What is glycogen?
Glycogen is the stored energy form of glucose found in the human body. Simply, glycogen is many glucose molecules strung together creating one large molecule that can be easily accessed when energy requirements increase.
Where is it found? How is glycogen measured? How much glycogen do we have?
Glycogen is primarily found in two locations in the body. It can be found in the liver and in skeletal muscle. While liver glycogen is not normally measured in humans, it is often measured in the muscle. The gold standard method to measure glycogen in the skeletal muscle is through muscle biopsies. Various reports in literature suggest that the amount of glycogen in the liver ranges from 50 - 100g, roughly equating to 200-400 kcal. This is much less than skeletal muscle which typically has 250 - 750g of glycogen equating to roughly 1000-3000 kcal. As you can imagine, during exercise you can rapidly deplete these stores, hence why intra and inter workout fueling is absolutely critical for maintaining optimal performance.
How is glycogen made?
Glycogen is created by a specific enzyme called glycogenin. Glycogenin is responsible for acting as the core of glycogen and linking together the initial glucose molecules. Once this “core” is created, glycogen synthase acts to extend the glucose branches observed in the glycogen structure. At rest, the carbohydrates that we consume are broken down into glucose and transported into our blood. These glucose molecules are then absorbed by our muscles to create glycogen. During exercise, glycogen is not created from the carbohydrates that we consume. Rather the glucose is absorbed by our muscles and directly broken down into the energy the muscle needs.
How is glycogen broken down and used?
Glycogen acts as one of the key sources of glucose for the exercising muscle. As exercise commences and the muscles begin to contract, there are molecules released that initiate the breakdown of glycogen. The breakdown of glycogen is facilitated by an enzyme which allows the release of glucose molecules that can be subsequently utilized for energy by the muscle. During exercise, liver glycogen can also be broken down into glucose and released into the blood. The glucose in the blood can then be absorbed by the muscle to be used as energy in the same way as muscle glycogen.
How does glycogen change during training?
The rate at which you deplete your glycogen stores is highly dependent on the intensity of your exercise and your level of fitness/training status. The lower the intensity, the less glycogen you will utilize while the higher the intensity the greater your glycogen utilization will be. When you are less fit, or less trained, your reliance on glycogen to provide energy to your muscles will be greater. However, as you become more trained and fit, your reliance on glycogen will decrease for the equivalent energy output. Now, how do you determine the exact amount of glycogen that you are using during training? Similar to how you would determine the amount of glycogen in your muscle, you would have to take a muscle biopsy before and after training to determine the amount of glycogen used (not very practical unfortunately).
How does glycogen change with training?
With training, your ability to store glycogen in your muscle increases significantly. So, the more fit you are, the more glycogen your muscles can store. For example, an untrained individual may have ~250g of glycogen found in their muscles while a highly trained endurance athlete may have ~700g of glycogen in their muscles. Of course, this is dependent on the size of the individual as smaller or larger individuals may have glycogen storage amounts that fall outside of this general range.
How long does it take to replenish glycogen?
Glycogen replenishment occurs at various rates. This range is highly dependent on the phases of glycogen resynthesis. Glycogen resynthesis is typically highest immediately after exercise and decreases over time. Furthermore, this rate can be augmented with increased carbohydrate intake; increasing carbohydrate ingestion increases the rate of glycogen replenishment. However, the rate of glycogen resynthesis is also dependent on how trained you are and your diet. The more trained the individual, the more rapid the replenishment occurs. More details and exact values can be found in this publication by Murray et al., 2018.
How can you modify glycogen stores with diet?
Muscle glycogen stores have been shown to be modifiable with diet. That is, ingesting a large amount of carbohydrates (8-12 g/kg body weight; for a 165lb or 75kg person, this is 600-900g) can increase the maximal amount of glycogen that your muscle can hold. While carbohydrate loading is less beneficial in shorter events, there is a plethora of data that suggests significant improvements to the time to exhaustion in those that carbohydrate load versus those that do not. Essentially, you can go longer before “bonking” (if you don’t fuel of course). However, with increased ingestion of carbohydrates all comes with the associated increase in water weight. For every 1 gram of glycogen stored, about 3 grams of water weight is also included. While this may be a concern for those who are weight conscious, this weight is quickly lost as you exercise. However, the larger concern is the bloated feeling some athletes may experience when carbohydrate loading, so this slight discomfort should be considered.
What happens when we are low on glycogen?
When muscle glycogen levels begin to decline, performance follows suit (as many of us know). Hence, when glycogen stores in both the liver and muscle become depleted, blood glucose levels also decline below resting and exercise levels. This occurs as the muscle continues to extract the glucose in the blood to sustain the energy requirements of the muscle. This is why we experience the dreaded bonk; as blood sugar declines our brain becomes deprived of the necessary glucose that it requires to function causing this uncomfortable and helpless feeling. Hence, the importance of fueling!
What happens to the carbohydrate we take when we exercise?
When we consume carbohydrates during exercise, they will be broken down and absorbed in the stomach and gastrointestinal tract. From there, the carbohydrates will arrive at the liver, then distributed in the blood to the rest of the body. Glucose will leave the liver and enter the bloodstream, as is, to be absorbed by the exercising muscle. However, fructose is different. It must first be converted to glucose before being released into the blood as glucose. Glycogen is rarely, if ever, resynthesized during exercise. Glycogen is usually restored once exercise ceases. This is why it is also important to provide your body with the necessary carbohydrates it needs after exercise to help improve the rate you can replenish your glycogen stores.