Glycogen Primer: What it is and What it Means For Your Performance

You may have heard your riding buddies mumbling something about replacing glycogen stores while stuffing their faces with a personal-sized pizza post-ride. So what is glycogen, and why is it important? First, it’s important to understand the relationship between carbohydrates, glucose and glycogen:

Carbohydrates

Carbohydrates can also be referred to as saccharides and are a group of organic molecules that includes sugars, starches and cellulose (1). They can be made up of multiple saccharide molecules linked together (polysaccharides), two saccharide molecules (disaccharides) or a single saccharide molecule (monosaccharide).

Glucose

Monosaccharides and disaccharides can also be referred to as sugars. Glucose (blood sugar) is an important monosaccharide that provides energy for muscle contractions (1). Glucose is stored as a specific polysaccharide in our bodies called glycogen.

Glycogen

Many molecules of Glucose are chained together to form glycogen, which is stored in our muscles and liver (1). Glycogen is broken down into individual glucose molecules in muscle cells when needed for energy production.

Glycogen is essentially stored carbohydrate, and as we know, carbohydrate as a substrate for endurance exercise is very important. Glycogen is mainly stored in our muscle fibers and liver (1) and is readily available for use during exercise.

A few landmark early studies have set the stage for why glycogen is so important. These studies showed that:

• Muscle glycogen was systematically depleted during heavy (77 percent V02 max) exercise (2).
• Glycogen content was near zero at the time of exhaustion (2).
• Work time to exhaustion was directly related to starting glycogen stores in muscles used (3).

These results have been backed up and confirmed by many related studies (4,11,12). It is clear that glycogen is important, and the amount of glycogen that you have is also important.

A normal, healthy 70 kg male eating a high carbohydrate diet might have around 600g (2,400 calories) of carbohydrate stored as glycogen in their muscles, plus another 90g in the liver (5,6,7). Compare this to about 10g of carbohydrates in the bloodstream (5,6,7), and you can quickly see why glycogen is vital as a source of carbohydrates during exercise.

Glycogen Before Exercise

A high carbohydrate diet is key to maintaining and maximizing glycogen stores (5,6,7). This is pretty straightforward, in order to store carbohydrates, you need to first eat carbohydrates.

There are some slightly more complex strategies for maximizing your glycogen stores, such as carbo-loading or carbohydrate periodization, that involve altering your carbohydrate intake in specific ways, but a more simple strategy will be easier to follow.

The simplest strategy is to maintain a high carbohydrate diet that reflects the energy requirements of your training or racing. The blog posts, Are you Fueling Enough on the Bike? and 3 Strategies to Get Lean and Stay Lean have more specific dietary recommendations.

Glycogen During Exercise

Once you start riding or racing, you will be using up your glycogen stores, especially during the first hour of exercise and during higher-intensity efforts (1). You will not be replacing glycogen stores as you ride, but rather you will increasingly rely on blood glucose and free fatty acids in the later stages of a longer ride or race (10).

Again, see Are You Fueling Enough on the Bike? for detailed nutritional recommendations, but understand that once you have started exercising, the glycogen you started with is all you have to work with, additional carbohydrate needs will need to be met with on-the-bike nutrition.

Glycogen After Exercise

Replacing glycogen stores should be one of your main concerns immediately post-ride or race:

• Chronic depletion of glycogen has been shown to be detrimental to performance during heavy training (9).
• The rate of glycogen replacement is highest immediately following exercise upon ingesting a high-carbohydrate meal (8).
• Glycogen levels should return to normal levels in about 24 hours if you eat a carbohydrate-rich meal post-ride and continue to ingest high levels of carbohydrates in your other meals (8).

In order to ensure that your glycogen stores return to normal levels for your next workout, eat a high-carbohydrate meal immediately post-exercise and continue to eat carbohydrates as a part of your normal meals.

Summary

• Glycogen is a stored carbohydrate and is mainly present in our muscles (1,5,6,7).
• Performance in endurance events is impacted by starting glycogen levels (2,3,5,6,7,11).
• Glycogen is a limited resource once exercise has begun (5,6,7).
• Replacing glycogen stores post-exercise is essential for maintaining performance during day-to-day training and racing (9, 11).
• Glycogen is most quickly replaced immediately post-exercise (8).
• It takes about 24 hours to completely replenish glycogen levels, assuming ingestion of a high carbohydrate meal post-exercise, and a consistently high carbohydrate diet (1,8,12)

Exercise nutrition is a very complex topic, but having a basic understanding of what your body needs in order to perform is an essential tool if you are serious about improving on the bike or in any athletic endeavor.

References

1. Powers, S.K. & Howley, E.T. (2012). Exercise physiology: Theory and application to fitness and performance (8th ed.). New York: McGraw-Hill Humanities/Social Sciences/Languages.
2. Hermansen, L. et al. (1967 October). Muscle glycogen during prolonged severe exercise. Retrieved from https://pubmed.ncbi.nlm.nih.gov/5584522/
3. Ahlborg, B. et al. (1967, June). Muscle glycogen and muscle electrolytes during prolonged physical exercise. Retrieved from https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1748-1716.1967.tb03608.x
4. Bergström J. et al. (1967, October). Diet, muscle glycogen and physical performance. Retrieved from https://pubmed.ncbi.nlm.nih.gov/5584523/
5. Davis, J.A. et al. (1976, October). Anaerobic threshold and maximal aerobic power for three modes of exercise. Retrieved from https://pubmed.ncbi.nlm.nih.gov/985399/
6. Dodd, S. et al. (1988, August). Effects of beta-adrenergic blockade on ventilation and gas exchange during incremental exercise. Retrieved from https://pubmed.ncbi.nlm.nih.gov/3178619/
7. Holloszy, J.O. (1982, May). Muscle metabolism during exercise. Retrieved from https://pubmed.ncbi.nlm.nih.gov/7073462/
8. Ivy, J.L. et al. (1988, April). Muscle glycogen synthesis after exercise: Effect of time of carbohydrate ingestion. Retrieved from https://pubmed.ncbi.nlm.nih.gov/3132449/
9. Costill, D.L. et al. (1988, June). Effects of repeated days of intensified training on muscle glycogen and swimming performance. Retrieved from https://pubmed.ncbi.nlm.nih.gov/3386503/
10. Holloszy, J. (1990). Utilization of fatty acids during exercise. In A. Taylor (Ed.), Biochemistry of Exercise VII. (pp. 319-328). Human Kinetics.
11. Karlsson, J. & Saltin, B. (1971, August 1). Diet, muscle glycogen, and endurance performance. Retrieved from https://journals.physiology.org/doi/abs/10.1152/jappl.1971.31.2.203
12. Conlee, R.K. (1987). Muscle glycogen and exercise endurance: a twenty-year perspective. Retrieved from https://pubmed.ncbi.nlm.nih.gov/3297721/