce insulin, a hormone that helps the body take up glucose from the blood for energy.

Insulin instructs the liver cells to produce an enzyme, glycogen synthase, that links chains of glucose together. As long as glucose and insulin remain plentiful, glycogen molecules can be delivered to the liver, muscle, and even fat cells for storage.

Glycogen makes up around 6 percent of the liver's total weight. Far less is stored in the muscles (only around 1 to 2 percent), which is why we run out of energy quickly during strenuous exercise.

The amount of glycogen stored in these cells can vary depending on how active you are, how much energy you burn at rest, and the types of food you eat. Glycogen stored in muscle is primarily used by the muscles themselves, while those stored in the liver are distributed throughout the body—mainly to the brain and spinal cord.

At any given time, there are about 4 grams of glucose in your blood. When the level begins to decline—either because you have not eaten or are burning glucose during exercise—insulin levels will also drop.

When this happens, an enzyme called glycogen phosphorylase starts breaking glycogen down to supply the body with glucose. For the next eight to 12 hours, glucose derived from liver glycogen becomes the body's primary energy source.

Your brain consumes more than half of the body's blood glucose during periods of inactivity. Over the course of an average day, your brain's demand for glucose accounts for about 20 percent of your body's energy needs.

What you eat and how much you move around also influences glycogen production. The effects are especially felt if you're following a low-carb diet, where the primary source of glucose synthesis—carbohydrate—is suddenly restricted.

When first starting a low-carb diet, your body's glycogen stores can be severely depleted and you may experience symptoms of fatigue and mental dullness. Once your body adjusts and starts renewing its glycogen stores, these symptoms should begin to subside.

Additionally, any amount of weight loss can have the same effect on glycogen stores. Initially, you may experience a rapid drop in weight. After a period of time, your weight may plateau and possibly even increase.

The phenomenon is partly due to the composition of glycogen, which is primarily water. In fact, the water in these molecules accounts for three to four times the weight of the glucose itself.

As such, rapid depletion of glycogen at the onset of the diet triggers the loss of water weight. Over time, glycogen stores are renewed and the water weight begins to return. When this happens, weight loss may stall or plateau.

The body can store around 2,000 calories of glucose as glycogen. For endurance athletes who burn that many calories in a couple of hours, the amount of stored glucose can be an impediment. When these athletes run out of glycogen, their performance almost immediately begins to suffer—a state commonly described as "hitting the wall."

If you're undertaking a strenuous exercise routine, there are several strategies endurance athletes use to avoid decreased performance you may find helpful.

• Carbo-loading: Some athletes eat excessive amounts of carbohydrate before an endurance event. While extra carbs will provide ample fuel, the method has largely fallen out of favor as it can also lead to excess water weight and digestive issues.
• Consuming glucose gels: Energy gels containing glycogen can be consumed in advance of or as-needed during an endurance event to increase blood glucose levels.
• Eating a low-carb ketogenic diet: Eating a diet high in fat and low in carbs can put your body in a keto-adaptative state. In this state, your body begins to access stored fat for energy and relies less on glucose as a fuel source.