Glucose, one of the major fuel of our body, is stored in our body as glycogen that basically consist of chains of glucose. Glycogen is the counterpart of starch found in plants. Due to the limiting storage capacity of our body for glycogen, mainly in the liver and muscles, we need a relatively constant supply of carbohydrates throughout the day. To control blood sugar levels for normal metabolic functions, the body constantly store and release glucose depending on the amount of glucose we ingest, in between meals, or during physical activity during which energy requirements are high.

Liver’s glycogen supply are mainly used to regulate blood sugar levels and it is the main energy source of our brain. The brain can use up to 400 calorie per day of glucose from liver’s glycogen. This is also the reason why often in the morning we could experience low energy levels due to the depletion of liver’s glycogen overnight between the long period from dinner to breakfast. Eating a good level of complex carbohydrates, especially at night, will replenish glycogen supply improving mental alertness and physical energy.  Fructose-containing foods and drinks also support replenish of liver’s glycogen reservoir.

It is important to notice that glycogen is not stored alone but it comes with water. For every ounce of glycogen, there are 3 ounces of water. This means that when glycogen is used, water is also removed from the body. Based on this physiological mechanism, many fad diets claim weight loss by reducing the carbohydrate intake and increase the protein intake. This cause a quick replenishment of liver and muscle glycogen with 24-48 hours and loss of several pounds of water, which dieters mistake for loss of body fat. Therefore a week or two of fad diets might result in significant loss of water and gastrointestinal bulk with only insignificant fat loss. This is the reason why weight is quickly regained when going back to a normal, balanced diet.

For athletes understanding how glycogen is used is utmost important to maintain top performance. Glycogen depletion followed by glycogen replenishment is called glycogen loading. Glycogen depletion can cause muscle to become stiff and feel heavy interfering with athletic performance. This is especially true for endurance sports than last for more than 60-90 minutes. Thus, for athletes, carbohydrate management skills are very important (pre-, during, post-exercise) and a constant (complex) carbohydrate intake by eating several serving during the day (meals and snacks).