Glycogen – A Basic Understanding

In many of our articles, we mention glycogen as our most effective stored fuel. However, we've rarely explained what glycogen actually is and how it works. This article covers what glycogen is, where it's stored (in the liver and muscles, as well as in different "zones" within the muscle fiber), and how it's used as intensity increases. The main question is how glycogen levels relate to fatigue and why you might feel drained even though the body still has energy left. The conclusion? It's not just about how much glycogen you have, but also where it is and how quickly it becomes available.

Glycogen is a polysaccharide, meaning a molecule composed of many glucose units that are linked together and tightly packed in storage. A polysaccharide contains ten or more glucose molecules and therefore holds a significant amount of energy.

Glycogen is present in small quantities in most cells, but our large energy reserves are located in the liver and muscles. In the liver, we have around 100 grams (400 kcal), which is about 10% of the liver's weight. In muscles, we have approximately 500g (2000 kcal) in a normal state. Considering that muscles make up at least 35-40% of body weight (woman–male), each kilogram of muscle mass in a 75 kg person contains about 15 g of glycogen (1.5%).

The approximately 15 g of glycogen per kilogram of muscle are energy-efficient; it requires about 7% less oxygen to extract energy (adenosine triphosphate – ATP) from glycogen compared to fatty acids. There is a clear connection between low glycogen stores and muscular fatigue, as we can generate less energy when the stores are depleted. This is where it becomes really interesting for those of us who want to avoid fatigue. Without going deep into the sarcoplasmic reticulum's (SR) calcium management, the principle is simple: depleted glycogen stores = less calcium release in the muscle = increased fatigue and reduced potential to exert force.

If you want to explore further, there's an explanatory, slightly nerdy video at the bottom.

Glycogen is found in three locations:

1. Subsarcolemmal glycogen: just under the cell membrane that surrounds a fiber bundle. It is thus connected to the outer fibers.
2. Intermyofibrillar glycogen: between the myofibrils, meaning in the area around the strands that contract when the muscle tenses.
3. Intramyofibrillar glycogen: closest to the actual contraction motor. Located on the inside of the small filaments (actin and myosin) that slide over each other when the muscle shortens.

Interestingly, about 75% of all muscle glycogen exists as intermyofibrillar glycogen (point 2). The other two locations share the remaining amount and account for between 5 and 15% each.

Studies indicate that intramyofibrillar (point 3) glycogen is depleted first because it's closest to the contraction motors. For elite cross-country skiers, this type of glycogen was reduced by about 90% during a one-hour test, while the other two stores decreased by 75-83%.

Researchers still don't fully understand if we can control exactly where glycogen is depleted from or how this might affect performance. However, intramyofibrillar glycogen appears to have a strong connection to muscle fatigue, as it influences calcium release in the SR.

Yes and no, but mostly yes. Firstly, glycogen is stored precisely where it's required: next to the working muscle. This means we bypass the transport step from the mouth through the stomach and intestine to the bloodstream and ultimately to the muscle. It's one of the reasons glycogen is our preferred fuel as soon as more energy is needed. In short: glycogen is always the fastest fuel.

Secondly, glycogen, just like maltodextrin, is a polysaccharide. Since these molecules are highly branched, there are multiple ends on the chain. The enzymes (amylase) that break down the molecule can attach to one end and start working, so the more ends there are, the more enzymes can work simultaneously, and energy is released faster.

Let's dive into a bit of a nerdy tangent: polysaccharides can be constructed from alpha-glucose or beta-glucose. The bond between glucose molecules is known as an alpha or beta bond. In our bodies, we have amylase, which can break down alpha-glucose with alpha bonds—this is glycogen. Beta-glucose with beta bonds, however, forms cellulose, a structure we lack the enzymes (cellulase) to digest. This is what we refer to as dietary fiber: it’s beneficial, passes through the system, and doesn't provide energy.

Since starch and glycogen are highly branched, they can be broken down quickly. The same applies to maltodextrin, a highly branched type of carbohydrate, making it common in sports drinks and energy products for endurance athletes.

The speed at which your glycogen is depleted primarily depends on the intensity of your training: the higher the intensity, the quicker the depletion. A specific example is a well-executed study from 1974 by Karin Piehl (link). In this study, four individuals trained intensively for two hours after consuming carb-rich food (thus starting with full stores). Over the two hours, glycogen levels decreased from 125 to 22, then refilled to 64 after five hours and 86 after ten hours with carb-rich energy intake post-session. Participants returned to the initial level only after 46 hours.

Among the three muscle locations, only intramyofibrillar glycogen can be completely emptied.

Can we pinpoint exactly when you hit the wall? Not quite. An interesting study on elite skiers had participants perform 4 × 4 minute sprints on roller skis (with 45 minutes of rest between each interval) while consuming 1.2 g of carbohydrates per kg of body weight during the rest periods. Already after the first sprint, intramyofibrillar glycogen in type I fibers (the most important, according to the study) was depleted by 50%, while type II fibers (fast, explosive) showed no depletion at all. The two other stores, intermyofibrillar and subsarcolemmal, were depleted by 20-35% each in both type I and type II.

After the fourth interval, the levels had balanced and each glycogen store had been depleted roughly equally. It seems the muscle starts from the store closest and then utilizes the other stores more.

Another observation: the total amount of glycogen decreased by 35% from the first to the fourth interval, but performance was similar between intervals 1 and 4. This suggests that a 35% decrease does not directly impair performance.

The researchers speculate that the sports drink the skiers consumed during the rest period contributed. The muscles likely absorbed glucose from the blood and liver, and the liver replenishes faster than muscles, which may have helped during the rest periods. So: we didn't get an exact time for when the wall hits. But for elite skiers, it seems to take longer than four minutes before it becomes critical. 😉

Uncovering the precise regulatory pathways and control mechanisms that govern glycogen breakdown in a site and fiber-type specific manner may have important implications to enhance exercise performance and muscle function not only in athletes, but also in the general population and in people with glycogen storage diseases. In the future, it may turn out that exercise training and nutritional strategies that ensure the right amount of glycogen in the right place at the right time are more important to enhance muscle function and performance than simply maximizing muscle glycogen loading.