Maximising glycogen storage: the key to athletic success?

The speed read: The fast fuel tank of the muscles - Glycogen

Are all carbohydrates equal? When fuelling intense exercise, probably not. As exercise gets more intense, glucose becomes the predominant fuel for respiration. It can either be exogenous (eaten) or endogenous (stored). Glycogen, the stored form of glucose in our bodies, is a critical energy source for athletes engaging in prolonged or high-intensity exercise. Its availability directly influences performance, endurance, and recovery. While glycogen is predominantly stored in skeletal muscles, the liver also plays a crucial role in maintaining blood glucose levels during exercise and fasting. In this blog, we’ll explore the importance of glycogen storage in both muscle and liver, and how athletes can optimise these stores to enhance performance.

What is Glycogen?

Glycogen is a large, branched molecule made of glucose units. It is stored primarily in muscle and liver cells, acting as the body’s quick-access energy reserve. When carbohydrates are consumed, they are broken down into glucose, which is either used for immediate energy needs or stored as glycogen for future use.

While skeletal muscle glycogen provides the energy needed for muscle contraction during exercise, liver glycogen is essential for maintaining stable blood glucose levels, particularly during periods of fasting or prolonged exercise. Each gram of glycogen is stored with about 3 grams of water, which is why athletes often experience weight fluctuations based on their glycogen levels.

Glycogen storage in muscles

Muscle glycogen serves as the primary fuel for muscle contractions during both aerobic and anaerobic exercise. The amount of glycogen stored in muscles varies depending on the individual’s fitness level, diet, and body size, but on average, a well-trained individual can store between 300 to 700 grams of glycogen in their muscles.

In muscles, glycogen is stored in three distinct locations:

1. Intermyofibrillar Glycogen: The majority of muscle glycogen (about 75%) is stored in this location, near the mitochondria and contractile elements, providing energy for sustained muscle contractions during exercise.

2. Intramyofibrillar Glycogen: This makes up about 5-15% of muscle glycogen and is located within the contractile fibers. Its proximity to the sarcoplasmic reticulum allows it to play a key role in high-intensity exercise by supporting muscle contraction through rapid energy release.

3. Subsarcolemmal Glycogen: This type also constitutes 5-15% of total muscle glycogen and is located just beneath the muscle membrane. It helps support cellular energy needs during lower-intensity exercise and contributes to maintaining muscle function.

The availability of glycogen in these specific compartments ensures that athletes can sustain various intensities of exercise, from long endurance efforts to short, explosive activities. Once glycogen stores in the muscles are depleted, performance decreases sharply, and the athlete may experience muscle fatigue, often described as “hitting the wall.”

Glycogen storage in the liver

Liver glycogen plays a different but equally critical role in energy management. The liver stores about 80-100 grams of glycogen, depending on diet and body size. Unlike muscle glycogen, which is used exclusively by the muscles, liver glycogen is released into the bloodstream to maintain normal blood glucose levels, especially during periods of fasting, rest, or prolonged exercise when dietary carbohydrate intake is low.

Fun fact: the glycogen particles in the liver are 10 times larger than those in muscle and are composed of over 50000 glucose units.

During extended physical activity, liver glycogen ensures that the brain and other organs receive a steady supply of glucose, preventing hypoglycemia (low blood sugar), which can impair cognitive and physical performance. When liver glycogen stores become depleted, the body begins to rely on gluconeogenesis (the production of glucose from non-carbohydrate sources) to maintain blood sugar, but this process is slower and less efficient, making it vital for athletes to replenish liver glycogen regularly.

The role of Glycogen in athletic performance

Both muscle and liver glycogen play vital roles during exercise:

• Muscle glycogen provides energy directly to the working muscles, enabling sustained contraction and higher exercise intensity.

• Liver glycogen helps maintain blood glucose levels, ensuring the brain and muscles have a constant energy supply during prolonged activities.

When glycogen stores in either the liver or muscles become depleted, performance suffers. This is why athletes must not only focus on replenishing muscle glycogen but also ensure liver glycogen is restored for optimal performance and endurance.

Levels of stored glycogen vary between people but 800g of stored glycogen is around 3200 calories of energy, which (on its own) is enough to pedal for around 4 hours at an average of 200w. It's not quite that simple though! The body is always using multiple fuel sources, and research shows that muscle glycogen does not drop below around 10% of maximum levels.

How to maximise Glycogen storage in muscle and liver

For athletes, maximizing glycogen storage in both the liver and muscles is essential for maintaining high performance levels. Here’s how to do it:

1. Consume a High-Carbohydrate Diet

Athletes need to consume sufficient carbohydrates to maximise glycogen stores. Current recommendations suggest 6 to 10 grams of carbohydrate per kilogram of body weight daily, depending on the intensity and duration of their training. Carbohydrates should come from a variety of sources, including grains, fruits, vegetables, and dairy.

To fully load glycogen in both muscles and the liver, carbohydrate intake could be increased to 10-12 grams per kilogram of body weight for 48 to 72 hours before a major competition, a practice known as carbohydrate loading. This not only enhances muscle glycogen stores but also ensures that liver glycogen is maximised, reducing the risk of hypoglycemia during prolonged efforts.

2. Post-Exercise Glycogen Replenishment

After exercise, glycogen stores are significantly depleted, particularly in the muscles. The immediate post-exercise period is the most critical time for glycogen resynthesis. Athletes should consume 1.0 to 1.2 grams of carbohydrates per kilogram of body weight within 30 minutes of finishing exercise, as this is when muscles are most receptive to glycogen replenishment.

Combining carbohydrates with protein (around 0.3 to 0.4 grams per kilogram of body weight) helps boost insulin levels, which further enhances glycogen storage in both muscles and the liver. High-glycaemic index foods, such as white rice, potatoes, and sports drinks, are ideal for quick glycogen restoration during this period.

3. Frequent Carbohydrate Intake Throughout the Day

Glycogen resynthesis occurs in two phases. The first phase happens rapidly within the first 30-60 minutes post-exercise, and the second, slower phase occurs over the next several hours. To sustain glycogen replenishment, athletes should consume frequent carbohydrate-rich meals and snacks every 2-4 hours after intense exercise. This helps restore both muscle and liver glycogen fully.

4. Adapt Training for Glycogen Storage Efficiency

Over time, regular training enhances the body’s ability to store glycogen. Endurance training, in particular, increases both muscle and liver glycogen stores, allowing athletes to perform for longer durations at higher intensities. Athletes who frequently train at higher intensities or perform endurance exercises are better able to spare glycogen, relying more on fat as a fuel source during exercise.

The consequences of glycogen depletion

When glycogen stores in either the liver or muscles run low, athletes experience a sharp drop in performance. Muscle glycogen depletion leads to muscle fatigue and an inability to sustain high-intensity efforts. Depletion of liver glycogen results in a drop in blood sugar levels, causing symptoms of hypoglycemia, such as dizziness, confusion, and reduced cognitive function.

For athletes engaged in multi-day competitions or frequent training sessions, maintaining glycogen stores is crucial to avoid cumulative depletion, which can impair recovery and reduce performance over time.

Conclusion

Glycogen storage in both muscles and the liver is fundamental to athletic performance, endurance, and recovery. By following a high-carbohydrate diet, prioritizing post-exercise nutrition, and adjusting training regimens, athletes can maximise their glycogen reserves, ensuring they have the energy to perform at their best. Understanding how glycogen functions in both muscle and liver can help athletes tailor their diets and training strategies to ensure they have the fuel necessary for sustained athletic success.

Source:

Thomas, D.T., Erdman, K.A. and Burke, L.M. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. Nutrition Reviews, 2016. Available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC6019055/pdf/nuy001.pdf