Post exercise muscle glycogen resynthesis in humans
Bottom Line
Glycogen is an important fuel source for high intensity training and inadequate glycogen is associated with fatigue. Glycogen replenishment is especially important for athletes who complete more than one intense training session per day. Glycogen replenishment should be planned as a part of training for competitive athletes.
Key Ideas
Glycogen is a key fuel source for athletes who participate in endurance and high intensity exercise. A training plan should include a strategy to align carbohydrate intake with glycogen needs.
Glycogen resynthesis occurs in two phases: an immediate post-exercise phase (0-4 hours) and a longer-term phase (4-24 hours).
Immediate carbohydrate intake within the first two hours after exercise results in higher glycogen storage during that period. There is a glycogen storage threshold of around 7-10 grams per kilogram of body weight.
Factors like energy intake, co-ingestion of other nutrients (especially protein), and substances like creatine can influence glycogen synthesis. Factors such as muscle damage, eccentric exercise, and post-training activities (heat and cold applications) can affect glycogen storage. These effects are generally small.
Supercompensation – the ability to store above normal amounts of glycogen – can be accomplished by modulating carbohydrate intake and exercise intensity. There are several common supercompensation strageies.
Summary
Glycogen is a key fuel source for athletes, and a steady glycogen supply is critical. After training or competition, athletes should have a strategy for refueling. Glycogen resynthesis process occurs in two phases: an immediate post-exercise phase (0-4 hours) and a longer-term phase (4-24 hours).
During the immediate phase, 1 gram of carbohydrates per kilogram of body weight is suggested. In the secondary phase, carbohydrate intake should align with anticipated fuel needs, and total carb intake takes precedence over the type or form of carbohydrates. Co-ingestion of protein can improve glycogen storage in some cases.
This review focuses on matching glycogen stores to fuel requirements and outlines strategies for rapid restoration, with specific attention to conditions where optimal refueling opportunities are limited.
Dietary Carbohydrate and Muscle Glycogen Synthesis
There appears to be a glycogen storage threshold at around 7-10 grams per kilogram of body weight. Carbs above that level offer no additional benefit. During the first four hours after exercise, glycogen storage rates are higher. Evidence shows that intake of larger quantities of carbohydrates consumed in multiple small feedings may be optimal.
Timing of Carbohydrate Intake
Immediate carbohydrate intake within the first two hours after exercise leads to higher rates of glycogen storage during this initial period. Delaying the initial post-exercise feeding likely has minimal adverse effect if there is 24+ hours between sessions. However, with limited time, this initial feeding is critical. As glycogen content increases, the rate of resynthesis decreases. This feedback loop may contribute to the equalization of muscle glycogen content over time among different feeding schedules.
Types of Carbohydrate Intake
Glycogen synthesis is more effective when dietary carbohydrates induce higher blood glucose and insulin responses. Liquid and solid carbohydrate intake similarly affects muscle glycogen storage over 2 to 24 hours. Special carbohydrate drinks with specific properties may enhance glycogen storage in the first two hours.
Energy Intake and Energy Availability
Inadequate energy intake can impair glycogen storage, as glucose is diverted to immediate energy production. The relationship between carbohydrate intake and glycogen storage is influenced by total energy intake. It’s important for athletes to eat enough to permit glucose to be stored as glycogen.
Co-Ingestion of Other Macronutrients
The co-ingestion of protein can enhance post-meal insulin release, stimulating glucose uptake and glycogen synthesis. This effect is most pronounced when protein is co-ingested with carbohydrates below the glycogen storage threshold. Co-ingesting fats has not been extensively studied, and their impact on glycogen synthesis remains unclear.
Other Dietary Agents that Promote Glycogen Storage
Creatine supplementation, when combined with a high-carbohydrate diet, has shown promising results in increasing post-exercise muscle glycogen stores. The mechanism is not fully understood, but it appears to have an impact on glycogen storage, particularly following intense exercise. However, more research is needed to confirm its effectiveness in well-trained athletes.
Non-Dietary Factors Affecting Glycogen Storage
Exercise-induced muscle damage can affect glucose transporter translocation which reduces glucose uptake. Eccentric exercise and exhaustive running have been linked to reduced glycogen restoration during the 24 to 72-hour period after exercise. The mechanisms behind this reduction are attributed to muscle fiber damage and inflammation. Delays in glycogen resynthesis have been observed in studies related to competitive sports such as soccer, potentially due to the specific demands of the sport, especially the stresses associated with deceleration and changing direction.
Post-training activities like heat and cold applications can impact glycogen synthesis. Some studies show reduced storage after intermittent ice application and improved storage with heat. The underlying mechanisms are not fully understood and require further investigation.
Glycogen Supercompensation Strategies
Supercompensation refers to maximal levels of glycogen storage and is broadly used for endurance athletes and others with high energy needs. Traditional models for glycogen loading include depletion phases followed by high-carbohydrate loading.
Recent research suggests that trained athletes can super-compensate glycogen with a shorter taper and high carbohydrate intake without a depletion phase. This newer approach is more practical and avoids the stress of extreme diet and training regimens. Achieving supercompensation typically occurs within 36 to 48 hours of the last exercise session, when the athlete rests and consumes additional carbohydrates. However, complete inactivity pre-competition may not be feasible for all athletes.
Implications for Athlete Practices
An always-high-carb diet to maximize glycogen storage is no longer recommended. Athletes can adapt their carbohydrate intake according to their training intensity. Lower carbohydrate intake during periods of lighter training can lead to greater adaptive responses, while reducing carbohydrates can extend the time during which adaptive responses occur.
Supercompensation strategies can be valuable for pre-competition preparation, normalizing activity schedules, or rapidly restoring glycogen levels in scenarios with limited time windows.
Terms for Further Study
Glycogen: A complex carbohydrate stored in muscles and the liver, serving as a primary energy source during high-intensity exercise.
Glycogen Resynthesis: The process of replenishing muscle glycogen stores after exercise, critical for athletes to recover and perform optimally.
Biphasic Process: Refers to the two distinct phases of muscle glycogen resynthesis: the immediate post-exercise phase (0-4 hours) and the longer-term phase (4-24 hours).
Supercompensation: The process of increasing glycogen stores above normal levels, often used by athletes to enhance performance.
Monosaccharides: Simple sugars such as glucose, fructose, and galactose, which are the building blocks of carbohydrates.
Hexokinase: An enzyme that phosphorylates glucose upon entry into cells, directing it toward glycolysis or glycogen synthesis.
Glycogen Synthase: The enzyme responsible for catalyzing the addition of glucose molecules to the glycogen polymer.
Gluconeogenesis: The process by which the body synthesizes glucose from non-carbohydrate sources, such as amino acids and glycerol.
Glycemic Index: A measure of how quickly a carbohydrate-containing food raises blood sugar levels.
Hypoglycemia: A condition characterized by low blood sugar levels, which can result from depleted liver glycogen stores.
Subcellular Location: The specific areas within muscle cells where glycogen is stored, influencing its utilization during exercise.
Carbohydrate Loading: A dietary strategy where athletes increase their carbohydrate intake to enhance glycogen stores before competition.
Original Paper