Glycogen

If you have no idea what glycogen is or if you have some idea, but you’d like more knowledge on the subject, you’ve come to the right place. Here we will examine several key points for any person hoping to understand how the body functions on a minute to minute basis during non-stressful environments to stressed environments. By the end of this article you will have a detailed understanding of how glycogen plays an incredibly important role in your body’s functioning and be able to answer any relatively detailed question brought to your attention. The article will be designed so that under each bold heading you will find the general information for those that want easy answers, and under the italicized subheadings, you will find the specifics for those more inclined to learning everything there is to know. OK, Class is officially in session.

Disclaimer: A few pieces of information in this article are labeled according to simplicity as to avoid confusion. More detailed, future articles on various sections will take these simpler labels and expound on them to make them fully relatable.

 

What is glycogen?
I would guess that knowing what glycogen is an invaluable starting point for educated and uneducated alike.

Glycogen is stored glucose – simple as that. Restated, glycogen is stored energy.


What does it look like? Structure?
Glycogen, is a polymer made up of chains of activated glucose monomers (single units of glucose).

Specifically, as glycogen is a series of 8-10 activated glucose monomer branches, the branches have their own specific anchor points that play an important role when understanding glycogenolysis (discussed later)(1). These anchor points do two things:

1. a 1g4 glycosidic bond
This is the first of the two bonds. This bond binds individual glucose molecules together into the 8-10 molecule branches. It is found between each glucose molecule that makes up the glycogen chain.

2. a 1g6 glycosidic bond
The second of the two bonds, this bond binds the 8-10 molecule branches held together by the a 1g4 glycosidic bonds between each molecule to one another. So, if there are multiple branches made up of 8-10 glucose molecules, those branches would be held together, at the base, by this a 1g6 glycosidic bond.

 

2. a 1g6 glycosidic

2

1. a 1g4 glycosidic

1

Where is it stored?
As previously mentioned, glycogen is an energy substrate that is stored. It is stored in the muscle and liver cells, primarily.  Approximately 10% of the liver’s weight is due to glycogen, while only 2% of muscle’s weight is due to glycogen (2). However, as we hold far more muscle than we do liver, glycogen is found in higher total amounts in muscles.

Cellular Storage
For those of you interested, glycogen is found in the cytoplasm of the cell; this is the fluid-like section that surrounds the other organelles of a cell (2).

 

Organelles

Why is it important?
This could certainly be an expansive section, but at any given time during a normal day, glycogen plays the key role in regulating your blood sugar. If your blood sugar falls below a certain amount, you die, if it goes above a certain amount, you die (one way or another). So, glycogen (along with other substrates) keeps you from dying by keeping blood sugar within a safe range between meals and during high intensity moments like exercise.

As it is the fastest form of fuel when your body needs fuel immediately (for example, resistance training, or running away from a serial killer at top speed – it happens), it is the primary source of energy in most instances. Essentially, if your body needs a lot of energy in a very short amount of time, it relies on the quick source of glycogen to give itself enough energy for immediate, strenuous work. Also, as your brain usually runs on glucose, glycogen allows the brain to continuously work without interruption due to its constant break down (between meals or during high energy needs) and storage (during meal time or lower energy needs).


How do we release and store glycogen?
Glycogen is in a constant state of flux and as such, we need to discuss its release and storage to fully examine how glycogen keeps blood sugar stable as well as providing high amounts of energy when needed. The two processes that glycogen goes through are:

1. Glycogenolysis (Link)
This is a fancy term for glycogen break down.

So, once glucose has been stored to form glycogen, glycogen is then used by breaking down said glycogen into different chemical compounds with the help of two enzymes that we will not discuss in this article, but will appear in a more detailed article describing this exact process.

2. Glycogenesis (Link)
This is a fancy term for glycogen storage.

If the body is a state of high energy (during a meal, for example), it will begin to store activated glucose into glycogen with the aid of a further two enzymes that, again, will be discussed in more appropriate articles in detail.


 

How do we build glycogen?
Glycogen production is a product of having an overabundance of glucose in one’s system. So, if a person were to consume a meal, glucose levels would rise, and glycogen production would occur. This is not forever true, however. While it is true that glycogen is broken down to stabilize blood sugar levels, a meal would help rebuild glycogen as glucose remains stable due to the ingestion of food, but this only occurs up to a capacity – after reaching said glycogen capacity, adipose tissue (fat) is produced. So, being in an energy surplus (caloric surplus) will likely have your glycogen levels at normal capacity (4).

How much glycogen do we have in our body?
On average, we hold 450-550 grams of glycogen between the muscles and the liver, this can change depending on your physiology (6).

Can we maximize glycogen?
Yes, through a procedure called "super compensation". Those who reduce exercise and maximize carbohydrate intake coming up to an event can increase muscle glycogen stores (7)(8).

How quickly does our body build glycogen?
This is likely dependent on how depleted one’s glycogen stores are, but if completely depleted and favorable nutrition is implemented, it can take more than 20 hours to recover glycogen levels (6).

How quickly does our body use glycogen?
This is highly dependent on the circumstances. If you are in a low intensity state (sitting, sleeping, walking, etc) you are using low amounts of glycogen and your stored glucose will last longer. If you are, however, in a high intensity state (sprinting, lifting weights, etc) you will use glycogen quickly and therefor run out sooner. We usually have enough glycogen to keep us going for 12-14 hours of normal daily activity – this reduces significantly with increased intensity of work (9).

What happens when our body runs out of glycogen?
By now we should know that the body regulates blood sugar with the storage (when excess blood sugar is available) and break down (when too little blood sugar is available) of glycogen, but when glycogen runs out and blood glucose is low, called hypoglycemia, things get interesting.

Essentially, when our body runs out of its reserves (glycogen), and as those reserves are especially important for quick, immediate energy, it shuts off processes that need quick, immediate energy like the ability to sprint effectively, and as brain function is dependent on blood sugar, we tend to be confused, less able to think, among a slew of other issues (10).

Have no fear, however, although it is true that we will die if blood sugar drops below life maintaining levels, the body has other ways of keeping us going. In terms of glycogen, however, any short duration, high intensity work is out of the question until glycogen is restored throughout the body (10). 


Water weight and glycogen?
Likely you have heard that when you lose weight, the first few pounds are the easiest; this is due to glycogen. As we know, glycogen is broken down and released to increase blood sugar levels and maintain energy needs in the short term. So, as we lose weight (a reduced calorie state), our body metabolizes glycogen reducing those levels across the body. However, glycogen is not stored alone, but with water; specifically, approximately 2.4-3.0 grams of water per gram of glycogen (5). So, the first few pounds are indeed water weight and will come back once glycogen is replenished post diet.

What impacts glycogen?
This is a pretty important topic, because glycogen has a strong impact on many aspects of life. Here are some of the main things that affect glycogen:

1. Exercise (FITT)
It absolutely matters in several aspects if we examine the FITT principle.

A. Type
The type of exercise will have a massive impact on your glycogen levels. Aerobic exercise tends to have less of an impact on glycogen than anaerobic exercise, but still uses glycogen depending on intensity (direct relationship between intensity and glycogen use) (11). Increasing intensity so that exercise emulates the effects of anaerobic exercise (for example, weight lifting or sprinting) leads to a majority fuel use from glycogen therefor depleting it quicker.

B. Duration (Time)
This seems pretty self-explanatory, but the longer one exercises, the more glycogen will be used and it will be used more readily depending on the intensity, again (10).

C. Intensity
We’ve already been over it, but the higher the intensity (leaning further toward anaerobic exercise), the faster and more readily glycogen is used (12).

D. Frequency
We know, from earlier sections that it takes hour, if not days, to fully recover glycogen from a depleted state, so taking that knowledge we know that multiple exercise sessions in one day will have a significant impact on glycogen levels, as well (12).

2. Nutrition
Consuming large amounts of carbohydrates as shown in super compensation, but even in reaching back up the normal levels is absolutely imperative for maximal glycogen levels (13).

3. Disease
There are a variety of diseases that can impact glycogen. We will not go through every single one, but Diabetes is one most are familiar with as the regulation of blood glucose levels is impaired. Another interesting disease is Glycogen Storage Disease which, in various ways, decreases the ability of the body to synthesize glycogen effectively (4).

 

SUMMARY

It is difficult to reduce all the information above into one or two concise statements, but glycogen is the primary fuel source during exercise and is the key substrate for blood glucose regulation. Without it, high intensity exercise is impossible and low intensity exercise is drastically reduced, our brain functions less well as glycogen acts as a glucose reserve for times of need (between meals, during exercise, etc).

 

Writer: Nicolas Verhoeven

                                                                                                           Citations

(1) Glycogen. (n.d.). Retrieved August 5, 2015, from http://cmgm.stanford.edu/biochem200/glycogen/

(2) Glycogen Metabolism. (2002). Biochemistry, 5, 75-75. Retrieved August 5, 2015, from http://www.ncbi.nlm.nih.gov/books/NBK21190/

(3) Kreitzman, S. (1992). Glycogen storage: Illusions of easy weight loss, excessive weight regain, and distortions in estimates of body composition. The American Journal of Clinical Nutrition, 56, 292-3. Retrieved August 5, 2015, from http://ajcn.nutrition.org/content/56/1/292S.full.pdf

(4) Glycogen and Glycogen Storage Diseases. (2015, June 13). Retrieved August 5, 2015, from http://www.agsdus.org/html/glycogenandgsds.html

(5) Puckett, H. (1931). The Relation of Glycogen to Water Storage in the Liver. Journal of Biological Chemistry. Retrieved August 5, 2015, from http://www.jbc.org/content/96/2/367.full.pdf

(6) Tardie, G. (2008, February 11). Glycogen Replenishment After Exhaustive Exercise | The Sport Journal. Retrieved August 5, 2015, from http://thesportjournal.org/article/glycogen-replenishment-after-exhaustive-exercise/

(7) Roedde, S. (1986). Supercompensation of muscle glycogen in trained and untrained subjects. Canadian Journal of Applied Sports Science, 11(1), 42-6. Retrieved August 5, 2015, from http://www.ncbi.nlm.nih.gov/pubmed/3698159


(8) Goforth, H., Laurent, D., Prusaczyk, W., Schneider, K., Petersen, K., & Shulman, G. (2003). Effects of depletion exercise and light training on muscle glycogen supercompensation in men. American Journal of Physiology - Endocrinology And Metabolism Am J Physiol Endocrinol Metab, 285(6), 1304-1311. Retrieved August 5, 2015, from http://ajpendo.physiology.org/content/285/6/E1304.full


(9) Vance, K. (2013, October 21). The Best Time to Replenish Glycogen & Exercise. Retrieved August 5, 2015, from http://www.livestrong.com/article/537246-the-best-time-to-replenish-glycogen-exercise/


(10) King, J. (2015, May 11). What Happens When Your Body Runs Out of Glycogen During a Long Workout? Retrieved August 5, 2015, from http://www.livestrong.com/article/415921-what-happens-when-your-body-runs-out-of-glycogen-during-a-long-workout/


(11) Rahnama, S. (2005, October 17). Timing is Everything: Why the Duration and Order of Your Exercise Matters (E. Rust, Ed.). Retrieved August 5, 2015, from http://www.umich.edu/~medfit/resistancetraining/timingiseverything101705.html

(12) Astornio, T., & Kravitz, L. (n.d.). Glycogen and Resistance Training. Retrieved August 5, 2015, from http://www.unm.edu/~lkravitz/Article folder/glycogen.html

(13) Costill, D. (1981). The role of dietary carbohydrates in muscle glycogen resynthesis after strenuous running. The American Journal of Clinical Nutrition, 34, 1831-1836. Retrieved August 5, 2015, from https://www.researchgate.net/profile/David_Costill/publication/15915711_The_role_of_dietary_carbohydrates_in_muscle_glycogen_resynthesis_after_strenuous_running/links/0c9605280dd1bc67e6000000.pdf

(14) Glycogen. (n.d.). Retrieved August 5, 2015, from http://www.diabetes.co.uk/body/glycogen.html

Illustrations:
 

1. http://cmgm.stanford.edu/biochem200/glycogen/

2. https://ajweinmann.wordpress.com/microscope/

3. http://www.theguardian.com/lifeandstyle/the-running-blog/2014/sep/25/bonk-run-long-distance-runner-athlete-beat

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