What is glycogen?

Glycogen is how our bodies store carbohydrate. It is predominantly stored in our muscles and liver and provides the glucose needed for exercise, particularly the high intensity type.

What is glycogen?

Glycogen is a branched polymer of glucose used as fuel storage. Plants store glucose in starch, we do it in glycogen (1).

Most glycogen in the body is located in the muscle (~300-700g) and liver (~100-200g) (2, 3). Each gram of glycogen is stored in our muscles with 3-5g of water. Therefore, carbohydrate loading will lead to weight gain (3, 4).

Roles of glycogen

Some of the roles of glycogen include:

• Fuel source, providing carbohydrate to working muscles (2, 4)
• Senses fuel levels (4)
• Modulates muscle contraction (2, 4)
• Regulates signalling pathways involved in training adaptation (4)

Glycogen depletion

The higher the intensity of exercise, the greater the breakdown of glycogen (2, 4) and utilisation of glucose for fuel. In muscle, type II (“fast twitch”) fibres contain more glycogen than type I (“slow”) fibres. Each type of fibre will see glycogen depleted depending on how much they are used. Therefore, glycogen from type I fibres will be depleted more quickly during prolonged endurance-type exercise and from type II during intense exercise. At the same time, glycogen utilisation means the body relies less on fat for fuel (2).

The depletion of muscle glycogen causes fatigue and inability to maintain peak levels of muscle function and performance (4).

Glycogen synthesis

The factors that influence the rate of glycogen synthesis are:

• Muscle glycogen depletion, especially through intense exercise (3, 4)
• Carbohydrate intake after exercise (3, 4)
• Increased insulin sensitivity due to exercise (3)
• Increased muscle cell membrane permeability to glucose (3)
• Improved physical fitness (4)
• Age: muscle glycogen synthesis in older athletes might be slower (4)

The body can take up to 24 hours to replenish muscle glycogen after extreme depletion (3).

Timing of carbohydrate intake

The rate of glycogen synthesis is higher during the first 4 hours of recovery (3, 4). Carbohydrate should be taken during this window if the next exercise session is less than 8 hours away (3).

Type of carbohydrate

High glycaemic index (GI) foods increase glycogen storage to a further extent than low-GI foods. It does not matter whether the foods are in solid or liquid format (3, 4).

Quantity of carbohydrate

The literature suggests that 1-1.2g of carbohydrate per kg of body weight per hour is the optimal rate to stimulate glycogen synthesis in the recovery period. The total for a 24 hour recovery period is 10g of carbohydrate per kg when glycogen stores are severely depleted (4).

Protein + carbohydrate

When carbohydrate intake is less than 1g per kg of bodyweight per hour, the consumption of 0.3-0.4g of protein per kg can augment glycogen synthesis similar to larger doses of carbohydrate. In addition, the protein will help with muscle protein synthesis (3, 4).

Creatine monohydrate

Creatine monohydrate can play a role in upregulating glycogen synthesis (3, 4).

Carbohydrate before exercise

Eating carbohydrate 3-4 hours before exercise can help top up muscle and liver glycogen. The latter is related to exercise capacity (2).

Carbohydrate during exercise

Carbohydrate intake during exercise will be used as fuel, therefore sparing glycogen from the liver and muscle for later during the exercise bout. It also keeps the rates of carbohydrate oxidation and subsequent production of ATP high. (2) Exercise sessions that are not long enough to require carbohydrate intake can benefit from a mouthwash rinse (2, 4).

Carbohydrate manipulation protocols

Glycogen supercompensation

This strategy involves a few days of a low carbohydrate diet in combination with intense exercise sessions to deplete glycogen followed by a few days of high carbohydrate with less intense exercise. This protocol can cause a greater increase in muscle glycogen stores through supercompensation (3).

Train-low, compete high

In this protocol the athlete restricts carbohydrate intake during training to improve fat metabolism, but increases carbohydrate intake before and during competition for increased performance (2).

Fasted training

When training in a fasted state liver glycogen will be low. At the same time, fatty acid availability and metabolism will be increased. This model, however, does not guarantee improved performance (2).

Sleep-low, train-low

This protocol involves performing a training session in the evening followed by carbohydrate restriction overnight and a morning training session with low muscle glycogen. Note that all other meals should contain enough carbohydrate to replenish glycogen prior to the evening session. This protocol seems to be beneficial for improving performance (2).

Cons of carbohydrate restriction

• Chronic carbohydrate restriction can suppress immune function (2).
• Carbohydrate restriction can increase protein breakdown, potentially leading to muscle loss (2).

Summary and recommendations

• Current recommendations are to adjust carbohydrate intake based on exercise intensity. Low intensity exercise will not deplete glycogen to an extent that requires large amounts of carbohydrate post-exercise.
• Periods of low carbohydrate intake can be used in combination with high intensity exercise to enhance glycogen synthesis.
• After severe glycogen depletion, replenishment can be enhanced by consuming large amounts of carbohydrate (1-1.2g per kg per hour) particularly within 4 hours post-exercise.
• Consuming 0.3-0.4g of protein in addition to carbohydrate in the recovery period allows for a lower dose of carbohydrate to be consumed while still enhancing glycogen (and muscle) synthesis.

References

1. Roach PJ, Depaoli-Roach AA, Hurley TD, Tagliabracci VS. Glycogen and its metabolism: some new developments and old themes. The Biochemical journal. 2012;441(3):763-87.
2. Hearris MA, Hammond KM, Fell JM, Morton JP. Regulation of Muscle Glycogen Metabolism during Exercise: Implications for Endurance Performance and Training Adaptations. Nutrients. 2018;10(3).
3. Burke LM, van Loon LJC, Hawley JA. Postexercise muscle glycogen resynthesis in humans. Journal of applied physiology (Bethesda, Md : 1985). 2017;122(5):1055-67.
4. Murray B, Rosenbloom C. Fundamentals of glycogen metabolism for coaches and athletes. Nutr Rev. 2018;76(4):243-59.

[Photo by Tom Roberts]

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