Fundamentals of Glycogen Metabolism for Coaches and Athletes

Abstract

• Athlete performance relies on the restoration of muscle glycogen stores.
• Restoration requires adequate dietary carbohydrates and time.
• Understanding muscle glycogen variations, restoration time, carbohydrate intake, and nutrient impact is crucial.
• Highlights practical implications of glycogen metabolism research for sports professionals.

Introduction

• Muscle glycogen is crucial during intense or prolonged exercise for ATP production.
• Glycogen degradation rate corresponds with exercise intensity.
• High-intensity and endurance sports deplete glycogen differently.
• Carbohydrate intake recommendations vary with training loads.
• Optimal glycogen restoration requires adequate carbohydrate consumption.
• Glycogen stored in cells, notably in the liver and muscles.
• Weight fluctuations can occur with glycogen storage changes.
• Glycogen resynthesis is critical for training and competition readiness.

Carbohydrate Oxidation During Exercise

• Skeletal muscles rely on ATP from oxidative processes during exercise.
• Blood glucose and muscle glycogen are primary fuels at >60% VO2 max.
• Fast-twitch fibers increasingly rely on carbohydrates with intensity.

Glycogen as a Fuel Source

• Glycogen stored in muscle, liver, and other cells.
• Brain uses glucose for ATP; liver glycogen maintains blood glucose levels.
• Carbohydrate intake during exercise helps maintain glycogen stores.
• Astrocytes in the brain store glycogen, aiding neuronal function.

Carbohydrates and Performance

• Carbohydrates have been known since the 1920s as a critical exercise fuel.
• Consuming carbohydrate-rich diets and during exercise enhances performance and recovery.
• Glycogen storage is crucial for performance; its restoration is key to exercise capacity.

Whole-body Glycogen Stores

• Around 600g of glycogen stored in body, mostly in muscles.
• Muscle glycogen is primary fuel during moderate to high-intensity exercise.

Shape of the Glycogen Particle

• Glycogen particles initiated by glycogenin enzyme.
• Proglycogen and macroglycogen are two forms based on size.
• Glycogen repletion shows biphasic nature.

Location of Glycogen Storage

• Glycogen is distributed to meet cellular energy needs during exercise.
• Intramyofibrillar glycogen is prioritized during exhaustive activity.

Roles of Glycogen

• Beyond being a fuel, glycogen is a sensor, influences osmolality, and regulates training adaptations.

Measurement of Glycogen

• Muscle biopsies are used to study glycogen behavior.
• Liver glycogen estimated through noninvasive techniques.

Factors Influencing Glycogen Stores

• Training, diet, and fitness influence glycogen levels.

• Glycogen supercompensation can occur with high carbohydrate diets.

  • Training and Diet Interventions
    • High carbohydrate diet leads to supercompensated glycogen.
    • Glycogen loading strategies enhance glycogen stores.
    • "Train low, compete high" can influence glycogen storage.
    • Protein or creatine supplements can affect glycogen synthesis.

Muscle Glycogen Storage

• Glycogen synthesis relies on glucose intake, facilitated by insulin.
• Glycogen synthesis is a biphasic process post-exercise.
• Glycogen degradation rate is exercise intensity-dependent.

Glycogen and Adaptation to Training

• Low glycogen enhances signaling and training adaptations.
• Low glycogen levels don't impair resistance training adaptations.

Normal Variation in Glycogen Concentration

• Glycogen levels vary with training, often below supercompensated levels.
• Adequate glycogen levels are necessary for optimal muscle function.

Time Course of Glycogen Replenishment

• Consuming adequate carbohydrates post-exercise restores glycogen.
• High carbohydrate diets lead to complete restoration within 24 hours.

Glycogen Stores and Training Capacity

• Many athletes train without fully restored glycogen.
• Moderate carbohydrate intake can maintain training capacity but may impair performance.

Glycogen Resynthesis After Exercise

• Rapid glycogen synthesis occurs if carbohydrates are ingested after exercise.
• Adequate energy intake is necessary for glycogen repletion.

Amount and Type of Dietary Carbohydrate

• Total carbohydrate intake is crucial; timing and type are secondary.
• High-GI foods can speed glycogen restoration post-exercise.

High-Glycemic Carbohydrates

• High-GI foods can enhance glycogen storage and utilization.
• Important for rapid glycogen resynthesis in demanding training.

Waxy Starch Versus Resistant Starch

• Waxy starches may better support glycogen resynthesis and performance than resistant starch.

Extended Fasting and Ketosis

• Fasting and low-carb diets reduce glycogen and may impair performance.
• Evidence for ketosis as an ergogenic aid is inconclusive.

Other Nutrient Interventions

• Protein with carbohydrates can stimulate glycogenesis post-exercise.
• Creatine and caffeine have shown mixed results in glycogen storage enhancement.

Effects of Sex and Age

• Both sexes restore glycogen similarly if carb intake is adequate.
• Older athletes may require more protein for recovery and glycogen synthesis.

High-Quality Carbohydrate Foods

• Foods high in carbs, such as grains, fruits, and vegetables, support glycogen restoration.
• Carbohydrate-rich diets also ensure intake of essential nutrients.

Helping Athletes with Food and Portion Sizes

• Athletes benefit from understanding carbohydrate content in foods.
• Portion size awareness is crucial for meeting carbohydrate goals.

Practical Tips

• Pack carbohydrate-rich snacks for between training sessions.
• Variety of sports foods available for carbohydrate intake.

Conclusion

• High-carb diets are essential for athletes to maintain glycogen.
• Glycogen restoration is crucial for sustained performance.