The Role of Glycogen in Diet and Exercise

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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, all deplete glycogen stores, causing the body to metabolize fat for energy.

Glycogen Production and Storage

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's cells take up glucose from the bloodstream for energy or storage.
  • Insulin also instructs the liver and muscle cells to produce an enzyme called glycogen synthase that links chains of glucose together.
  • As long as glucose and insulin remain plentiful, glycogen molecules can be delivered to the liver, muscles, and even fat cells for storage.

Most glycogen is found in the muscles and the liver. 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.

Glycogen should not be confused with the hormone glucagon, which is also important in carbohydrate metabolism and blood glucose control.

How Your Body Uses Glycogen

At any given time, there is a set amount of glucose in the 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. Glucose derived from liver glycogen becomes the body's primary energy source.

Glycogen and Diet

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

Fatigue and Mental Dullness

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

Water Weight

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 also contains water. 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.

Gains experienced in the beginning come from water loss, not fat loss, and are only temporary. Fat loss can continue despite the short-term plateau effect.

Glycogen and Exercise

For endurance athletes who burn a lot of 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 carbohydrates 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.
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  1. D'anci KE, Watts KL, Kanarek RB, Taylor HA. Low-carbohydrate weight-loss diets. Effects on cognition and mood. Appetite. 2009;52(1):96-103. doi:10.1016/j.appet.2008.08.009

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